Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 663

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New Fullerene Materials Obtained in Solution and by High Pressure

High Temperature Treatment

BY

ALEXANDR TALYZIN

ACTA UNIVERSITATIS UPSALIENSISUPPSALA 2001

Dissertation for the Degree of Doctor of Philosophy in Inorganic Chemistry presented atUppsala University in 2001

ABSTRACT

Talyzin, A.V. 2001. New Fullerene Materials Obtained in Solution and by High PressureHigh Temperature Treatment. Acta Universitatis Upsaliensis. Comprehensive Summaries ofUppsala Dissertations from the Faculty of Science and Technology 663. 54 pp. Uppsala.ISBN 91-554-5138-1.

Crystallization of C60 and C70 from organic solution often leads to the formation of newsolvates and other fullerene compounds. In the present thesis, a number of such solvates wereobtained and their phase transitions studied using in situ "in solution" techniques. Newfullerene materials can be also obtained using High Pressure High Temperature (HPHT)treatment. The formation of C60 polymers in thin films and bulk samples has been studiedin situ over a wide pressure-temperature range.New methods for single-crystal growth of fullerenes and their compounds have beendeveloped. It was found by in situ "in solution" XRD and Raman spectroscopy that solvateC60 crystals with benzene, toluene and hexane are stable only in equilibrium with theirsolution. Their melting points coincide with the maximum in the temperature dependence ofsolubility. C70 solvates grown from these solutions are stable out of solution, and decomposeabove the boiling points of the solvents. Vibrational signatures were found for the C60 and C70solvates which are very similar to these for fullerene-sulfur compounds obtained as thin filmsand single crystals. A new C70S8 compound was obtained as relatively large single crystals. C60 polymerisation under HPHT conditions was studied on thin films and showed a thicknesseffect on the phase transition around 20 GPa. Superhard and superelastic films were obtainedby treatment at 23 GPa and 570K. In situ Raman and XRD studies were performed on bulksamples at pressures up to 27 GPa and temperatures up to 850K. Below 13 GPa, only one-and two-dimensional polymers were found to form during the heating. The observedpolymerisation pathway suggests a gradual increase in polymerisation. Above 18 GPa, the insitu Raman spectra obtained during heating remained almost unchanged. The XRD studyshowed that heating at 830K and 13 GPa leads to the formation of a rhombohedral phase witha volume per C60 molecule of 560-570 Å3/M, which is below the value for two-dimensionalpolymers. Nevertheless, no superhard, highly dense phases were observed under theseconditions, in contrast to previous studies.

Key words: fullerene, solution, high pressure, Raman spectroscopy, polymerisation, XRD.

Alexandr Talyzin, Department of Materials Chemistry, The Ångström Laboratory, UppsalaUniversity, Box 538, SE-751 21, Uppsala, Sweden

© Alexandr Talyzin 2001

ISSN 1104-232XISBN 91-554-5138-1

Printed in Sweden by Eklundshofs Grafiska AB, Uppsala 2001

Preface.

This thesis comprises the present summary and the following papers, which are referred to inthe summary by their Roman numerals.

I Molecular dynamics of C70S48: dielectric function and NMR study

A.V. Talyzin, A.S. Grell, F. Masin, A.B. Sherman, V.V. Lemanov, P. Lunkenheimer, R. Brand and A. Loidl, J. Phys. Chem. B, 105, 1162, (2001).

II C60 and C70 solvates studied by Raman spectroscopy

A. Talyzin and U. Jansson, J. Phys. Chem. B, 104, 5064, (2000).

III Single crystal growth of C70S8 -a new phase in the C70-sulphur system

A.V. Talyzin, L.-E. Tergenius, and U. Jansson, J. Cryst. Growth, 213, 63, (2000).

IV Preparation and characterization of C60S16 and C70S48 thin films

A.V.Talyzin and U.Jansson, Thin Solid Films, 350, 113, (1999).

V C70 in benzene, toluene and hexane solutions

A.V. Talyzin and I Engström, J. Phys. Chem. B, 102, 6477, (1998).

VI In situ Raman Study of C60 films at High Pressures,

A.V. Talyzin, L.S. Dubrovinsky, U. Jansson , Phys. Rev. B., 64, (2001).

VII Superhard and superelastic films of polymeric C60.

A.V. Talyzin, L.S. Dubrovinsky, M. Oden and U. Jansson, Diamond and Related Materials, 11, (2001).

VIII Pressure-induced polymerization of C60 at high temperatures-an in situ study

A.V. Talyzin, L.S. Dubrovinsky, T. Le Bihan, U. Jansson submitted to Phys.Rev.B.

IX In situ Raman study of C60 polymerisation at high pressure high temperature conditions."

A.V. Talyzin, L.S. Dubrovinsky, T. Le Bihan and U. Jansson, submitted to J. Chem. Phys.

X In situ XRD study of C60 polymerisation at High Pressure High Temperature Conditions.

A.V. Talyzin, L.S. Dubrovinsky, M. Oden, T. Le Bihan and U. Jansson, in manuscript.

Publications not included in the thesis:

1. Deposition and characterisation of NbxC60 fulleride

A.V. Talyzin, H. Högberg and U. Jansson, submitted to Thin Solid Films

2. Bonding mechanism in the transition-metal fullerides studied by symmetry-selective resonant x-ray inelastic scattering

L. Qian, M. Nyberg, Y. Luo, J.-E. Rubensson, A. V. Talyzin, C. Sathe, D. Ding,4 J.-H. Guo, H. Högberg,T. Kambre, U. Jansson, and J. Nordgren, Phys. Rev. B, 63, 12 667, (2001).

3. Comparative study by Raman spectroscopy of the Ti complex Cp2Ti(η2-C60)⋅C6H5CH3 and TixC60 films

A.V. Talyzin, U. Jansson, A.V. Usatov, V.V. Burlakov, V.B. Shur and Novikov Y.N. to be published in proceedings of the International Workshop "Fullerenes and Atomic Clusters" (IWFAC) 2001, St-Petersburg.

4. Deposition of transition metal carbides and superlattices using C60 as carbon source

H. Högberg, J.O. Malm, A.V. Talyzin, L. Norin, J. Lu and U. Jansson J. Electr. Soc., 147,

3361, (2000).

5. Dielectric properties of C70-solvate crystals grown from a benzene solution

A. Sherman, A.V. Talyzin, M.El. Gholabzouri, P. Likenheimer, R. Brand and A. Loidl, J. Phys. Chem B.,102 ,7511, (1998).

Table of content.

1. Introduction 2. C60 and C70 molecular compounds 2.1 Structures and phase transitions of C60 and C70 2.2 Fullerenes solvates and their role in the "anomalous" temperature dependence of solubility 2.3 C60 and C70 compounds with sulfur 3. C60 polymerisation at High Pressure High Temperature Conditions 3.1 The 2+2 cycloaddition mechanism of polymerization and photopolymerization of C60 3.2 One-and two-dimentional polymers obtained at HPHT conditions 3.3 Superhard materials and three-dimensional polymerisation at HPHT conditions4. Experimental 4.1 Synthesis 4.1.1 Crystal growth of C60 and C70 compounds from solution 4.1.2 Preparation of C60 and C70 sulfur films 4.1.3 In situ high pressure studies 4.1.4 Study of the C60 and C70 solvates "in-solution" 4.2 Characterization 5. Study of C60 and C70 molecular solids prepared in solution 5.1 C60S16 and C70S48 thin films 5.2 Crystal growth of C70S48 and new compound C70S8 5.3 XRD study of C70 solvates. 5.4 Raman spectra of C60 and C70 solvates compared to sulfur compounds 5.5 Rotation of C60 and C70 in molecular solids 6. High Pressure C60 studies 6.1 High Pressure study of C60 films. 6.2 In situ Raman study of the C60 polymerisation at HPHT conditions 6.2.1 Pressure region below 8 GPa 6.2.2 Pressure region 8-18 GPa 6.2.3 Pressure region above 18 GPa 6.3 In situ XRD study of C60 polymerisation using synchrotron radiatio 6.4 A revised P-T diagram for C60 6.5 Reproducibility problem in C60 polymerisation studies 6.6 Can compounds with covalent C60-S bonds be prepared by high pressuretreatment?7. Concluding remarks and Future Outlook Acknowledgements References

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1

Chapter 1

Introduction

Fullerene-related research has exploded after the discovery of a method for preparation ofgram quantities of solid C60 by Krätschmer et al. in 1990 [1]. More than 10000 papers havebeen published during the last ten years and the publication rate is still about 1000 per year. Itis clear that fullerene science has the potential to produce many new materials with a largeimpact on modern technology. It is rare in the modern science that a completely new type ofmaterials emerges with such intriguing properties. This can only be compared to the wave ofactivity caused by the discovery of the high temperature superconductors in the 80's.

One example of new fullerene materials is the superconductive alkali metal-doped C60compounds, which have attracted a lot of attention in recent years. Another example of amaterial with unique properties is TDAE-C60, which is ferromagnetic with the highesttransition temperature of all known non-polymeric organic materials.

The purpose of my work has been to search for new fullerene materials. In the present thesistwo different approaches were attempted. The first is based on a solution method.Crystallization experiments with C60 and C70 as well as studies of the phase transitions ofsolvates have been conducted as a further development of my previous work, which wasstarted at A. F. Ioffe Institute (St.-Petersburg). Several sulfur-fullerene compounds were alsosynthesized from solution and studies of them showed many similarities to fullerene solvates.In general, the solution method resulted in studies of compounds with weak van der Vaalsbonding between C60 and the solvent or sulfur molecules. In contrast, the high pressureapproach leads to formation of materials where C60 molecules are linked by strong covalentbonds. A number of polymers with C60 molecules linked by strong covalent bonds are knownto form at High Pressure High Temperature (HPHT) conditions, but most studies have beenperformed ex situ on quenched samples. Therefore, I performed a study at HPHT conditionsin situ using Raman spectroscopy and XRD. A special attention was paid to the pressure-temperature region where "superhard" fullerites have been reported.

Although these two approaches, solution and high pressure, seems to be very different, thereis a connection point. When all attempts to synthesize a C60 compound with covalent bondingto sulfur using solution and vapor methods failed, the high pressure techniques seemed to be avery good next step in this direction. Using C60S16 grown from solution as a precursor forhigh pressure synthesis of sulfur-C60 with covalent bonds appeared to be a link from solutionwork to high pressure research.

2

Chapter 2

C60 and C70 molecular compounds

2.1 Structures and phase transitions of C60 and C70

The C60 molecule has the shape of a truncated icosahedron (point group Ih) with a meandiameter of 7.1Å. A truncated icosahedron has 60 vertices, 12 pentagonal faces, 20hexagonal faces and 90 edges. Although each C site is equivalent, the C-C bonds are ofdifferent lengths, which result in a slightly distorted icosahedron. The C60 molecule has twodifferent bonds; the fusion between two hexagons ("6:6 bond") with a length of 1.40Å and thefusion between a pentagon and hexagon ("5:6" bond) with a length of 1.45Å. The 6:6 and 5:6bonds can also be referred to as double and single bonds, respectively [2].

At room temperature, the C60 molecules in the solid are rotating rapidly with three degrees ofrotational freedom and an average rotation period t = 10-11s. [3] The molecular centersthemselves are arranged on a face-centered cubic lattice (a=14.17Å). Below a characteristictemperature of T01=261K, the C60 molecules lose two of their three degrees of rotationalfreedom [4,5]. The structure of the ordered phase is simple cubic with a cell parameter of14.13Å [6].

Evidence for another "phase" transition at lower temperatures (90 K to 165 K) has beenprovided by a number of different experimental techniques. A detailed study showed that thetemperature T02 obtained for this phase transition depends on the time scale of themeasurement [7]. This phase transition occurs due to a further decrease of the rotationalmotion and is often referred to as a "glassy" transition.

The second most abundant fullerene molecule is C70, which has the shape of a rugby ball.The C70 molecule has five inequivalent sites and eight distinct bond lengths. At hightemperatures the C70 molecules in the solid has a free rotation similar to C60. During thecooling solid C70 exhibits a number of phase transitions which occur due to the step by stepfreezing of rotational freedom. At high temperatures C70 grows most often as a mixture of fccand hcp phases. The sequence of phase transitions upon cooling is complicated and yet notfully understood. Different researches have reported different phase transition temperatures.The most frequently reported sequence of transitions is [8,9]: fcc - rhombohedral - monoclinic 340K 275K

However, other studies have shown up to five different solid C70 phases and four phasetransitions, (see e.g. Verheijen et al. [10]). It has been argued also that the high temperaturehcp phase has its own set of phase transitions and since it is almost always obtained as amixture with the fcc phase the total picture can be very complicated. Most of the studiesagree, that while in the high temperature phases the C70 molecules have a free rotation, atroom temperature only rotation along the long axis is possible and below approximately 275Krotation is frozen [11].

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2.2 Fullerenes solvates and their role in the "anomalous" temperature dependence ofsolubility

C60 and C70 are soluble in a number of organic solvents, but the solubility is rather low [12].Typically, when C60 crystallizes from a solution in organic solvents, the resulting structureappears to be different from the pristine C60 solid due to formation of low symmetry solvates.It is known that C60 forms solvates with a number of different organic solvents [13-15]. Manyof these solvates are unstable at room temperature and lose solvent very fast immediately aftertaking them out of solution. Nevertheless, structures of many C60 solvates have beensuccessfully solved at low temperatures. These studies showed that each of the solventsproduces its own kind of phases with weak van der Waals bonding and different symmetriesranging from triclinic to cubic. A review on known structures of C60 solvates has beenpublished by Ceolin et al. [16].

