Experimental and computer simulation study of the vibrational

spectra of vermiculite

Mehdi Arab, Daniel Bougeard and Konstantin S. Smirnov*

Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 CNRS), Centre d’Etudes et deRecherches Lasers et Applications, USTL, Bat. C5, F-59655 Villeneuve d’Ascq Cedex, France.E-mail: Konstantin.Smirnov@univ-lille1.fr

Received 23rd November 2001, Accepted 1st March 2002First published as an Advance Article on the web 15th April 2002

Infrared and Raman spectra of vermiculite clay have been recorded, and an assignment of the observed featuresin the spectra is proposed on the basis of molecular dynamics calculations. For this purpose, a force fieldyielding a structure and vibrational spectra in good agreement with the experimental data was developed withthe help of ab initio calculations of models of vermiculite structural units. The calculated spectra of the periodicclay model are analysed in terms of symmetry adapted internal coordinates of the building units of the claystructure.

1 Introduction

Raman spectroscopy has been used extensively for studies ofclay minerals, especially since the advent of near infrared lasersas excitation sources.1–4 However, the reliable assignment offeatures in the spectra of clay systems is often hampered bythe complex structure of these solids, and theoretical methodsare therefore necessary to facilitate the interpretation of theresults of measurements.5,6 Among modern computationaltechniques, the methods of quantum chemistry have a limitedapplication to clay systems, because the large number ofatoms, their disordered distribution in atomic positions, andthe low symmetry of clay unit cells demand large computerresources. Force-field-based methods are an alternative, whichcan provide a detailed analysis of the systems at the micro-scopic level. Monte Carlo and molecular dynamics (MD) stu-dies of clays have already been done successfully on severalstructures like kaolinite or smectite.7–13

Vermiculite occurs in nature in the form of macroscopiccrystals. Due to its structural features, vermiculite clays playan important role in many natural processes and industrialapplications. These include oil and gas production,14 theremoval of organic contaminants from water,15,16 and radio-active waste disposal.17 Recently, these clays have also beenused to produce new nanoporous materials.18 Vermiculite crys-tals are constituted of stacking negatively charged sheets,which are held together by charge balancing interlayer coun-terions.19 The sizeable interlayer spacing of the clay plays animportant role in the transport processes of natural and con-taminant species in the soil. Modelling of clay minerals istherefore of significant interest for understanding the underly-ing physico-chemical processes in these phenomena, particu-larly clay swelling and the behavior of metallic ions andorganic compounds in the interlamellar space. This behaviormay well depend on the dynamics of the clay structure thatis generally probed by the methods of vibrational spectro-scopy.In the present work we employed infrared and Raman spec-

troscopy to characterize vermiculite structure. The experimen-tal works were complemented by modelling studies, which aimat interpretation of the vibrational spectra of the clay. For thispurpose, as a first step, it was necessary to develop a force field

that is able to describe the structural characteristics and thedynamical behavior of the solid. Ab initio calculations of smallbuilding units of the vermiculite lattice (octahedral sites) wereundertaken in order to derive the force-field parameters. Theobtained force constants were then added to a force field devel-oped for aluminosilicate following the same strategy.8,20–22

Finally, the complete force field was used in moleculardynamics simulations and the assignment of the bandsobserved in the vibrational spectra of the vermiculite sampleswas carried out with the help of symmetry coordinates of thebuilding units constituting the clay structure.

2 Experimental and computational methodologies

2.1 Experimental details

The vermiculite samples were obtained from Sigma Aldrich.This clay was extracted from the mountains of South Carolina(USA) and studied without further purification. The providerstates that vermiculite is the pure geological ore. The chemicalanalysis of metal atoms in the samples resulted in the followingcomposition (at.%): 33.14% Si, 25.95% Mg, 16.83% K, 13.24%Fe, 8.99% Al, 1.87% Ti. The comparison of these values withthose expected for an ideal vermiculite structure with theatoms in either tetrahedral or octahedral positions shows anexcess of Fe and K atoms, in agreement with the light yellow-ish color of the samples indicating the presence of iron oxidesor hydroxides. Despite the abundance of iron, the measuredRaman spectra do not reveal any pattern characteristic ofthe presence of well defined iron compounds. Further, themicro-Raman measurements showed no difference in the spec-tra for different parts of the sample and all the spectra are verysimilar to the Raman spectrum of vermiculite found in theliterature.23