An interesting property of the solvates was discovered by studying the temperaturedependence of the C60 solubility. Ruoff et al. reported that this temperature dependence has amaximum near room temperature for several solvents [17]. Smith et al. showed that thisanomalous behavior of the solubility can be explained thermodynamically [18]. According totheir model, two phases can exist in equilibrium with solution; one of them with a positiveenthalpy of dissolution, another with a negative. These phases can be either both solvatedwith different amounts of solvent, or one of them can be pure C60. Usually at low temperaturethe solvate is stable in the solution but at some temperature a phase transition between the twophases occurs. The solvated crystal melts and another phase forms such as pure C60 or C60with a smaller amount of solvent. This phase transition gives a change in the slope of thetemperature dependence of solubility. Such a phenomenon is not unusual and example of suchsystem can be found even in basic course books on Physical Chemistry. (See Fig.2.1)

Figure 2.1 Temperature dependence of solubility for several solids in water. Solubility ofNa2SO4 shows a maximum due to the existence of the solid solvate Na2SO4·10H2O. From ref.[19]

4

Differential scanning calorimetric (DSC) experiments made for a number of solvents showedthat such transitions could be seen as a maximum in the temperature dependence of the heatcapacity [20-22]. At that time DSC data were the only available evidence for a phasetransition but the nature of the phases below and above the transition point was unknown. Forexample, it was argued that this phase transition may occur between two solvates withdifferent amount of solvent. My study showed for the first time direct observations of thephase transition C60 -C60⋅4C6H6 in benzene [23]. The phase transition was observed by opticalmicroscopy and X-ray powder diffraction The C60⋅4C6H6 solvate is stable in equilibrium witha saturated solution at temperatures below 313K while the pure C60 fcc phase is stable abovethis temperature. Crystals of the benzene solvate can be easily obtained from C60 powder byspontaneous recrystallization of a C60 powder in equilibrium with a solution cooled to 278-280K. These crystals remain stable upon heating to 313K where a phase transition to pure fccC60 occurs [23]. Although a maximum on a solubility dependence is very typical for C60 indifferent solvents (e.g. benzene, toluene, hexane), there are also examples of solvents such asCS2 which shows an usual linear dependence of solubility without a maximum [18].

Even less information have been available for C70 solvates formed by crystallization fromdifferent solutions. The first remarkable difference from the C60 solvates is the absence ofmaxima in the solubility curves of C70 for the same solvents [24]. The second importantdifference is the stability of C70 solvates out of solution. The C70-toluene solvate was firstdescribed by Agafonov et al. [25] and studied later by Takahashi [26] with high-resolutiontransmission electron microscopy. This solvate phase exhibits an orthorhombic structure withthe Amm2 space group. No other C70 solvates were studied at the starting point of my studies.At the same time there were reports that pure unsolvated C70 crystals have been grown fromtoluene near the boiling point which pointed towards the possible existence of phasetransitions similar to those observed for C60 in different solutions [27].

2.3 C60 and C70 compounds with sulfur

Compared to other groups of fullerene compounds, relatively little attention has been paid tocompounds of fullerenes with sulfur. The properties of these compounds are determined bythe strong tendency of sulfur to form Sn rings (n=6-20). In most modifications, sulfur form S8rings, which are maintained also when sulfur is dissolved in organic solvents such as CS2. S8rings are also present in the vapor during sublimation and vaporization of most sulfurmodifications or of melts thereof. Hitherto, all known fullerene-sulfur compounds consist offullerenes and S8 rings which are weakly bonded to each other with van der Waals bonds (seee.g. ref. [28-31]). For example, Roth and Adelmann have synthesized and determined thestructure of C60S16 grown from a solution of C60 in a mixture of CCl4 and CS2 [28]. TheC70S48 and C76S48 compounds have also been synthesized and their structures have beendetermined [29-31]. The structure of the C70S48 is shown in Fig.2.2.

5

Figure 2.2 Crystal structure of the C70(S8)6. From ref. [29].

Theoretical predictions have shown that a phase with atomic sulfur connected by covalentbonds to two carbon atoms of C60 (similar to well known C60O phase) may exist [32,33]. Sofar, all attempts to synthesize this compound have failed. Nevertheless the recent success insynthesis of the dimeric C120SO compound have showed that covalent bonding of sulfur tocarbon atoms of the C60 cage is possible, although attempts to synthesize C120S2 failed [34].

6

Chapter 3

C60 polymerization at High Pressure High Temperature Conditions

3.1 The 2+2 cycloaddition mechanism of polymerization and photopolymerization of C60

It is now well established that C60 molecules can polymerize by breaking double bonds of theneighboring molecules and joining two molecules by a four-membered ring (see Fig.3.1). It isalso clear that more then one square ring per C60 molecule can be formed to produce astronger polymerization.

Figure 3.2 Dimer of C60 molecules connected by a 2+2 cycloaddition mechanism [35].

C60 can be polymerized by laser light or by prolonged exposure to UV light [35-37]. Thesepolymers are insoluble in the organic solvents typically used for C60 (e.g. benzene, toluene)and show a small decrease of the cell parameters of the cubic fcc structure from 14.17Å to14.05 Å [38]. Recent results show that the phototransformation can lead to the formation ofchains with two square rings per molecule and branched chains (three square rings permolecule)[39]. Polymerization by the 2+2 cycloaddition mechanism has also been observedin C60 doped with alkali metals. The alkali metal in these compounds catalyses thepolymerization process [40]. Finally, C60 polymerization occurs easily also under HighPressure High Temperature conditions (HPHT) as will be discussed below. The square ringconnecting the C60 molecules can be broken by heat treatment and C60 polymers can bereverted back to monomeric C60 at moderate temperatures and ambient pressure [41].

3.2 One and two-dimensional polymers obtained at HPHT conditions

It is well known that high-pressure high-temperature treatment (HPHT) of C60 below 9 GPaand 900K leads to the formation of several kinds of one- and two-dimensional polymers [42-46]. One-dimensional orthorhombic C60 polymers (chain-like) have been obtained over awide range of pressures (up to 8 GPa) and relatively low temperatures (starting already from370 K) [45,46]. At higher temperatures, two different two-dimensional polymers have been

7

reported which are tetragonal at lower pressures and rhombohedral at higher pressures [44,45](see Fig.3.3). Studies of two-dimensional polymers have recently also been carried out onsingle-crystals [47]. Today, Raman spectra have been recorded for all one- and twodimensional polymeric phases and characteristic features of the spectra have been identifiedfor each phase. Some of the most typical signatures for polymerization are: (i) a shift of theAg(2) mode proportional to the number of square rings connecting neighboring C60 molecules,(ii) peaks originating from square rings vibrations around 900-1000 cm-1 and (iii) new peaksbelow 200 cm-1 due to intercage vibrations [39,45-48].

Figure 3.3 Schematic structural arrangement of C60 molecules in the two-dimensionalrhombohedral (R) and tetragonal (T) networks; and in the one-dimensional orthorhombic (O)chains. From ref. [43].

Sundqvist has recently presented a review of the P-T phase diagram for C60 [46]. A similar P-T diagram was also presented by Davydov et al. [45] but with addition of a dimeric phaseinstead of orthorhombic for pressures above 2-3 GPa and moderate temperatures. All resultssummarized in this diagrams originated from ex situ studies where the samples were heated toa certain temperature and usually cooled down to room temperature and analyzed with Ramanspectroscopy after a quick release of pressure (quenching). A problem with this type ofstudies is that the phase composition of the quenched samples can be different from the phasecomposition at the HPHT conditions. This means that the P-T diagrams recorded ex situshould be considered rather as a map showing the conditions under which a certain phase canbe produced. It must be also noted that a discussion of the real phase diagram for fullerenepolymers is not correct since C60 is a metastable modification of carbon and the term"equilibrium phases" should therefore not be used.

3.3 Superhard materials and three-dimensional polymerization at HPHT conditions

Three-dimensionally polymerized fullerites have been claimed to exist at pressures above 12-13 GPa and temperatures above 800K [49-57]. The structural characterization of these phases

8

so far remains very poor but their extremely high hardness has attracted a lot of attention.Several structural models have been proposed for these "superhard" phases, includingdifferent hypothetical kinds of bonding between C60 molecules but so far none of them is wellproven [56-58]. The problem with characterization of these "superhard" phases is that theyare either completely amorphous or exhibit very few lines in XRD which allow a quiteambiguous interpretation. Raman spectra of these phases are also typically almost featurelessand have been interpreted by some researchers as "collapsed" fullerite with only fragments ofC60 cages remaining [59]. Nevertheless, the most recent review of the P-T diagram showed anumber of different phases in the pressure range 12-13 GPa and high temperatures althoughthe existence of some of them have been based on single observations. The review by Blanket al. reported four different structures above 9.5 GPa and 700K, which were suggested fromfitting conventional XRD patterns (see Fig. 3.4) [60].

Figure 3.4 P-T diagram from ref. [60] constructed from ex situ data.

Later results obtained with synchrotron radiation showed strongly elliptical Debye-Scherrerrings in 2D XRD images from a sample treated at 13 GPa and 830K [57]. This suggests thatstructural analysis with conventional XRD patterns is not correct since they represent sectionsof 2D pattern with unknown ellipticity. For such conditions the fitting of conventional pattern(without using 2D images) may give different results depending on orientation of the sampleetc. Nevertheless, some recently published structural models have been based on Rietveldanalysis of XRD patterns obtained by such conventional methods [55,56]. Superhard phase

9

has been also obtained without heat treatment at much higher pressures, above 20-22 GPa[50]. This amorphous phase was suggested to be the same as the phase obtained by HPHTtreatment at 12-13 GPa (so called "Amorphous II" phase) [46].

It should to be noted that data reported by different groups in this pressure region aresignificantly different. One of the reasons for such a difference is that all studies above 12GPa have been performed ex situ and with an unknown pressure variation during the heatingprocess. Furthermore, in many HTHP studies rather short time of heating has been used (1min) and therefore it is not known whether the transformations were completed or not. Thesamples obtained at these conditions were inhomogeneous and the authors faced problems inseparating between different phases [55].

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Chapter 4

Experimental

4.1 Synthesis

4.1.1 Crystal growth of C60 and C70 compounds from solution

There are several common difficulties when growing fullerene compound crystals fromorganic solvents:

1. A low solubility in solvents such as benzene and toluene makes it necessary to use a largeamount of solvent.

2. A weak temperature dependence of solubility makes it necessary to use an evaporationmethod although many organic solvents are poisonous and harmful to the environment.

3. If the evaporation is carried out in an open system the crystallization takes place on thewalls of the vessel in a thin film of the solution, a little bit above the main level. In thiscase very long evaporation times are required and the regulation of growth conditions ispractically impossible.

Figure 4.1 Scheme of the first crystallization method: 1-heater, 2-inner vessel with solution,3-outer vessel.

To avoid the difficulties listed above, two new methods were developed in an earlier study[61]. The principal scheme of the first growth method is shown in Fig. 4.1. The evaporationis carried out in a closed vessel. The growth rate is regulated by the temperature gradientbetween the inner and outer vessels denoted 2 and 3 in Fig.4.1 and depends on the shape and

1

2

3

11

the size of the outer vessel as well. After evaporation from the inner vessel the solventcondenses on the walls of the outer vessel and runs off to the bottom, where it is collected andcan be used again. A condensation takes place also on the walls of the inner vessel. Theformation of a permanent flowing film of pure solvent on the walls prevents formation ofnuclei at the walls and restrict crystallization to the bottom of the vessel. This device allowsthe use of wide range of supersaturations, growth on seed crystal, stirring of solution andother facilities. At the same time environmental pollution is avoided. However, the methodhas the disadvantage that large volumes of solvent are required. To avoid this problemanother method was developed. In this device there is only one vessel which is divided intotwo chambers, separated by a filter (Fig. 4.2). The fullerene powder is situated on this filter.A small volume of benzene (10-15ml) is heated from the bottom. The evaporated solventcondenses on the top and on the walls of the upper chamber and runs off to the filter,dissolves the powder and drops into the lower chamber again. As a result the concentration ofthe fullerene solution is increasing with time. The rate of growth is regulated by thetemperature gradient. This very simple method allows the growth of crystals of a large size invery small volumes of solvent. An additional advantage is that the increased concentration ofimpurities during growth, typical for a simple evaporation method, can be avoided.

Figure 4.2. Scheme of the crystallization method: 1-heater, 2- vessel with solution, 3-filterwith C60 powder.

With the methods described above millimeter-sized crystals of different fullerene compoundshave been grown. This includes C60, C70 and C70 solvates, needles of C70S48 up to 10 mm long

1

2

3

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and crystals of a new C70S8 compound with sizes up to 2x1x0.5 mm [III,V]. So far, these arethe largest fullerene crystals grown from solution. An even more simple method to grow large crystals of the C60 solvates was developedduring studies of phase transitions in the C60-benzene system [23]. Below 300K the onlysolvated structure, C60⋅4C6H6, is stable in equilibrium with solution. As a result, if we putsome fcc C60 powder into solution it must recrystallize to the solvated phase. A small closedvessel with a powder of C60 and filled with benzene was cooled to 280K and after 10 daysseveral needles with sizes up to 3 mm grew on the bottom of the vessel. The new crystals arered and transparent unlike the black opaque crystals of pristine C60. This method was used forRaman studies of C60 and C70 solvates in paper [II].