The infrared spectra of the samples in the 5000–400 cm�1

range were recorded at room temperature by using the KBrpellet technique with a Bruker IFS 45 spectrometer, and inthe 700–100 cm�1 range by using the polyethylene techniquewith a Bruker IFS 113 spectrometer. All spectra were regis-tered with a spectral resolution of 4.0 cm�1 in both spectralregions and were averaged over 500 scans.

DOI: 10.1039/b110768b Phys. Chem. Chem. Phys., 2002, 4, 1957–1963 1957

This journal is # The Owner Societies 2002

PCCP

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online / Journal Homepage / Table of Contents for this issue

The Raman spectra were obtained with a LABRAM Ramanspectrometer manufactured by Dilor SA Instruments. TheRaman scattering of the samples was excited using the 632.8 nmline of a helium–neon laser. The laser beam was focusedthrough a high-aperture microscope objective and the scat-tered light was collected through the same objective. A lowlaser power of about 5 mW was used to prevent any structuralchange in the sample during the measurements.

2.2 Structure

Vermiculite is a trioctahedral 2:1 layered silicate. The nega-tively charged layer of its structure is formed from a sheet ofedge-connected XO6 octahedra, where X is an octahedral atom(XVI ¼ Mg, Al, Fe), which are symmetrically bound to twosheets of corner-connected TO4 tetrahedra, where T is a tetra-hedral atom (TIV¼ Si, Al), forming six-membered silicaterings.24 The vermiculite structure, which is shown in Fig. 1,contains three types of oxygen atoms. Oxygen atoms of thefirst type (OI) link two Si atoms and are located at the cornersof the SiO4 tetrahedra. Oxygen atoms of the second type (OII)are tetracoordinated oxygens bound to a silicon atom andthree magnesium atoms. Oxygen atoms of the third type (OIII)are tetracoordinated oxygens coordinated to one hydrogenatom and three magnesium atoms. The external surfaces ofthe layer are built from oxygens of the first kind (OI). Follow-ing Mathieson24 the structure of vermiculite has a monoclinicunit cell with Cc space group and the unit cell is characterizedby the following values of the lattice parameters: a ¼ 5.33 A,b ¼ 9.18 A, c ¼ 28.90 A, and b ¼ 97�. The interlamellar spaceis large enough (Fig. 1) to accommodate both inorganic andorganic counterions compensating the eventual charge of thelayer.

2.3 Computational methods

The vibrational spectra of vermiculite were modelled by a clas-sical MD technique. The force field used in the present work isa generalized valence force field defined in terms of internalcoordinates8,20–22

2U ¼X

i

X

j

Kijðsi � s0i Þðsj � s0j Þ ð1Þ

where U stands for the potential energy, and si and sj are inter-nal coordinates having the equilibrium values s0i and s0j , respec-tively. The parameter Kij� (@2U/@si@sj)0 denotes a forceconstant. As in previous studies,8,20–22 the internal coordinatesused are defined in terms of bonds and bond angles. For thebond angle potential Uy we used a cosine harmonic formallowing linear angles