Very good results have been obtained also for crystal growth of C70S48 crystals using analternative method: when a saturated C70 solution in benzene is mixed with a saturatedsolution of sulfur, a C70S48 powder forms very quickly. In order to grow large crystals, the rateof the reaction must be slowed down. In the case of C70 this turned out to be very easy usingthe extremely slow diffusion rate of such a large molecule. A mixture of C70 and a sulfursolution (with a sulfur concentration below saturation) was placed into thin vertical tube (1-2mm in diameter and 30-50 mm height). To start the reaction a small crystal of sulfur wasdropped into the tube. C70S48 crystals started to form immediately but as soon as all C70around the nucleation center was consumed, further growth could only occur by diffusion ofC70 through the tube. As a result the growth slowed down and a geometrical selection favoredthe growth of only a few needles oriented along the tube. This method allowed us to growplatelet-like needles with sizes up to 10x2x0.1mm.

4.1.2 Preparation of C60 and C70 sulfur films

The objective with this study was to investigate the possibilities to synthesize C60S16 andC70S48 thin films and to characterize them with different techniques [IV]. Several differentmethods were used to synthesize the fullerene-sulfur films but the best results were obtainedwith the following methods:

a) The reaction of a thin fullerene film which is placed in a saturated solution of sulfur intoluene. The fullerene films were deposited by the evaporation of C60 or C70 from aKnudsen cell in the vacuum chamber (sublimation temperature: 550-600 oC).Typically, the fullerene films were deposited on glass, Al2O3 or silicon substrates at20-100 oC. In the following this procedure will be referred to as the solution method.

b) The reaction between a thin fullerene film and a saturated sulfur vapor in a sealedampoule. The temperature of the ampoule was kept slightly higher than the sulfurmelting point (120-140 oC) In the following this procedure will be called the vapormethod.

The quality of the prepared films was found to be strongly dependent on the grain size of theinitial fullerene film, which can be varied by changing the deposition temperature. The bestresults were obtained with amorphous fullerene films deposited on a cold substrate at roomtemperature. Such amorphous C60 and C70 films typically yielded continuous fullerene-sulfurfilms. The developed methods of thin film growth can be used for variety of the C60 and C70compounds. The simplicity and the low temperature in the solution method are especially

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attractive for thin film growth of fullerenes compounds with low temperature ofdecomposition (e.g. organic derivatives).

4.1.3 In situ high pressure studies

The principle of the diamond anvil cell (DAC) is as follows: the heart of the DAC is a pair ofdiamond anvils, which squeeze a metal gasket with a hole. The hole contains the sample and apiece of ruby (for pressure measurement). Very often the sample is placed in a hydrostaticmedium, for example NaCl. Otherwise conditions of pressurizing are non-hydrostatic and thesample is subjected to a deviatoric stress. The Raman spectra and XRD patterns can berecorded in situ through the anvils. Such in situ studies at high pressure conditions and roomtemperature are a routine procedure today. However, in situ Raman experiments at HighTemperature High Pressure (HPHT) conditions are technically more difficult. Prior to thework in this thesis no such studies have been published. In papers [VIII-X] a special design ofthe externally heated DAC was used. Powder samples of freshly sublimed C60 (99.95% purity,MER Corporation) were studied using the TAU type diamond anvil (DAC) cell with 250 µmflat culets at non-hydrostatic conditions. The cell was placed into a resistively heated ceramicshell with a glass window where a nitrogen flow was used inside the heater to prevent celloxidation (see Fig.4.3). The temperature was controlled by a K-type thermocouple insertedinside the cell at the diamond/gasket interface. The combination of a rather small size of theDAC (~15mm) and a long focus objective (50×) made it possible to record Raman spectraduring heating.

Figure 4.3. Scheme of experimental setup for in situ Raman HPHT studies using the TAUtype DAC.

14

The design of the externally heated DAC in the in situ XRD study [VIII] was modified toallow pressure corrections. The screws, which are used in DAC for pressure regulation, wereput through the cover of the ceramic heater. With this design the pressure could be correctedthrough the external ceramic heater during the experiments.

Figure 4.4. DAC design for thin films studies.

A slightly modified design of the DAC was also used for thin film studies of the C60polymerization in paper [VI-VII]. The stainless steel gaskets were indented to a depth 0.07-0.075 mm and used as a substrate for the deposition of 300-10000Å thick C60 films at highvacuum conditions (10-6 Torr) by evaporation from a Knudsen cell. Two films were preparedfor each experiment using both sides of the gasket. The thickness of these films wassignificantly different (sample a: 300Å and 6000Å, sample b: 500Å and 4000Å and sample c:2000Å and 10000Å). This was important since we could not provide exactly the sameconditions in each experiment. The pressure (non-hydrostatic) was increased step by step andthe in situ Raman analysis at each pressure varied from 1 to 24 hours depending on thepressure and film thickness.

4.1.4 Study of the C60 and C70 solvates "in-solution"

C60 solvates often lose solvent very quickly when taken out of the solution. Thereforeexperiments with C60 solvates must be performed using "in-solution" methods. For this

15

purpose we used small cells made from glass and sealed with silicate glue to preventevaporation of solvent. This kind of cells was also used for XRD studies (transmission mode)of the C60-benzene solvates in my previous work. [23]. In paper [II] study this method wasused for XRD of C70 solvates and for Raman studies by recording spectra through the glass.Crystals and solution were typically loaded at room temperature and then cooled toappropriate temperature to allow pristine fcc C60 to recrystallize into solvated crystals. Forbenzene and hexane the recrystallization temperature was 280K, while for toluene 233K wasused. At these conditions the black non-transparent powder of pristine C60 slowly dissolvesand new red transparent crystals are formed.

The phase transitions (or more exactly incongruent melting of the C60 solvates) during theheating were examined in paper [II] using optical microscopy and Raman spectroscopy. Inthe optical microscope the phase transitions look like a melting of the red transparent crystalsof the solvates followed by the formation of new black opaque crystallites with the octahedralshape typical for fcc C60. For C60-benzene this melting was observed around 315K and forC60-hexane around 318K. For toluene we observed melting in two steps: a low temperaturesolvate I phase transformed to a solvate II phase around 280K and if further heating wasconducted rapidly (20-30 min) to temperatures higher then 320K, a second transition wasobserved. The solvate II appeared to be metastable. If the sample was stored at roomtemperature for several hours, the solvate II phase disappeared completely. Followingobservations with optical microscopy, the Raman spectra were measured from crystals withdifferent kind of morphology. The Raman spectra of these solvates will be discussed in detailin section 5.3. For C70 the change of structure was observed by XRD and Raman spectroscopywithout any changes in the morphology of the crystals (see section 5.2).

4.2 Characterization

In paper [V], the "in solution" X-ray diffraction data on C70 solvates were obtained using aSiemens system consisting of a Smart CCD Area Detector and a direct-drive rotating anode asx-ray generator. Mo(Kα) radiation (tube voltage 50 kV, tube current 24 mA, cathode gun0.1×1 mm), monochromatized by using an incident beam graphite monochromator, waspassed through a collimator of 0.5 mm diameter to the sample. The diffracted x-rays werecollected on a 512×512 pixels area detector. This setup was used in transmission mode andprovided measurements of a large portion of the Debye rings at different 2θ settings. Theadvantage of this system is the very short time required to provide satisfactory XRD patterns(1-2 min) but a drawback is the lower resolution compared to conventional diffractometers(without 2D XRD images). The powder samples and films were also analyzed with a Siemens5000 diffractometer in the θ-2θ mode using Cu Kα radiation [I-V]. In paper [III] a Rigakudiffractometer was used to collect data from single-crystals of C70S8.

Due to the very small size and often poor crystallinity of the samples studied at HPHTconditions synchrotron radiation was necessary for their characterization. Two-dimensionalXRD images of the samples in situ during the heating and ex situ after quenching [VI-VIII]were taken in transmission geometry on the ID30 and BM01A beamlines at the EuropeanSynchrotron Radiation Facility (ESRF, Grenoble, France) with the MAR345 detector using anX-ray beam of with wavelengths 0.3738 Å and 0.7Å and a beam size of 20x10 µm. Detector-to-sample distance was 350 mm. XRD images were taken with incident x-ray beam parallel to

16

the direction of compression (ϕ=00). At some temperatures and on quenched samples XRDimages were also taken with ϕ=300 to investigate the presence of internal stresses.

Raman spectra were recorded by a Renishaw Raman 2000 spectrometer using 780,787 and514 nm excitation wavelengths with a resolution of 2 cm-1. Precautions were taken to avoidlaser-induced photopolymerization during the measurements. Only a low laser power (<5W/cm2) was used for all experiments and no traces of photopolymerization were found in therecorded spectra. One of the main advantages with Raman spectroscopy is the very smallillumination area (1-2µm). Unlike calorimetry and X-ray powder diffraction which give anaverage information over many crystals (so far calorimetry experiments have been performedonly for powder due to problems with growth of large crystals), we were able to focus thelaser beam on individual small crystals and record Raman spectra for crystals with visualdifferences in morphology [II-IV]. The small illumination area is also a strong advantage inhigh pressure studies using DAC, since the size of samples was only 100-300 µm [VI-X].

In paper [IV,V] some samples were characterised with IR-spectroscopy using a Bio-Rad/Digilab FTS-45 FTIR spectrometer with a resolution of 2 cm-1. Elementary compositionof the C70S8 crystals in the paper [III] was analyzed by a commercial laboratory (MIKROKEMI AB, Uppsala, Sweden). They used a flush combustion gas chromatography methodwhere the samples are completely oxidized into gaseous CO2 and SO2. The gases are analyzedby gas chromatography and their concentration determined by a thermal conductivity detector(TCD). The morphology of the C70S48 and C60S16 films were studied with scanning electronmicroscopy (SEM) using JEOL JSM-840 instrument [IV].

In paper [I] the real and imaginary parts of the complex dielectric constant, ε*=ε'-iε" ofthe C70S48 were recorded in the frequency range 20 Hz - 1 MHz using the autobalance bridgeHP4284A. To carry out measurements as a function of temperature, the samples weremounted in a helium refrigerator system in a vacuum chamber. Electrodes for dielectricmeasurements were painted on the main faces of the crystals with silver paint. When thinfilms were studied, coplanar gold electrodes were prepared by evaporation.

The 13C NMR measurements on C70S48 in paper [I] were performed by means of a BrukerMSL 300 spectrometer and a superconducting coil delivering a 7.05 T magnetic field(working frequency for 13C : 75.47 MHz). All chemical shifts are given with respect to the 13Cresonance frequency in tetramethylsilane (TMS). The 13C (in natural abundance) spin-latticerelaxation time, T1, was obtained by using a saturation-recovery pulse sequence. The sample(inside a 4 mm diameter rotor) was located in a Bruker low temperature probe (4 - 500 K)and the temperature was varied by means of an Oxford CF1200 cryostat. Magic AngleSpinning (MAS) experiments were run with a 4 mm Bruker MAS probe and the rotatingworking frequencies were 5 kHz and 1285 Hz. MAS simulation spectra were made usingBruker software.

In papers [VI,X], mechanical properties of HPHT treated C60 films were evaluated with amicroindentor and a Nano Indentor II equipped with a calibrated Berkowich diamond tip. Thenanoidentation measurements were performed with a maximum load of 5mN. A maximum of10 indents was used to evaluate the hardness and elastic modulus. Indentation was performedon both as-deposited and HPHT-treated films.

17

Chapter 5

Study of C60 and C70 molecular solids prepared in solution

5.1 C60S16 and C70S48 thin films

Thin film technology can be used to synthesize fullerene materials, which are difficult toobtain as bulk material from solution. One example of this can be found in paper [I] where thedielectric properties of C70S48 films were studied. The C70S48 crystals were grown only asplatelet-like needles, which were too thin for successful measurements of the dielectricconstant along the polar axis (directed 450 towards the elongation of the needle). Using thinfilms was an advantage since the crystallites in the C70S48 films were preferentially orientedwith the (h00) planes parallel to the substrate and oriented randomly in the plane of the films.Since coplanar electrode geometry was used in the dielectric studies of these films, thedielectric constant components along the [001] polar axis could be measured [I].

In paper [II], C60S16 and C70S48 thin films were prepared according to the methods describedin section 4.1.2. These films are formed by a reaction of pre-deposited fullerene films andeither sulfur vapor or a solution containing sulfur. As a consequence the microstructure of theresulting C60S16 and C70S48 films will be strongly dependent on the initial fullerene film. It iswell known that the microstructure of a thin film can be controlled by a fine-tuning of theexperimental parameters. Microstructures ranging from amorphous to single-crystalline canalso be obtained on suitable materials at a given temperature by simply changing the flux ofevaporated fullerene materials and the substrate temperature. In general, crystalline fullerenefilms yielded discontinuous fullerene-sulfur films with individual crystallites with a grain sizeof several microns. In contrast, continuous fullerene-sulfur films were obtained withamorphous C60 and C70 films deposited at low substrate temperatures. XRD of the C70S48films showed a clear texture where the (h00) planes are preferentially oriented parallel to thesubstrate surface [II]. The preferential orientation of the C70S48 film can be explained by thestrong tendency of this material to grow as thin platelets, which align along the substrate. Incontrast, C60S16, which typically forms bulky grains, produced films with a randomorientation (see Fig.5.1).

a) b)

Figure 5.1 SEM images of a) C70S48 and b) C60S16 films grown by solution method.

18

Figure 5.2. Raman spectrum of a C60S16 film compared with spectra of C60 and sulfur in therange 100-600 cm-1.

Figure 5.3. Raman spectrum of a C70S48 film compared with spectra of pure C70 and sulfur inthe range 100-500 cm-1.

Raman spectra of the C60S16 and C70S48 films are shown in figures 5.2 and 5.3,respectively. The results showed showed a strong similarity between the spectra of C70S48,C60S16 and the C60, C70 + sulfur spectra, which suggest that the interaction between fullereneand the S8 rings is very weak. The observed discrepancies below 500 cm-1 (splitting and smallshifts of some peaks), however, suggest some kind of C70-S8 ring interaction. It is known thatin pure solid C60 and C70 the molecules are rotating. A decrease of rotational freedom due tothe addition of sulfur can explain all observed changes. It can explain also the small changes(weak shifts of some peaks) observed in IR spectra of fullerenes-sulfur compounds. It isclear, however, that rotation in C70S48 and C60S16 is not hindered like in the low temperature

100 200 300 400 500 600

0

5 000

10 000

15 000

20 000

25 000

30 000

3 42 35 4 52 553 4

49 2

49 4

26 6 26 9

27 1

su lfur

C60S16

pure C60

Inte

nsity

, a.u

.

W avenumber cm-1

10 0 20 0 30 0 40 0 50 0

0

400 0

800 0

1200 0

1600 0

228

452

456

230

224

259

263

C70

sulfur

C70S48

Inte

nsity

, a.u

.

Wavenumber cm-1

19

phases of C60 and C70. Rotational state of the fullerenes molecules in their sulfur compoundswill be discussed in more detail in section 5.5.

The weak interaction between sulfur and fullerenes C60 and C70 was also confirmed by IRspectroscopy. These spectra showed small shifts of 1-2 cm-1 for some C60 and C70 modes, butno new peaks. Small traces of solvents were also found by IR spectroscopy not only forsamples obtained by solution method but also for initially deposited C70 films. This suggeststhat IR spectroscopy is more sensitive to solvent contamination than Raman spectroscopy.Furthermore, the weak interaction also explains the fast evaporation of S8 rings when thefullerene-sulfur compounds are placed in high vacuum. This behavior makes it impossible toanalyze the films by e.g. X-ray photoelectron spectroscopy (XPS).

5.2 Crystal growth of C70S48 and the new compound C70S8

The initial purpose of the study was to grow large crystals of C70S48 for dielectric studies. Noother phases have previously been reported in the C70-S8 system. Following the proceduredescribed in ref. [61], C70 and a sulfur solution were mixed with a stoichiometrycorresponding to the composition C70S48 [III]. Surprisingly, the result was different fromthose obtained in previous studies [29-30]. The initial saturated solution was almost black incolor and non-transparent but as crystallization started, the solution became more and moretransparent until it turned completely colorless. This remaining part of the solution containedonly sulfur, which suggests that the overall sulfur content in the crystals must be less than inC70S48. Furthermore, in the end of the process, crystals with two different morphologies wereformed. Most of the crystals were black, non-transparent and rectangular millimeter-sizedplatelets with very smooth shiny faces. The other kind of crystals formed in the solution werevery thin red transparent needles, which are typical for C70S48. Chemical analysis on twosamples of each kind of crystal confirmed that the red needles are the well-known C70S48compound, while the rectangular black platelets exhibited a composition close to C70S8 with aslight excess of sulfur. The results suggest that the crystallization occur by the followingway: initially, crystals of a new C70S8 compound are formed. This phase contains fewersulfurs than the initial stoichiometry of the solution (C70S48). As a result, the sulfurconcentration in the solution will increase, and after a while, needles of C70S48 start to form.However, since most of the C70 is already deposited in the C70S8 crystals some sulfur willremain in solution.

Several experiments were carried out to investigate how the sulfur concentration in thesolution influenced the crystallization. No other phase than C70S8 and C70S48 were formed.Furthermore, the results showed that C70S48 crystallize from a benzene solution only when thesulfur concentration is close to saturation for pure sulfur (i.e. much higher than thestoichiometric composition). An interesting observation was that C70 does not dissolve in asaturated sulfur solution. Adding a C70 solution to a saturated sulfur solution results in a fastformation of thin red platelets and needles of C70S48 without any formation of the C70S8 phase.This method is very useful for obtaining C70S48 powder samples. For growth of the C70S8compound, best way to obtain large crystals was to use a solution with a sulfur contentapproximately twice the stoichiometry of this compound. When a stoichiometric compositionwas used the crystals tend to grow as dendrites. This can be attributed to an insufficient rate

20

of sulfur diffusion to the surface of the growing crystals. Typically, the formation of apolycrystalline sample consisting of up to 1-2 mm large crystals with approximately similarorientations was observed, although large single crystals with sizes up to 2x1x0.5 mm werealso obtained in some cases [III].

The new phase C70S8 was characterized by XRD and Raman spectroscopy. The powder andsingle crystal data are consistent with an orthorhombic structure (space group Pbcn) and cellparameters a=30.18Å, b=30.41Å and c=28.32Å. However, room temperature structure datacould not be solved up to the positions of each carbon atom. Using the SHELX software, S8rings and parts of the C70 molecules were located but approximately 50% of the carbon atomscould not be localized. We attribute these problems to a high degree of disorder at roomtemperature. Further studies at low temperatures are needed to overcome this problem.Raman spectra of the C70S8 compared to spectra of the C70S48, pure C70 and sulfur are shownin Fig. 5.4.

Figure 5.4. Raman spectrum of C70S8 compared with spectra of C70S48, pure C70 and sulfur inthe range 100-600 cm-1.

The Raman spectra of C70S48 and C70 were discussed in the section 5.1. C70S8 exhibited asimilar spectrum, which can be described as a simple sum of the C70 and sulfur spectra withsome small but clear differences in the C70 peak positions below 500 cm-1. The C70S8compound contains six times less sulfur than C70S48 and we should therefore expect weakereffects of the C70-S8 interactions in the spectrum as well as lower relative intensities of thesulfur peaks. As can be seen in Fig.5.4, the relative intensities of the sulfur peaks are aboutthree times lower than in the C70S48 spectrum. Furthermore, there are no additional peaks inthe C70S8 spectrum suggesting weaker C70-S8 interactions in this phase than in C70S48. Fig. 5.4shows, however, that the trends in the peak shifts are not that simple in the new compound.For example, the peak originating from C70 at 456 cm-1 is only downshifted with 1 cm-1 in theC70S8 spectrum compared to 4 cm-1 in C70S48. This could be an indication of weaker C70-S8interactions. However, the situation is more complicated since the peak at 228 cm-1 exhibits asimilar downshift in both compounds and the 263 cm-1 peak shows a stronger downshift forC70S8 (6 cm-1) than for C70S48 (4 cm-1). Consequently, each of these three peaks shows its own

100 200 300 400 500 600

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5000

1 0000

1 5000

2 0000

224 259

228263

225 257

452

456

455C70S8

Inte

nsity

, a.u

.

C70

sulfu r

C70S48

Raman shift, cm -1

21

behavior with respect to peak shifts in C70S8 while all three peaks exhibit the same shift (4 cm-

1) in the C70S48 spectrum. Finally, it should be noted that no peaks originating from thebenzene solvent could be detected in the Raman spectra.

5.3 XRD study of C70 solvates

As was discussed in section 2.2, very few data have been published on C70 crystallizationfrom solutions and about C70 solvates. Data obtained by several groups studyingcrystallization at different temperatures suggested some phase transition between the solvateand pure C70 similar to what has been observed in the C60-benzene system [23]. However, thesolubility dependence of the C70 in toluene showed no "anomalous" change of slope [24]. Inpaper [V] crystallization experiments at different temperatures were performed using themethods of controlled evaporation and in situ XRD diffraction studies of C70 in its ownsaturated solution.

As expected, C70 crystallization from a benzene solution was temperature dependent. Crystalsof different morphology and structure were obtained below and above 313K. Although thesecrystals were stable out of the solution (unlike to the C60-benzene solvate), powder XRDshowed that crushing of the crystals lead to degradation of the sample. Only very gentlegrinding allowed the recording of good quality XRD patterns, but even with all precautionsthe different samples gave cell parameters with a variation of 0.2-0.4 Å.

At temperatures below 313K large black platelets with a rectangular shape were grown (witha size up to 3×2 mm). XRD patterns of the sample prepared at room temperature wereindexed with an orthorhombic structure and the cell parameters were calculated to: a=20.99Å,b=32.85Å, c=11.01Å. These values are very similar to data by Agafonov et al. obtained for atoluene-C70 solvate (a=21.075, b=32.99, c=10.84Å [25]). Oscillation pictures taken fromsingle crystals showed extremely bad packing along b direction due to multitwinning andstacking faults but a much better packing along a and c directions (a=21.00, c=10.80Å) [V].

Above 313K crystals with a different morphology were obtained. Some of these crystals hadthe shape of a hexagonal prism, while others exhibited the shape of a distorted hexagonalprism. XRD patterns from a powder sample obtained at 338K can be described as a mixtureof the minor hcpI phase with cell parameters a=10.17Å,c=18.52Å which is close to the hcpphase of pure C70 and as a main phase a monoclinic structure with cell parameters a=9.89Å,b=10.76Å, c=37.44Å, β=1200 which can be interpreted as a distorted hcp.

XRD and DTG studies of samples grown at 308K and 338K confirmed their solvated nature.Both samples showed a slow mass loss starting from 363K with a sharp anomaly at 410-420Kfollowed by a slow mass loss up to 570K. The total mass loss for the first sample was about15% compared to 11% for the second sample. This corresponds to approximately a 1:1composition of the C70-benzene solvate. XRD of the heated samples after the DTGexperiments showed that they both consist of the pure C70 phases (fcc with a=14.93Å, hcpwith a=10.57Å, c=17.27Å). Comparison of XRD patterns of pure C70 with pattern of thesolvate grown at 308K is shown in Fig.5.5.

22

Figure 5.5. XRD patterns for (1) C70 powder provided by MER corporation, (2) crushedcrystals grown from benzene solution at room temperature, and (3) sample shown in curve 2after heat treatment at 600K.

The next step of was to perform "in solution" XRD experiments with pure C70 powder addedto a benzene solution. The results were significantly different from those obtained in thecrystallization experiments. Since the solvated C70 phase was obtained by crystallization froma benzene solution (as described above) a recrystallization of the C70 powder into solvatecrystals was expected. However, after the C70 powder was filled with solution no changes inthe morphology of the grains were observed and no new crystallites were formed. This wasobserved both for samples stored at room temperature and at 280K for several weeks.Nevertheless, the X-ray powder analysis showed that a transformation to the solvate structureoccurred without changes in the morphology of grains. The same experiments were carriedout for benzene, toluene and hexane solutions and the resulting XRD patterns are shown inFig. 5.6. The changes of the C70 pattern showed some similarities for all three solvents, but inthe case of benzene and toluene these changes were very fast, (less than 30 minutes), whilethe pattern of C70 in hexane came to equilibrium only after two hours. The solubility of C70 inhexane is approximately 100 times lower compared to the solubility in toluene and benzene.Therefore, it is reasonable to suggest that the interaction should be slower in the C70-hexanesystem. The changes in the X-ray powder diffraction patterns appeared to be reversible upondrying of the samples. It took several days at room temperature conditions to return back tothe pure C70 patterns for all three solvents. Heating experiments from room temperature to a temperature close to the boiling point werecarried out for all three solvents. In contrast to the C60 system, no changes were found in theX-ray patterns during heating. These data are in agreement with the data on the solubility

Diffraction angle, 2Θ0 5 10 15 20 25 30 35 40

Inte

nsity

, a.u

.

-0.01

0.99

1.99

2.99

3.99

4.99

5.99

Si (111)

1

2

3

23

dependence of C70 and calorimetric measurements, which show that no anomalies are foundin those systems. On the other hand the crystallization experiments showed that there aresome structural differences in crystals grown at different temperatures both for toluene andbenzene. It is clear that solvates of C70 are very different from C60 solvates and haveproperties more similar to pure C70. Relatively small amounts of solvent incorporated into thestructure (1:1 compared to 1:4 for C60-benzene) give a higher stability and a density of thesolvated phase that is very similar to the density of pure C70. The fact that the phase transitionaround 313K can be observed only by crystallization experiments could be explained by arather small difference in chemical potentials between solvated and nonsolvated phases. Inthat case only the crystallization in conditions close to equilibrium could produce differentphases and the fields of metastable existence of the phases should be large.

Figure 5.6. X-ray powder patterns obtained at room temperature for: (1) pure C70 powder, (2)C70 in benzene, (3) C70 in toluene, (4) C70 in hexane.

5.4 Raman spectra of C60 and C70 solvates compared to sulfur compounds

Our results clearly showed that Raman spectra from different solvate phases as well asfullerene-sulfur compounds exhibit some common features and are different compared tospectra from pure C60 and C70 [II, IV].

Raman spectra of C60 solvates with benzene, hexane and toluene solvate II (see section4.1.4) appeared to be very similar. In general these spectra are almost the same as spectrumof pure C60 with some small shifts for peaks below 600 cm-1.

Diffraction angle, 2Θ0 2 4 6 8 10 12 14 16

Inte

nsity

, a.u

.

0

2000

4000

6000

8000

10000

12000

1

2

3

4

Si

24

250 300 350 400 450 500 550 600

S

C60-benzene

C60-toluene

C60-hexane

C60S16

C60

Inte

nsity

(a.

u.)

Raman shift (cm-1)

Figure 5.7. Raman spectra of C60-toluene, n-hexane and benzene solvates compared withspectra of C60S16 and pure C60 in the range 200-600 cm-1.

In Fig.5.7 spectra of the solvates are compared with a spectrum from pure C60 and aspectrum from the C60S16 compound described in chapter 5.1. New weak peaks at 342, 353and 534 cm-1 are typical for all three solvents. In addition, a very weak peak at 579 cm-1 wasobserved for the toluene and the benzene solvates while for hexane this peak was found at577 cm-1. Furthermore, a downshift of about 3 cm-1 can be seen for the Ag(1) C60 peak at 492cm-1 and the Hg(1) peak at about 269 cm-1 It is clear that the new peaks and the shiftsdescribed above can be considered as vibrational signatures of C60 in its solvate phases.Heating of these samples to temperatures above the phase transition (313K for benzene,318K for hexane, and 280K for toluene) leads to a melting of the solvates and the formationof pure C60. This process can be followed in the Raman spectra as all new peaks and peakshifts disappear and only peaks from pure fcc C60 can be seen above the transitiontemperature. The Raman spectrum of the toluene solvate recorded at 233K (solvate I) issomewhat different from the solvate II but the difference is very small. It shows no peaks at342 and 353 cm-1, stronger shift of the Ag(1) mode (4 cm-1) and a new weak peak at 521cm-1.