2Uy ¼ Ciiðcos yi � cos y0i Þ2;

where Cii ¼ Kii/sin2 y0i . In eqn. (1) the interactions between

coordinates are limited to coordinates having either a commonatom or a common bond.To obtain the parameters Kij of the force field, it was

assumed that the dynamics of the periodic clay lattice can bedescribed in terms of the dynamics of the constituent structuralsubunits.20–22,25–27 Following this approach, the parameters ofthe force field are obtained by ab initio calculations of molecu-lar models of these building blocks of the lattice. In the case ofvermiculite, such primary building units are SiO4 tetrahedra,MgO6 octahedra connected via common oxygen atoms, andOH groups. Force-field parameters for the SiO4 tetrahedrahave already been determined by Ermoshin et al. in studieson zeolites20,21 and successfully applied to modelling silica sur-faces and kaolinite.8,22 Thus, only terms for internal coordi-nates involving magnesium atoms in octahedral coordinationand oxygen atoms presenting an unusual tetracoordinationhave to be added to the force field. For this purpose the matrixof the second derivatives of the total energy with respect to theCartesian coordinates (Fx) for Mg(OH)2 and Mg(OH)4

2�

models was calculated using the GAUSSIAN 94 package.28

In these molecular models the hydrogen atoms were addedin order to keep the divalence of the oxygen atoms. No symme-try constraints were applied to the models during the geometryoptimization. Preliminary calculations were carried out usingthe 3-21G* basis set at the Hartree–Fock (HF) level. The finalcalculations were performed at both Hartree–Fock and MP2levels employing the 6-311G** basis set, which is identical tothat used in the previous work on AlO6 octahedra.8 The Fx

matrices obtained were then transformed into the matrices ofthe second derivatives in internal coordinates (FR) using theprogram REDONG29 so that the redundancies existingbetween internal coordinates were taken into account. Theforce-field parameters for MgO6 tetrahedra were obtainedfrom the results of the calculations by correcting the systematicerror between the HF and MP2 levels like in ref. 20. As thebasis set was large enough, no scaling procedure was appliedto the results of quantum chemical calculations. The para-meters obtained in this way were added to the aluminosilicateforce field20–22,25,26 and the resulting potential model was usedin MD simulations of the structure and the vibrational spectraof vermiculite. No long-range interactions were explicitlyemployed in this potential model besides those which wereimplicitly included in the force constants Kij .The simulated system was built from 3� 2� 1 vermiculite

unit cells and it contained two sheets of the vermiculitestructure (Fig. 1) in a simulation box of size 15.99 A�18.36 A� 28.90 A. The unit cell used had the compositionMg3Si4O10(OH)2 assuming that all aluminium atoms werereplaced by either magnesium or silicon, and no charge-balan-cing cations and water molecules were included. The chemical

Fig. 1 Structure of vermiculite: a view of one unit cell in the bc-planeis shown.

1958 Phys. Chem. Chem. Phys., 2002, 4, 1957–1963

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online

formula used corresponds to talc, which has a layer structuresimilar to that of vermiculite. However, the symmetry, inter-layer spacing, and the local geometry of building units in bothstructures are different. To be sure that the computed modelmimics the experimental samples, the initial atomic coordi-nates were taken from the work of Mathieson24 on vermiculite.The initial atomic velocities of the particles were chosen fromthe Maxwell distribution at 300 K. The equations of motionwere integrated with the velocity form of the Verlet algo-rithm30 with time steps of 0.4 fs under periodic boundary con-ditions. During the first 50 000 time steps the velocities of theatoms were rescaled to the reference temperature and thenthe simulation was continued in the NVE (microcanonical) sta-tistical ensemble. The coordinates and velocities of all atomswere stored every tenth step for the last 30 000 steps fromthe total of 100 000 steps (40.00 ps). The relative deviation ofthe total energy from the initial value over the NVE run wasless than 0.4%, revealing a good conservation of energy.The infrared spectra were calculated from the MD run via

Fourier transformation of the autocorrelation function of thetotal dipole moment.31 The effective atomic charges were setto 1.564 e for Si, 1.047 e for Mg, and �0.795 e for O atoms.These values correspond to charges obtained by the Mullikenpopulation analysis in 6-311G**/HF calculation of the Mg[O-Si(OH)3]4(OH)2 cluster of vermiculite with the geometrydescribed by Mathieson24 without geometry optimization.The charge value of the hydrogen atoms was set to 0.072 ein order to keep the system neutral.Raman spectra were calculated by Fourier transformation

of the autocorrelation function of the polarizability tensorassuming that the polarizability of the system can be computedas a sum of bond polarizabilities.32,33 Electro-optical para-meters of the Si–O and O–H bonds were taken from previouswork.8,33 Parameters for the Mg–O bond were adjusted inorder to fit qualitatively the relative intensities of the experi-mental Raman spectrum. The electro-optical parameters usedto compute the Raman spectrum are listed in Table 1.To get a better insight into the participation of atoms of dif-

ferent types in the vibrational spectra, a power spectrum of thewhole system and a spectrum for each kind of atom were com-puted by Fourier transformation of the atomic velocity auto-correlation functions. In addition, the dynamics wasanalysed using the local symmetry of the building units ofthe vermiculite structure. Such an analysis in terms of symme-try coordinates has already been used in refs. 34 and 35 toelucidate features in the vibrational spectra of complexaluminosilicates.The SiO4 tetrahedra were assumed to have C3v symmetry

since one of the four oxygen atoms (OII) points toward a mag-nesium atom (Si–O bond internal coordinate Dr1), whereas thethree other oxygens (OI) are bound to two silicon atoms (coor-dinates Dr2–Dr4). The symmetry coordinates which involvevariation of the Si–O bonds of the SiO4 unit can then bewritten as

T1 ¼ Dr1 ð2Þ

T2ðA1Þ ¼ ð1=ffiffiffi3

pÞðDr2 þ Dr3 þ Dr4Þ ð3Þ

T3ðEÞ ¼ ð1=ffiffiffi2

pÞðDr3 � Dr4Þ ð4Þ

T 03ðEÞ ¼ ð1=

ffiffiffi6

pÞð2Dr2 � Dr3 � Dr4Þ ð5Þ

In the case of the MgO6 octahedra, an octahedron is builtof four OII and two OIII , and consequently it was split intotwo subunits, a subunit with C4 symmetry constituted of amagnesium atom and four OII (DR1–DR4 coordinates) anda linear OIII–Mg–OIII subunit (DR5 , DR6 coordinates). Thesymmetry coordinates which involve variation of the Mg–O bonds in the subunits are described with Si notationand read

S1ðAÞ ¼ ð1=2ÞðDR1 þ DR2 þ DR3 þ DR4Þ ð6ÞS2ðBÞ ¼ ð1=2ÞðDR1 � DR2 þ DR3 � DR4Þ ð7Þ

S3ðEÞ ¼ ð1=ffiffiffi2

pÞðDR1 � DR3Þ ð8Þ

S03ðEÞ ¼ ð1=

ffiffiffi2

pÞðDR2 � DR4Þ ð9Þ

S4ðA1Þ ¼ ð1=ffiffiffi2

pÞðDR5 þ DR6Þ ð10Þ

S5ðB1Þ ¼ ð1=ffiffiffi2

pÞðDR5 � DR6Þ ð11Þ

Spectra of the symmetry coordinates were then computed byFourier transformation of the corresponding autocorrelationfunctions. The frequency step in all the calculated spectra isequal to 4.07 cm�1 and the spectra were averaged over 32time origins.

3 Results and discussion

3.1 Force-field development

The force-field parameters for the silicate building units havealready been described in detail elsewhere,20,21 and numericalvalues of the force constants for Si(OI)4 tetrahedra and forthe Si–OI–Si bridge can be found in these publications. Thus,we can focus on the terms for MgO6 octahedra needed formodelling vermiculite.All optimized bond lengths and the corresponding force

constants calculated for Mg(OH)2 and Mg(OH)42� models at

different levels of theory are summarized in Table 2. The differ-ent values of the O–H force constants obtained at the HF andMP2 levels (Table 2) are due to the inclusion of electron corre-lation at the MP2 level. The MP2 calculations also result in alonger O–H bond length, which correlates well with a smallervalue of the force constant. The same, though less pronounced,effect is also obtained for the Mg–O bond. The MgO stretchingforce constants obtained for these models closely follow Bad-ger’s relation36–38 between T–O stretching force constant andT–O bond length as seen for SiO4 and AlO4 tetrahedra inaluminosilicates.21 Making use of the parameters of Badger’srule determined by Ermoshin et al.21 and the experimentalMg–O bond length of 2.06 A given by Mathieson,24 a valueof 1.40 mdyn A�1 was chosen for the Mg–O bond force con-