The C70 solvates also exhibited weak effects due to the interaction with solvent molecules[IV]. All the peaks, which can be observed for a pure C70 sample, are present in these spectratoo. Nevertheless, there are some clear changes, which allow us to confirm the formation ofnew solvate phases in solution. The main difference was observed in a region below 450 cm-

1. Peaks of C70 at 263 and 228 cm-1 are downshifted to 258 and 225 cm-1, respectively, for allthree solvates (Fig 5.8). Also three weak peaks between 400 cm-1 and 450 cm-1 showedsome clear changes in relative intensity. Once again, the spectrum of the C70S48 compound isvery similar to the spectra of the C70 solvates with typical shifts of some peaks.

25

Figure 5.8. Raman spectra of C70-hexane, benzene and toluene compared with spectra of pureC70 and C70S48 in the spectral range 160-300 cm-1.

The C60 and C70 solvates were also studied with respect to possible phase transitions in thetemperature interval from the room temperature down to 80K. No evidence for any phasetransitions was found for all three C60 solvates and for C70 solvates with toluene and hexane.Although the structures of all three studied C70 solvates seems to be very similar, the only theC70-benzene solvate showed a clear phase transition around 278K. Fig. 5.9 shows clear peakshifts in the spectra from the C70-benzene solvate during cooling which can be attributed to aphase transition. This was also observed in our studies of large crystals out of solution as ananomaly of the dielectric properties around 275K [62].

The peaks of the room temperature C70-benzene solvate at 258 cm-1, 225 cm-1 and 1180 cm-1

are shifted to 264 cm-1, 230 cm-1 and 1186 cm-1, respectively, during cooling. The spectrumtaken at 278K shows both shifted and unshifted peaks, which are overlapped. The shifts arereversible upon heating. Nevertheless some kind of irreversibility could be observed bycomparing the intensities of the peaks at 258 cm-1 and 250 cm-1. The relative intensity of thepeak at 250 cm-1 is clearly increased after the cooling-heating cycle. The nature of this phasetransition is not clear and further XRD studies are required.

Figure 5.4.3. Fragments of Raman spectra of C70-benzene recorded during the cooling. Thefigures only show peaks, which are shifted.

1140 1160 1180 1200

303KC70-benzene

263K

278K

223K

173K

78K

Inte

nsity

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.)

Raman shift (cm-1)

200 220 240 260 280 300

303K

C70-benzene

263K

278K

223K

173K

78K

Inte

nsity

(a.

u.)

Raman shift (cm-1)

180 200 220 240 260 280 300

S C70S48

C70-hexane

C70-benzene

C70-toluene

C70

Inte

nsity

(a.

u.)

Raman shift (cm-1)

26

5.5 Rotation of C60 and C70 in the molecular solids

An interesting question is: what is the rotational state of the C60 and C70 molecules in thesulfur compounds and solvates? The structure of C60S16 has been solved by XRD at roomtemperature with the positions of all carbon atoms determined and it is assumed that therotation of the C60 molecules is frozen in this compound [27]. However, Raman spectra fromC60S16 showed only a splitting of the Hg(1) mode, while up to ten peaks are split in sc C60 dueto the freezing of rotation [63]. Shifts of six modes were observed for simple cubic C60 phasecompared to fcc C60, while only two modes are shifted for the C60-sulfur compounds andsolvates. The results suggest that the rotation of the C60 molecules is somewhat decreased butalmost free as in the fcc C60 phase. This conclusion is also confirmed by recently publishedNMR data, which also showed free rotation of the C60 molecules in C60S16 at roomtemperature with a step change of this motion at 150K [64]. The same group performed NMRstudy of our samples of C70S48 and reached a similar conclusion: the rotational state of the C70molecules is uniaxial at room temperature both in the C70S48 and the pure C70 solid.

For the solvate phases the question is more complicated. Attempts to solve the structure ofmany solvates at room temperature have failed because of strong disorder. This suggests thatthe C60 molecules in these compounds have a free rotation at room temperature. NMR studieshave also confirmed that the fullerene molecules in the C60-benzene solvate have a freerotation not only at room temperature but also at temperatures as low as 120K [65,66]. This isin contradiction, however, to X-ray diffraction studies carried out at low temperatures, wherethe structures of the toluene (103K) and benzene (173K) solvates have been successfullysolved and it has been shown that the C60 molecules are frozen [13,14]. Our results show nosplitting of the Raman speaks from the solvates at room temperature or when cooled down to78K. This suggests that crystal field effects are too weak to show up in the spectra.Nevertheless, the decrease of peak linewidth suggests that solvates have a decreased state ofrotational freedom. Similar results have been obtained for the C70 solvates. Although someweak changes in the spectra can be attributed to a decreased motion of the C70 molecules,there are no evidences of rotation freezing.

The conclusion about free rotation of fullerene molecules in the sulfur compounds and thesolvates seems to contradict XRD data. This fact can be explained by different time scale ofthe measurements if we suggest that the fullerene molecules undergo rotational jumpsbetween two positions, so that rotation is almost free with high jumping frequency. In thiscase short time-scale NMR technique would show an almost free rotation while XRDaverages the electron density over a long time and show a disorder over two orientations. Itshall be noted that such a disorder has been reported in XRD studies for C60 and C70compounds with sulfur, as well as for some solvates [30, 67].

27

Chapter 6

High Pressure C60 studies

6.1 High Pressure study of C60 films

Hitherto, most of the HPHT experiments have been carried out on bulk powder samples. Aproblem with this procedure is that it may be difficult to obtain a homogeneous sample or toinvestigate the influence of grain texture on the process. The advantage with thin films is thatthe grain size, grain texture and crystallinity can be controlled by a careful tuning of thedeposition parameters. For example, in a simple evaporation process, the substratetemperature and evaporation rate can be controlled to obtain a film with a predesignedmicrostructure. This can be important since, for example, the microstructure influencessignificantly the elastic properties of the C60 films [68]. Co-evaporation of C60 with otherelements such as alkali metals, transition metals or p-elements can also easily be applied tosynthesize well-defined thin films. It is, therefore, clear that HPHT experiments on this typeof precursor materials can give new and unexpected results.

Figure 6.1 In situ Raman spectra from a 4000Å film recorded at room temperature anddifferent pressures in the range 7.5-30 GPa.

Most of the thin film experiments in papers [VI, VII] were carried out above 10 GPa wherepolymerization is known to occur at room temperature and non-hydrostatic conditions [69].The typical transformations of the C60 Raman spectra during pressurizing are shown in Fig.6.1 The spectrum at 7.5 GPa is typical for C60 with a slight degree of polymerization. Thisshows that non-hydrostatic conditions allow polymerization which otherwise (at hydrostaticconditions) occurs only if additional heating is added [45,46]. At 22.5 GPa all the peaks above1400 cm-1 disappear. They are replaced with a broad asymmetric peak typical for the

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superhard phase denoted as "phase V" in ref [50]. Also at this pressure all peaks in the range700-770 cm-1 are transformed to a single broad and weak feature. As can be seen, all otherpeaks observed below 15 GPa disappear above this pressure.

An interesting question is whether the conditions for the formation of "phase V" are affectedby the film thickness. Fig. 6.2 shows spectra from films with different thickness at threedifferent pressures. As can be seen, the spectra recorded at 11 GPa are almost identical. Theyshow no influence of film thickness until the phase transformation to the "phase V" beginsaround 18-20 GPa. At 20 GPa, a clear thickness dependence could be observed. As can beseen, films thicker than 4000 Å exhibit spectra identical to "phase V" with two broad,asymmetric peaks at about 700 cm-1 and 1600 cm-1, respectively. In contrast, the thinner filmsare more similar to the orthorhombic phase observed in the 11 GPa spectra suggesting that notransition to "phase V" has occurred. At 22.5 GPa, however, also the thinner films exhibit thespectra typical for "phase V".

Figure 6.2. Effect of film thickness on in situ Raman spectra at different pressures.

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Another unresolved problem is to determine the nature of "phase V". It has been argued thatthis phase can be polymeric C60 while others have suggested a "collapsed fullerite" withbroken C60 cages [59]. Figure 6.3 shows Raman spectra from a 6000 Å thick C60 filmrecorded in situ at 22.5 GPa and after quenching to normal pressure. As can be seen, thespectrum recorded at 22.5 GPa shows only two broad asymmetric peaks centered at about 700cm-1 and 1600 cm-1, respectively. This spectrum is similar to those obtained from thesuperhard Am2 phase and phase V by Blank et al. [50-53] In contrast, the quenched sampleshows a spectrum with a number of well-resolved peaks identical to those obtained fromorthorhombic or dimeric C60 [45].

Figure 6.3. Raman spectra of a C60 film recorded in situ at 22.5 GPa and after quenching.

Fig. 6.3 clearly demonstrates that the high-pressure treatment does not destroy the C60 cagesand that high pressure fullerite can be transformed back to the well-known one-dimensionalpolymers. The result strongly suggests that phase V consists of unbroken C60 moleculesstrongly bonded to each other. Nevertheless, the samples pressurized above 27 GPa could notbe reverted to the low-polymeric state. It is clear that Raman spectra, which was reported for"superhard" phases indeed can be the same for materials with different properties andhardness. For example, a sample heated to 570K for 80 hours exhibits an in situ spectrumidentical to that in Fig. 6.3 but which remains unaffected by quenching. Hardnessmeasurements showed that this sample has a very high hardness of about 62 GPa combinedwith an extreme elastic recovery of about 90%. These properties seem to be rather uniquebecause the only harder material, single-crystalline diamond, does not show such elasticproperties [VII].

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6.2 In situ Raman study of the C60 polymerization at HPHT conditions

In papers [VIII, IX] seven samples were pressurized to 5.5, 10, 14, 18, 24, 27 GPa andheated slowly up to 780-850K. Raman spectra were recorded in situ during the heating andcooling. After the cooling the samples were quenched and studied ex situ by Ramanspectroscopy and XRD (using synchrotron radiation). At about 800K the pressure haddecreased by approximately 4-5 GPa. As a rule, the higher initial pressure, the higher decreaseof pressure was observed. Therefore samples with the initial pressures 5.5,10, 14, 18, 24, 27GPa at room temperature were estimated to have pressures at 800K of about 1.5, 6, 10, 13, 19and 22 (±1) GPa, respectively. This means that each experiment represents a diagonal sectionof the C60 P-T diagram. In the following the pressure regions 1-8 GPa, 8-18 GPa and 18-27GPa will be discussed separately since they showed different results.

6.2.1 Pressure region below 8 GPa.

Fig. 6.4a shows that a room temperature Raman spectrum of a sample pressurized to 5.5 GPais slightly different compared to the pristine C60 spectrum. An asymmetric shape of the Ag(2)mode and a strong change in relative intensity of some peaks indicate that a fraction ofpolymeric phase (dimers or chains) is already present at room temperature. The spectra in Fig.6.4a also show the appearance of a number of new peaks at higher temperatures. Thisindicates that the degree of polymerization increases as a function of temperature.Surprisingly, however, a maximum in polymerization can be seen at about 670K. A furtherincrease in temperature leads to the reverse process and at 780K all polymers havedecomposed and the Raman spectrum is very similar to that of pure, unpolymerized C60 (seeFig. 6.4b). The decomposition of the polymeric phases at high temperatures is explained bythe pressure decrease during the heating. It is known that C60 polymers decompose at ambientpressure if heated to 400-500K [14]. We know from our experiments that the pressure at780K was about 1.5 GPa. This is in a good agreement with the phase diagram recentlypublished by Sundqvist which shows that the one and two -dimensional polymeric C60 phaseswill decompose to monomeric C60 at about 1-1.5 GPa at 780-800 K [5,8].

a) b)

Figure 6.4 In situ Raman spectra recorded during the heating for two temperature intervals:a)290K (5.5 GPa)-670K (2.3 GPa) b) 670K (2.3 GPa)-780K (1.5GPa).

Determination of the phase composition during heating and cooling is possible since it isknown that the position of the Ag(2) mode is downshifted compared to pure C60 by 5, 10 and21 cm-1 for dimeric, one-dimensional orthorhombic and two-dimensional tetragonal phases,

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respectively [45-47]. For rhombohedral phase this downshift is significantly stronger and theAg(2) mode can be found at ambient pressure at about 1410 cm-1. It was found that therelative positions of these peaks remain similar under HPHT conditions but a clear referencepoint is required since even at room temperature the sample is partly polymerized. As shownabove, the sample at 780K and 1.5 GPa consists mainly of monomeric C60. We have,therefore, used the Ag(2) peak of unpolymerised C60 at these conditions as reference peak andidentified phases for all spectra taken during heating and cooling. The pressure-temperaturepathway during the heating and cooling is shown in Fig.6.5. Phase transformations shown inthis figure will be discussed in more detail below. During cooling, the pressure and thedegree of polymerization increased again as it shown in Fig.6.5.

Figure 6.5. P-T diagram showing phase transformations observed during the heating-coolingcycle. The phases are: fcc-monomeric C60, D-dimeric, O-orthorhombic, T-tetragonal, R-rhombohedral. The phase with the highest intensity in the Raman spectra is listed first.

The second important reference point for analysis of the phase composition is a quenchedsample. A fitting of the spectra from the quenched sample in Fig. 6.6 shows that the mostintense component of the Ag(2) mode belongs to the orthorhombic phase (1460 cm-1),followed by the tetragonal phase (1448 cm-1), while less intense peaks of the rhombohedralphase are situated at 1410 cm-1 and 1432 cm-1. Peaks from monomeric C60 are very weakwhich means that most of the sample is polymerized. The same phases can be identified in theroom temperature Raman spectrum of the sample at 7.5 GPa before quenching, although thedifferences in relative intensity of some peaks are quite remarkable.

The results from the Raman analysis of the quenched sample in Fig. 6.6 are in a goodagreement with data obtained by synchrotron X-ray diffraction [VI]. X-ray diffraction imageswere taken with a beam direction nearly parallel to the uniaxial direction of compression andshowed a number of diffraction spots and arcs due to the strong preferential orientation of thesample. Analysis of the integrated pattern shows that it could be indexed as a mixture ofmainly orthorhombic phase together with the tetragonal phase. The cell parameters of theorthorhombic phase were determined to a= 9.08 Å, b=9.58 Å, c=14.85 Å using the 13 best

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peaks. These values are in good agreement with literature data. It should be noted, however,that the sample has a b-parameter shorter than the previously reported values of 9.8-10.0 Å[43,45]. The shortening of the b-parameter can be explained by a partial tetragonalpolymerization that is present as a disorder in the orthorhombic chain structure. Using a valueof 9.08Å as a reference for the b parameter of pure tetragonal phase and 9.9Å for pureorthorhombic phase we could estimate the ratio of the orthorhombic to tetragonal phases ofabout 1.5.

Figure 6.6. Raman spectrum of the quenched sample recorded ex situ (a) compared with insitu spectrum of the sample after the heating-cooling cycle at 290K (b). Inset shows a fittingof the spectrum of the quenched sample in the spectral region around Ag(2) mode.

The results from the in situ Raman analysis have been summarized in a P-T diagram shownin Fig. 6.5. This diagram is in fact a diagonal section in the P-T space and a number of suchsections are required to construct a complete diagram similar to that presented by e.g.Sundqvist [46,48]. Nevertheless, the diagram allows us to compare the in situ experimentswith previously published results obtained in ex situ experiments. The phase relationsobserved in situ (see Fig. 6.5) and the P-T diagram based on ex situ data in refs [45,46,48]show a very good agreement. We found no other phases besides those obtained in the ex situstudies. Heating from room temperature leads first to the formation of dimers (D) and one-dimensional orthorhombic polymer (O) and at T>470K (4.1 GPa) a mixture of theorthorhombic phase and two-dimensional tetragonal and rhombohedral phases (O+T+R) wasobserved. This seems to be a difference compared to the ex situ P-T diagram presented bySundqvist where only T+R were observed together in the multiphase region [46,48]. Also weobserved the formation of a dimeric polymer already at room temperature which continuouslywas transformed to the orthorhombic phase at higher temperatures. This is also a differencecompared to the results presented by Sundqvist et al. where no region for a dimeric phase isgiven in the P-T diagrams [46] although samples containing a dimeric polymer have beenidentified in their studies [47,48]. Our result is also different from the P-T diagram byDavydov et al. [45], which shows a direct transition from dimeric phase to a tetragonal phasewhile no orthorhombic, or rhombohedral phases were observed in this section of the P-Tspace. However, the transition lines in the P-T diagram presented by Davydov et al. are veryapproximate and will fit to our data with only a small change in the slope of the lines [45].

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Another difference can be seen above 670K, where we during both the heating and coolingsteps observed the one-dimensional orthorhombic phase. In contrast, the ex situ P-T diagramby Sundqvist suggests a direct transition from the monomeric fcc C60 phase at hightemperature (T>500K, P=0-2GPa) to the two-dimensional T and R phases without theformation of any orthorhombic phase [46,48]. In this respect our observations are moresimilar to the P-T diagram presented by Davydov et al. which shows an orthorhombic phasein the region around 550-750K and 1-2 GPa. Therefore our study suggests that our results arecomplementary to the P-T diagrams presented by Sundqvist [46-48] and by Davydov et al.[45].

It is also interesting to note that the evolution of phase composition during cooling wasdifferent compared to heating. For example, much less amounts of the rhombohedral phasewere observed during cooling. This is in good agreement with previous studies, which haveobserved that different pathways for applying pressure and temperature may lead to differentproportions of polymeric phases [45,48]. The preferential pathway for the preparation of thetetragonal phase is to start with a high temperature and then apply pressure (T-P) while therhombohedral phase is favored by first applying pressure followed by a higher temperature(P-T).

6.2.2. Pressure region 8-18 GPa

The phase transformations observed for samples with initial pressure 10-18 GPa weresurprisingly similar (see Fig 6.7 for 10 GPa sample). The general observation in this pressurerange was that new phases appears during heating. It is very interesting, that even after a slowcooling no reversible phase transformations can be observed. In fact, the spectra of thesamples with initial pressure 10-18 GPa remained almost the same during cooling to roomtemperature with only some changes in relative intensity of the peaks (Fig. 6.7).

Figure 6.7. In situ Raman spectra recorded during heating-cooling of a sample with initialpressure of 10 GPa.

In general, a similar observation can be applied to the quenching of cooled samples. Afterrelease of pressure we observed similar spectra for all samples (with the exception of changes

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in relative intensity of some peaks). Therefore, first the polymeric phases in the quenchedsamples were identified using signature peaks from literature data. This identification wasextended to the in situ spectra recorded after cooling. Finally, the in situ spectra duringheating and cooling could be analyzed.

The best quality spectra were obtained for the 10 GPa sample. Spectra of this sample beforeand after quenching are shown in Fig.6.8. The results show that this sample after quenchingconsists of several phases. The strongest peaks can be attributed to the a rhombohedral two-dimensional phase. This phase was obtained in reasonably pure state and studied by Ramanspectroscopy by Davydov et al. [45].

Figure 6.8. Raman spectrum of the 10 GPa sample recorded in situ after the heating-coolingcycle and after quenching. The seven peaks above 1400 cm-1 have been numbered in bothspectra.

The strongest peaks of this phase are observed at 1410 cm-1, which can be assigned to astrongly downshifted Ag(2) mode, and at 1625 cm-1 with an unclear assignment. Some weakerpeaks very typical and unique for the rhombohedral phase can be seen at 1497 cm-1, 858 cm-

1, 728 cm-1 and 242 cm-1. The spectrum of the quenched sample in Fig. 6.8, shows alsoevidences for the presence of the other minor phases represented by the peaks at 1468 cm-1

and 1448 cm-1. The first peak corresponds to the Ag(2) mode of unpolymerised C60 while thesecond peak typically has been reported for the Ag(2) mode of a phase with four square ringsper each C60 molecule (tetragonal phase [44-48]). The peaks found in the quenched samplecan be identified with their counterparts in the in situ spectrum recorded prior to quenching at10 GPa as shown in Fig.6.8.

After analysis of the phases in the room temperature spectra, the phase transformations in situduring the heating-cooling cycle (Fig.6.7) can be followed. For the 10 GPa sample therhombohedral phase starts to form at 470K. This can be seen by the appearance of severalpeaks, e.g. at 729 cm-1. The position of this peak is more or less unaffected by the pressure-temperature changes and it is therefore a good indicator for the rhombohedral phase. Also apeak at 1640 cm-1 (1625 cm-1 for quenched sample) as well as the appearance of the Ag(2)mode of the rhombohedral phase can be recognized already at 470K. These peaks stronglyincreased in intensity at 570K and at 670K they have the highest intensity of all peaks. At 770K the quality of the spectra is less good but the Ag(2) peaks from different polymeric phases

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can still be recognized. It is more difficult to determine the temperature where the Ag(2) peakfrom tetragonal phase starts to form. It is definitely present at 570K and above. However, at470K we see only one broad feature and it is unclear whether it is the Ag(2) peak of thetetragonal phase, chain polymer or their mixture with the monomeric phase, since only onebroad peak can be seen. Most probably we have a monotonous shift of this peak from aposition typical for monomers to dimers and at higher temperature to chains and finally ateven higher temperature to the tetragonal phase. Since we can not separate shifts due to thePT changes from the shifts due to polymerization it is impossible to follow this process indetail using our data.

The in situ heating of the 14 and 18 GPa samples gave results similar to that obtained on the10 GPa sample but the phase transformations occur at approximately 100K highertemperature. As a result, for example, the spectra obtained for the14 GPa and 18 GPa samplesat 570 K looks very similar to spectra from 10 GPa sample obtained at 470K. It is especiallyinteresting that the 18 GPa sample shows the same phase transformation to rhombohedralphase since the starting material prior the heating at 18 GPa is different compared to 10 and14 GPa. A similar spectrum was interpreted previously as a separate phase, so called "phaseIV" by Blank et al. [10] Our results suggest that C60 at 18 GPa is actually in the similar stateas the 10-14 GPa samples but the Raman lines are strongly broadened, probably due to thehigher deformation of the C60 molecules. Nevertheless, it must be noted that peaks fromtetragonal phase and chain polymers are stronger in the 18 GPa sample indicating that theamount of the rhombohedral phase is less compared to samples treated at lower pressures. Thereasons for such behavior are not quite clear. It is likely that higher temperatures would leadto a more complete polymerization.

The quenched samples were also studied using XRD. Two dimensional XRD imagesobtained from all quenched samples after heat treatment are shown in Fig. 6.9. These imagesshow not only Debye-Scherrer rings but also a number of diffraction spots due to preferentialorientation. It is remarkable that higher pressures seems to result in more oriented sampleswith many diffraction spots and arcs together with weak diffraction rings. In particular, theXRD image from the 18 GPa sample seems to contain mostly diffraction spots and arcs whichis evidence for very strong preferred orientation. It is clear that preferential orientation maylead to the disappearance of some peaks and broadening of others. Therefore, having onlyaccess to conventional XRD patterns may lead to erroneous interpretations. The orientationaleffects make it also practically impossible to use Rietveld analysis of the diffraction patterns.

A detailed study of the XRD images and integrated patterns shows that the 10 GPa and 14GPa samples consist of the rhombohedral phase with cell parameters a=9.19(1) Å, c=24.50(2)Å respectively and for the 18 GPa sample a=9.2(1) Å, c=24.4(1) Å. The problem withintegrated XRD patterns of the highly oriented samples can be seen in the diffraction from the18 GPa sample which shows broader peaks and a strong decrease of the intensity for somepeaks, for example, for (110). It is interesting to note that the structural orientation of thesample is correlated with the direction of compression. We could not find any indications ofthe tetragonal and orthorhombic phases in XRD of the quenched samples. The fact thattetragonal and chain polymers were observed by Raman spectroscopy suggest that a fractionof the sample consist of C60 with four square rings per molecule and two square rings permolecule, which are distributed in the matrix of the rhombohedral phase as some kind ofdefects.

36

Figure 6.9 Two-dimensional XRD patterns from the 10-27 GPa samples.

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6.2.3 Pressure region above 18 GPa.

The samples with initial pressures of 24 and 27 GPa showed very different results comparedto the samples heated at lower pressures. In situ Raman spectra of these samples are shown inFig. 6.10.

a) b)

Figure 6.10 In situ Raman spectra recorded during heating-cooling of the samples with initialpressures: a) 24 GPa, b) 27 GPa. As can be seen, the spectra typically exhibit two broad features at around 1500 cm-1 andabout 700 cm-1. During heating the feature at about 700 cm-1 become weaker but otherwise nomajor changes can be observed (the peaks at about 1200-1300 cm-1 is an experimental artifactoriginating from the diamond anvils). The spectra in Fig. 6.10 exhibit clear similarities to thesuperhard fullerites and the phase denoted as “phase V” by Blank et al. [49-54]. It is alsoknown that this type of spectra has been observed for materials with rather different properties[49-54]. The thin film studies presented above in Chapter 6.1 showed that even moderateheating can result in significant increase of the sample hardness without visible changes in theRaman spectra. Therefore, XRD results provided most of the information about thesesamples.

Most previous XRD studies have suggested an amorphous structure for samples pressurizedto above 20 GPa [50,70]. Our data obtained using synchrotron radiation, however, showedthat the quenched 24 and 27 GPa samples have a crystalline structure. The 2D XRD images(see Fig. 6.9) exhibited several lines with a few spots although the quality of the obtainedimages is worse compared to the low-pressure samples. The integrated diffraction pattern ofthe samples are shown in Fig. 6.11 and compared to the 18 GPa sample with a rhombohedralstructure. The diffractogram from the 24 GPa sample contains 6-7 rather broad peaks whichcan be attributed to a rhombohedral structure with a=9.4(1) Å and c=23.1(2) Å. Thiscorresponds to a cell volume only slightly lower than for the 10-18 GPa samples. It should benoted, however, that the c/a ratio is about 2.45. This means that the diffractogram also can beinterpreted as a cubic fcc structure with a cell parameter a=13.3(1) Å and with a volume perC60 molecule of about 590 cm3.

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Figure. 6.11 Integrated XRD patterns from samples with initial pressures of 18,24 and 27GPa. The 27 GPa sample is shown for two different sample orientations (see text).

The analysis of the XRD patterns from the quenched 27 GPa sample is even morecomplicated. XRD images taken with an incident x-ray beam parallel to the direction ofcompression (ϕ=00) and those with ϕ=300 showed a strong difference in contrast to all otherstudied samples. The XRD pattern recorded with ϕ=300 revealed a strong ellipticity of theDebye-Scherrer rings (about 8 %). It must be noted that most of the intensity in the integratedpattern comes from the diffraction arcs and relatively little from the corresponding Debye-Scherrer rings. The integrated pattern from the 27 GPa sample recorded in the direction of thecompression (ϕ=00) can also be attributed to a cubic fcc phase with a= 13.1(1) Å and a cellvolume per C60 of 568 cm3. The pattern recorded at ϕ=300 can not be fitted to a cubicstructure. A more or less good fit, however is achieved for a rhombohedral structure witha=9.2(3) Å, c=20.9(3) Å (V=508Å/M). Another explanation is that the structure is cubic witha=12-13Å but since the major intensity in the integrated pattern originates from fewdiffraction arcs and these arcs are situated on different sections of the ellipses, they giveinformation about parts of the sample with other cell parameters. Taking into account thatonly three peaks are available for analysis the structure of this sample can not be determined.It is clear that strong anisotropic deformation is a reason for the differences between XRDpatterns obtained using two different orientations of sample (ϕ=30o and ϕ=0o). The 27 GPa sample exhibited a strong similarity to literature data by Marques et al. [57]. Theelliptical shape of the Debye-Scherrer diffraction patterns was attributed in this study to three-dimensional polymerization. The very high density found by Marques et al. [57] is consistentwith previous reports on a very high hardness of such samples (as high as diamond) [51,52].Our sample also exhibited elliptical diffraction patterns and a high density calculated fromXRD data. Nevertheless, measurements showed a hardness of only 17 GPa for this sample.This shows that elliptical Debye-Scherrer rings can not be considered as a sign of a superhardphase.

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6.3 In situ XRD study of C60 polymerization using synchrotron radiation

According to literature data three-dimensionally polymerized superhard C60 samples havebeen obtained around 820K and 13 GPa [57]. However, characterization of this phase byXRD has been performed only ex situ on quenched samples. Therefore, in paper [X] severalXRD experiments at HPHT conditions were performed in situ using synchrotron radiation.

Figure 6.12 P-T pathway for in situ XRD experiments.

Three samples, a) b) and c), were pressurized to 13, 16.5 and 18 GPa, respectively, followedby a slow heating to 750-830K. Several pressure corrections were applied during the heating.Samples b) and c) exhibited strong variations in pressure up to 5-8 GPa while sample a) washold between 11.6 and 13 GPa using a larger number of smaller pressure corrections.Therefore each sample has its own history of pressure-temperature treatment as shown in Fig.6.12.

Surprisingly, already at room temperature the samples exhibited significant differences. TheDebye-Scherrer diffraction rings from samples a) and b) remained non-elliptical at roomtemperature, during heating up to 830K and also after quenching. Nevertheless, ellipse-likediffraction patterns were found for sample c) already at room temperature starting from 13GPa. The patterns of sample c) are elliptical (~8%) at 13 GPa which is evidence for astrong anisotropic deformation of the sample. This anisotropy became even stronger at 18GPa (~10%). Due to the combination of a strong deformation and a higher pressure, samplec) become amorphous already at about 500K. In contrast, samples a) and b) remainedcrystalline at all temperatures.. XRD patterns collected from sample a) during the heating areshown in Fig.6.13. At low temperatures the peaks can be fitted to fcc structure. As thetemperature increases, this fit become less good (see standard mean deviation, σ, for

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calculated volume in inset in Fig. 6.13). In contrast, a much better fit is obtained at hightemperatures assuming a rhombohedral structure. At 830 K the cell parameters of therhombohedral phase are a=9.13 (2) Å and c = 23.42(3) Å which gives a cell volume per C60 of563 Å/M.

Figure 6.13 In situ XRD patterns recorded during the heating of the sample a). The insetshows the temperature dependence of the cell volume standard mean deviation, σ, for fittingwith fcc (squares) and rhombohedral (circles) structures.

Cooling of the sample and quenching to ambient pressure changes the cell parametersslightly to a=9.18(3) Å and c=23.33(5) Å. Further evidence that a rhombohedral structuregives a better description of the sample at high temperatures can be seen by the appearance ofa (110) peak of the rhombohedral phase.

Sample b) showed a similar behavior. At room temperature and 16.5 GPa the sample can beindexed as fcc with a=13.37(2) Å. The smallest cell parameter a=12.97(2) Å was observed at18 GPa and 600K, further heating with simultaneous decrease of the pressure resulted inslight increase of the cell parameter. At the same time, starting from 15.6 GPa and 670K abetter fit is obtained by assuming a rhombohedral structure with a=9.13(4) Å, c=22.9(1) Å.This corresponds to a slightly distorted cubic structure with a=12.98(4) Å. At the highesttemperature, 800 K, and 9.5 GPa, the cell parameters are a=9.28(1) Å and c=23.04(2) Å (c/a=2.48). This corresponds to a cell volume of 572 Å3 per C60 molecule and is about 4.7 %smaller than observed in known two-dimensional polymers [45]. The decrease of the c-parameter of rhombohedral structure has been considered as a sign for three-dimensionalpolymerization in some studies [43]. The increased density of our sample is consistent withsuch a suggestion and will be discussed in more detail in section 6.5. Further support for athree-dimensional bonding in this phaseis an increased hardness of samples a) and b) (37 and29 GPa respectively). In contrast, the amorphous sample c) was rather soft with a hardness ofonly 1 GPa.

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6.4 A revised P-T diagram for C60

Figure 6.14 P-T diagram constructed using in situ Raman and XRD data in sections 6.2-6.3.

The data on C60 polymerization discussed in sections 6.2-6.3 are summarized in the P-Tdiagram shown in Fig.6.14. It shall be noted that somewhat different P-T conditions for sometransformations were observed in paper [IX] compared to paper [X]. The reasons for such adifference will be discussed below. The lower pressure region where mostly one- and two-dimensional C60 polymers were observed is based on Raman spectroscopy data. In the higherpressure region (above the section between 18 GPa, 290K and 13 GPa 830K) Ramanspectroscopy gives less information and most of the conclusions are based on in situ and exsitu XRD data. It should be noted that Raman gives insight into the structure of the polymericbonds while XRD gives a macroscopical information on crystal structure. As a consequencewe can expect some differences between P-T diagrams based on Raman spectroscopy andXRD. To emphasize this difference we are using the names hexa-coordinated polymer, tetra-coordinated polymer and chain polymer instead of rhombohedral, tetragonal andorthorhombic since the latter names are based on XRD results. The proposed names refer to alocal coordination of the C60 molecules as described in the introduction. It is also need to beemphasized that Fig.6.14 is not a "true" phase diagram since C60 and C60 polymers aremetastable modifications of carbon. The estimated pressure-temperature pathways for in situRaman studies (paper [IX]) are shown by hatched lines. It shall be noted that therhombohedral phase shown in the high pressure region (Fig. 6.14) is completely differentfrom two-dimensional rhombohedral polymer known from literature [43,45]. The probablenature of this phase will be discussed below.

The strongest differences compared to literature data were observed for pressures above 9GPa. First of all, the experiment with an initial pressure of 18 GPa (Raman data, paper [IX])

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showed only usual two-dimensional polymers and therefore we have extended the fields ofthese phases in our P-T diagram to higher pressures. Below 18 GPa different kinds ofpolymers were obtained at different temperatures. At high temperatures, we have a region ofmainly hexa-coordinated polymer and small amounts of tetra-coordinated and chainpolymers. At low temperatures the samples consist of dimers and chain polymers, whilechain- and tetra-coordinated polymers dominate at intermediate temperatures. It should benoted that there seems to be a gradual transition from dimers and chain polymers to the tetra-coordinated and hexa-coordinated polymers and that we never have observed a completelypure single phase polymer sample. It is therefore very difficult to give any transition linesbetween the regions in the P-T diagram. Nevertheless, we have given approximate transitionlines in Fig. 6.14.

Another difference is that previous studies have observed only dimeric or orthorhombic(chain polymer) phases at 3-9 GPa and 700K and the rhombohedral phase above 700K [44-48]. Our P-T diagram is complementary to these two literature diagrams in a way that itshows both dimeric and chain polymers, but in addition we also observed a tetra-coordinatedpolymer at intermediate temperatures. It must be noted that the tetragonal phase reported inliterature was observed only below 5 GPa in a relatively small P-T region. Fig.14 also showssome fraction of dimeric phase already at room temperature in the pressure range 5-10 GPasimilar to the P-T diagram of Davydov et al. [45], while the P-T diagram by Sundqvist [48]shows no polymerization at these conditions. An interesting question is to determine the polymerization pathway for pressures below 18GPa. According to our data the polymerization sequence during heating is:

monomer→dimer→chain polymers→tetrapolymer→hexapolymer

A problem with this reaction pathway is that the tetragonal phase reported in literature isassumed to form along the {110} direction and the hexapolymer along the {111} direction[43-45]. This means that the tetra-coordinated polymer (tetragonal phase) cannot betransformed into the hexa-coordinated polymer (rhombohedral phase) or vice versa withoutbreaking intermolecular bonds. Nevertheless, our data showed strong evidence that tetra-coordinated polymer transforms directly to the hexa-coordinated polymer. Additionalevidence for this direct formation is that the rhombohedral phase decomposed into tetra-coordinated and chain polymers by simply heating the sample by increasing the laser power.This means that we need an alternative model to explain the formation and decomposition ofthe rhombohedral hexa-coordinated polymer. Fig. 6.15 shows that if we break some bonds inthe hexa-coordinated polymer, a tetra-coordinated polymer with four square rings per C60molecule will be formed.

43

Figure 6.15 Structural diagram illustrating the depolymerisation process in the (111) plane.

If we continue to break bonds a complex polymer with four and two square rings are formedand finally further bond breaking will lead to a chain polymer with two square rings per C60.The proposed reaction pathway suggests that the polymerization process for all polymersproceed along the {111} planes of the fcc cell. During the HPHT treatment the number ofpolymeric bonds in the (111) plane increases. This will give the pathway monomer →dimer→chain polymer →tetra-coordinated polymer →hexa-coordinated polymer, which wasobserved in our experiments. It also allows a direct transition from the tetra-coordinatedpolymer to the hexa-coordinated polymer without bond breaking. The shift of the Ag(2) modein the Raman spectra is proportional to the number of intermolecular bonds and does not referdirectly to the structure of the sample determined by XRD [47]. Therefore, the structure of thetetra-coordinated polymer observed in our study can be different from the tetragonal phasereported in literature [44,45], while the number of the intermolecular bonds and the Ag(2)mode downshift is the same. The preferential pathway for formation of the literaturetetragonal phase is first to increase the temperature and then pressurize the sample. A similareffect was observed in the 5.5 GPa sample on cooling (see chapter 6.3.1) since the monomericphase was formed at high temperature. The cooling part of this experiment showed much lessof rhombohedral phase compared to the heating halfcycle and well resolved peaks from chainand tetra-coordinated polymers. In that case the tetra-coordinated polymer was, in fact,identical to the tetragonal phase described in literature and evidence for formation of thisphase was also found in XRD from the quenched samples. It is clear that further in situ XRDstudies are required to confirm the proposed structural model. It is interesting that the onlyavailable in situ XRD study in this PT region do not report an orthorhombic phase at allshowing a direct transition from cubic to rhombohedral phase [71]. This observation can beexplained within the proposed model if dimeric, chain and tetra-coordinated polymers formsin a random directions within the cubic structure. In that case the macroscopic structureremains cubic during the formation of dimers and chains but with some decrease of the cellparameter.

As was noted above, the pressure region above 12GPa showed the strongest differencecompared to literature data. We have not observed the formation of the very dense superhardphase reported at 13GPa and 820K (V=437-464Å3/M) [57]. At these conditions we observedonly known two-dimensional polymers [IX] or a rhombohedral polymer with a decreased c-cell parameter [X]. Most probable the dense phase can be obtained only at strongly non-hydrostatic conditions. The high stress will provide three-dimensional polymerization at lowerpressures and temperatures.

44

Some suggestions about the nature of the new rhombohedral phase observed in the highpressure region (Fig. 6.14) can be done using established values for cell volumes permolecule of C60 found for different polymers [45]. For pure C60 this value is 711Å/M, for theorthorhombic chain polymer 650-660Å/M, for the tetragonal two-dimensional polymer witheach molecule bonded to four neighbors it is 600-620Å/M, and finally for the rhombohedralpolymer with six square rings per molecule it is about 600Å/M. This gives a trend ofapproximately 20 Å/M for each new square ring connecting two neighboring C60 molecules.If this trend can be extrapolated further, the cell volume of approximately 560Å/Mcalculated for the rhombohedral phase in samples a) and b) would correspond to eight squarerings per molecule [X]. A polymer with this number of bonds must be three-dimensional.Such a polymer can be formed, for example, from a two-dimensional rhombohedral phase bylinking the polymeric planes to each other. The maximal coordination of C60 in a three-dimensional polymer is obviously twelve according to the number of closest neighbors in theclose-packed structure. For such a polymer the calculated cell volume would be around 480Å/M which is not far from the values reported by Marques et al. for samples obtained at 13GPa and 830K (437-464Å/M) [57]. Hardness of this particular sample was not measured bythe authors, but samples prepared by the same group at identical conditions have beenreported to be as hard as diamond [52,53]. The polymeric structure with each C60 moleculeconnected to eight neighbors must be harder compared to 2D polymers but softer than thesuperhard phase studied by Blank et al. [52,53]. Hardness values of 37 and 29 GPa found forsamples a) and b) in paper [X] seem to confirm the suggested structural model. It is also clearthat elliptical Debye-Scherrer rings in XRD patterns can not be used as a sign of three-dimensional polymerization. The 27 GPa sample in paper [IX] which exhibited such ellipticalpatterns after quenching (with cell parameter a= 12-13Å if indexed as cubic structure) showedhardness of only 17 GPa, far from the "superhard" fullerites which were reported to be harderthen diamond. In another experiment elliptical diffraction pattern was observed already atroom temperature (a=12.9-13.3Å) [X]. The heating of this sample resulted in quickamorphization and rather low hardness after quenching (1 GPa).

6.5 Reproducibility problems in C60 polymerization studies

As discussed above, the P-T diagram in Fig.6.14 is significantly different compared toliterature data, especially in the higher pressure region above 9-12 GPa. In general, a poorreproducibility has been acknowledged in many HPHT studies [49]. Some probable reasonsfor this are:

- Poor control of the pressure during heating. Most of the HPHT C60 studies have beenperformed on quenched samples. A precise control of pressure variations during heating isimpossible for many experimental techniques.

- Different heating times. All the studies above 12 GPa have been performed with a heatingtime of 1 min. It is not clear if such a short heating time could produce completetransformations. Most of the samples prepared in this way were inhomogeneous. A study atlower pressure (9 GPa) showed a non-trivial dependence of hardness as a function of heatingtime. A maximum in hardness was observed at 20 minutes and a lower harness with longer orshorter times [ ].

- Several experimental techniques with different stress combinations have been used in thesame P-T regions and often such data have been combined to a single P-T diagram. Forexample, the hardest fullerite samples have been obtained in a "toroid" chamber at very strong

45

non-hydrostatic conditions. Nevertheless, these data were used in the same P-T diagram withexperimental results obtained at hydrostatic conditions [46,49]. Recent results have showedthat strongly anisotropic compression favors the formation of superhard phases (obtained atroom temperature using anvils with shear deformation) [72]. Obviously our experiments inthe DAC had much lower stress compared to studies where these superhard fullerites wereobtained, although conditions were also non-hydrostatic. That can explain why no superhardphases were obtained at the same P-T conditions. It is clear that deviatoric stress must beadded somehow to the P-T diagrams as a third parameter.

6.6 Can compounds with covalent C60-S bonds be prepared by high pressure treatment?

Theoretical calculations have shown that a phase with sulfur atoms connected to two carbonatoms of C60 with covalent bonds (similar to the well known C60O phase) may exist [33]. Sofar all attempts to synthesize this compound have failed. Nevertheless, the recent success inthe synthesis of a dimeric C120SO compound shows that covalent bonding of sulfur to carbonatoms in the C60 cage is possible, although attempts to synthesize C120S2 have beenunsuccessful [34]. It is likely that new types of C60-sulfur materials may be obtained byHPHT treatment either in the form of the predicted C60S compound or as some kind ofpolymer like C120S2.

Figure 6.16. Raman spectra of the C60S16 at different pressures: 1) at ambient pressure, 2) 3.9GPa, 3) 10.3 GPa, 4) 11.4 GPa.

There are two possible scenarios for high-pressure transformations, which can be consideredduring analysis of C60S16 Raman spectra. The first probable scenario is an almost independentchange in the sublattice of the C60 molecules and sulfur rings. In this case, C60 may betransformed to some kind of polymer while S8 rings may be transformed to some other kindsof rings (S6, S4, S3) without sulfur-carbon bonding. The second scenario suggests that at someconditions sulfur may bond with C60 molecules. Here several possibilities have to be takeninto account: (i) formation of C60S, (ii) some kind of C60 polymer connected with sulfurbridges or chains, (iii) S8 or other rings directly bonded to C60. It is also believed that solid

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sulfur undergoes a phase transition from S8 to S6 rings above 10 GPa [74]. If a similartransformation with S8 occurs in C60S16 then the suggestion that at least a part of the sulfurmay bond to C60 looks reasonable. It should be noted that the phase diagram of sulfur is notyet well known and different studies have reported significantly different results, whichcomplicates the analysis of data. The C60S16 in the present study was pressurized step by stepto 12.4 GPa and quenched. The spectra of C60S16 recorded at different pressures are shown inFig. 6.16. Most of the observed peaks can be assigned to C60 in some polymeric state.

At 3.9 GPa only one sulfur line can be recognized of the four observed for C60S16 at ambientpressure. According to high pressure studies carried out on pure sulfur, the peaks of S8 and S6rings are found below 600 cm-1 [74]. Nevertheless, the only line which may be assigned tosulfur S8 rings at 3.9 GPa is found at 502 cm-1, which suggests an upshift from 471 cm-1 in theuntreated sample. At 11.4 GPa the situation become even more complicated. The broadcontinuum which appeared in the spectral region 350-600 cm-1 obviously consists of manycomponents, where the most intense are centered at 490 and 518 cm-1. The new peaks alsocan not be assigned to S6 rings, which have been observed for pure sulfur at pressures above10 GPa [67]. However, it is possible that these lines are included as components into thebroad continuum in the spectral region 350-600 cm-1. A very interesting feature of the 11.3GPa spectra is the broad peak at around 900-1000 cm-1 since it is usually attributed to a four-membered carbon ring which connects C60 molecules by a ”2+2” cycloaddition mechanism[47]. The existence of this kind of rings do not exclude the possibility that sulfur participatesin the linking of C60 molecules since, for example, the C120OS compound (and probableC120S2) contain one square carbon ring and two S-bridges [34].

Figure 6.17 Raman spectra of C60S60 and pure C60 recorded in situ at 12.4 and 13.6 GPa,respectively.

The key point in the analysis of our spectra must be the identification of possible C-S-Cvibrations. Since this kind of bridge has longer bonds and a weaker force constant than the C-O-C bridge [34], this is expected to result in lower vibration frequencies. The epoxy groupvibrations are expected near 1265 cm-1 and between 800 and 900 cm-1, but were not clearlyidentified for C60O, C120O and other C60 oxides [75]. Fig. 6.17 shows Raman spectra from

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pure C60 and C60S16 recorded at 13.6 and 12.4 GPa, respectively. In paper VII, it was foundthat C60 at this pressure and room temperature transforms to mixture of dimeric and chainpolymers. As can be seen, the C60S16 spectrum is very similar but exhibit much sharper peakscompared to pressurized C60. The spectra of C60S16 recorded at high pressure (starting from7.2 GPa) showed peaks at about 798 cm-1 and 849 cm-1, which are probable candidates for C-S-C vibrations. These lines are not present in the spectra of the pure C60 pressurized to similarpressure and can not belong to modifications of sulfur rings because all sulfur lines are foundbelow 600 cm-1.

The spectrum of the quenched sample (see Fig. 6.18) is similar to pristine C60S16 in a sensethat we again observe clear peaks of S8 rings and lines of C60. However, there are some cleardifferences between the spectra. The state of C60 molecules has clearly changed, mostprobably due to polymerization. The Ag(2) mode is downshifted to 1464 cm-1, the Ag(1) modeis split into two peaks at 489 and 493 cm-1 while the Hg(1) mode shows splitting on threepeaks at 257, 260 and 297 cm-1. In general, the spectrum of quenched C60S16 is very similarto that of dimeric C60 [45, 47].

Figure 6.18. Raman spectra of pristine C60S16 (top) and the sample quenched from 12.4 GPa(bottom)).

The peaks at 945 and 974 cm-1 are typically assigned to vibrations of square rings connectingC60 molecules. On the other hand, the spectrum of quenched C60S16 looks very similar to thatof C60O. It is interesting that the C60O spectrum also shows the position of the Ag(2) mode at1464 cm-1, a splitting of the Hg(1) mode with the strongest peaks at 258, 268 and 289 cm-1

and splitting of the Ag(1) mode into two peaks at 475 and 488 cm-1. The C60O spectrum alsoshowed some broad feature around 950-100 cm-1. Finally, it shall be noted that specific peaksfound for C60S16 at high pressure were not observed in the quenched sample.

In conclusion, the in situ high pressure studies show some additional peaks, e.g. 849 and 798cm-1 (at 12.4 GPa) which cannot be attributed to any known polymeric phase. This can be due

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to the formation of C-S bonds. After quenching these peaks disappeared indicating that C-Sbonds are broken upon quenching. However, the presence of peak shifts and splitting suggestthat the sample consist of either C60S +S8 or that C60 in the C60S16 become polymeric. It shallbe noted that the data discussed in this section are yet unpublished and further studies arerequired.

49

Chapter 7

Concluding remarks and future outlook.

The most important results in my thesis are:

(i) Crystallization of fullerenes and their co-crystallization with sulfur from solutions resultsin the formation of solids with weak van der Vaals bonding between fullerene molecules andmolecules of solvent or sulfur. In contrast, HPHT treatment results in the formation ofpolymers with strong covalent bonding between the C60 molecules. Therefore, usingmolecular solids (e.g. C60S16) as a precursor, HPHT treatment may lead to the formation ofcovalent bonds between C60 and, for example, sulfur.

(ii) The anomalous temperature dependence of the C60 solubility in some solvents can beexplained by the existence of solvated phases. It was found that C70 also form solvates inbenzene, toluene and hexane solutions, but their melting occurs above the boiling points ofthese solutions which results in absence of maxima on solubility dependence.

(iii) Raman and NMR studies gave evidences that C60 and C70 molecules in the molecularsolids show almost the same rotational freedom as in pure fullerenes despite the fact thatprevious structural studies suggested that rotation must be frozen.

(iv) Vibrational signatures of the weak bonding in the studied molecular solids have beenestablished.

(v) New methods have been developed which allowed the growth of large crystals and toprepare sulfur-fullerenes films by solution and vapor techniques.

(vi) Large single-crystals of a new C70S8 compound were obtained and characterized.

(vii) The first studies of C60 films at high pressure conditions have been performed. It wasshown that the thickness of the films strongly effects the pressure of the phase transitionaround 20 GPa. Superhard and superelastic films were obtained by heat treatment at 20 GPaand 570K.

(vii) The first in situ Raman studies of the C60 polymerization have been performed in a broadpressure and temperature range. In general, this study showed results similar to those obtainedusing ex situ methods. Two-dimensional polymers were observed at significantly higherpressures compared to previous studies. When the pressurizing is followed by heat treatmentthe polymerization exhibited the following pathway:

monomer→dimer→chain polymers→tetra-coordinated polymer→hexa-coordinated polymer.

The tetra-polymer is suggested to be different from the known tetragonal phase.

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(viii) In situ and ex situ XRD studies of the C60 polymerization above 12 GPa showed noevidence for the superhard, very dense phases reported in previous studies. This can beexplained by very strong differences in stress conditions and heating times. (ix) It was shown that elliptical Debye-Scherrer diffraction patterns can not be considered asevidence for three-dimensional polymerization and can appear even in samples with relativelylow hardness.

(x) Raman spectra which previously has been considered to be typical for superhard fullerites(e.g Blank et al. [49-53]) was observed for both soft and hard C60 phases above 20 GPa. Forroom temperature samples a transformation to a orthorhombic chain polymer could beobserved after quenching. This shows that such spectra can also be associated to samples withunbroken C60 cages.

(xi) A new rhombohedral phase was observed by in situ XRD at pressures above 13 GPa and800K. The new phase was suggested to be a three-dimensional polymer where each C60molecule is bonded to eight neighbors.

It is clear that further studies are required to answer the many questions raised in the papersand to confirm some of the conclusions. In particular, this is true for the high pressureresearch. The C60 polymerization at HPHT conditions should be studied in situ at differentcontrolled stress conditions. This will make it possible to construct better P-T diagrams of theC60 polymers and to obtain homogeneous samples of the superhard fullerite. The superhardfullerite material is very interesting for different applications. It has a hardness at least ashigh as diamond, but has also extreme elastic properties. Such a combination of properties israther unusual. Also a lot remains to be done in High Pressure chemistry of fullerenes. It isclear that HPHT treatment is able to produce a number of new fullerene materials. This fielduntil now remains almost untouched and potentially able to produce a number of surprises.

51

Acknowledgements

First of all I would like to thank Professor Jan-Otto Carlson for giving me an opportunity tomake this thesis and for placing departments excellent facilities at my disposal. I would liketo express most sincere gratitude to my supervisor Prof. Ulf Jansson for giving me as muchfreedom as I wanted and as much help as needed. I especially appreciate his patience dealingwith my writings and during hot discussions of our papers.

I also would like to thank Prof. Leonid Dubrovinsky who became my second supervisorduring the last year. It is a real pleasure to work with somebody so enthusiastic andinspirational about science. Thanks to Prof. Ingvar Engström for the help with my first paperat the department.

Warmest thanks to my co-authors: Dr.Sherman from St-Petersburg for his support and forteaching me a lot of things, Ane-Sophie Grell from Bruxelles for excellent NMR studies, M.Oden from Linköping who made hardness measurements.

Many thanks also to my former colleagues from A.F.Ioffe Institute where I started my workon fullerenes in 1994: P.P.Syrnikov, V.V.Lemanov, V.Bahurin, A.P.Savelieva. Your helpwas so valuable for me in the beginning of my scientific career.

I would like to thank all friends at the Department for their support. In particular, JennyOlander for being always very kind and helpful, Lars Norin for all the nice conversationswhich we had last years and for returning to the department after long time of absence,Michael Ottosson who survived being my roommate almost year and significantly enrichedmy knowledge of brewery, Hans Högberg for teaching me a lot about UHV system and forthat special kind of nodding which now I sometimes unconsciously copy. I also would like tothank Anders Lund and Torvald for performing a lot of practical work with equipment.

My Russian friends in Uppsala, past and present, were realy great to me, thank you all: Mitjaand Nadja, Kristina, Natasha's (D.and M.), Dima T., Artem, Tanjana T., Yury and all others. Finally, the most thanks to my beloved wife Nina who brought me to Uppsala and wassupportive through all these years.

Alexandr Talyzin, Uppsala, October 2001.

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