Table 1 Electro-optical parameters used in the Raman spectra calcu-

lation

Si–Oa Mg–O O–Ha

Equilibrium bond length/A 1.64 2.06 0.93

Derivative of longitudinal bond

polarizability/A22.27 0.80 2.77

Derivative of transverse bond

polarizability/A20.64 0.20 0.57

a Values from ref. 8.

Table 2 Calculated bond lengths/A and the corresponding force

constantsa /mdyn A�1 for Mg models

HF 3-21G* HF 6-311G** MP2 6-311G**

Mg–O

Mg(OH)2 1.72 1.75 (3.98) 1.81 (3.38)

Mg(OH)42� 1.94 2.00 (1.41) 2.02 (1.40)

O–H

Mg(OH)2 0.95 0.93 (10.26) 0.95 (9.14)

Mg(OH)42� 0.98 0.94 (9.28) 0.96 (8.21)

a The force constants are given in brackets.

Phys. Chem. Chem. Phys., 2002, 4, 1957–1963 1959

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online

stant. Assuming a linear relation between the bending forceconstant of non-linear O–Mg–O angles and the value of thecorresponding angles, the value of 0.98 mdyn A rad�2 wasextrapolated from the calculations performed for Mg(OH)2and Mg(OH)4

2� models. In the MgO6 octahedra one can dis-tinguish two different MgO/MgO interaction force constants.The first one corresponds to bonds which are nearly perpendi-cular and thus are not mechanically coupled. The corres-ponding cross-terms were set to 0.00 mdyn A�1. The secondtype of interaction force constant characterizes two Mg–OII

bonds with a nearly linear angle between the bonds. The valueof the corresponding force constant was also set to 0.00 mdynA�1 because the quantum chemical calculations resulted in asmall value of 0.02 mdyn A�1, which is close to the precisionof the procedure used to obtain the parameters. In the caseof MgO/OMgO and OMgO/OMgO force constants, our cal-culations for Mg(OH)4

2� gave values which are close to thosefor octahedral aluminium.8 It was therefore assumed that theseforce constants are identical to the corresponding AlO6 forceconstants used in the kaolinite study.8 The OH stretching forceconstant was set to 8.2 mdyn A�1 as a result of the quantummechanical calculation at the MP2 level. All the force-fieldparameters for the Mg-containing units are summarized inTable 3.The remaining parameters to determine were angle-bending

force constants and interaction force constants for four-foldcoordinated oxygen atoms (OII and OIII). These oxygens arebound to three magnesium atoms and to either silicon orhydrogen. The force-field parameters for these oxygens wereset equal to the corresponding parameters in the SiO4 tetra-hedron20 since the geometric environment is similar.

3.2 Molecular dynamics calculations

After the 20 ps relaxation of the structure, a narrow peak at1.62 A is calculated for the bond distance in the Si–O radialdistribution function (RDF). This Si–O bond length agrees

well with the value 1.62 A given by Mathieson.24 Two distinctmaxima at 2.04 and 2.27 A are computed in the Mg–O RDFfor the Mg–O bonds. These maxima are due to the Mg–OII

and Mg–OIII distances, respectively. The Mg–OII bond lengthis in good agreement with the value of 2.06 A given by Ma-thieson,24 whereas the value obtained for the Mg–OIII bondsis significantly longer than the mean experimental Mg–O bondlength. This difference is explained by the fact that the silicatestructure of the clay limits relaxation of the Mg–OIII bonds.In the high-frequency range above 1200 cm�1 the only peaks

existing in the computed spectra are due to the O–H stretchingvibrations. Thus, in the OH stretching region, peaks at 3860and 3868 cm�1 and peaks at 3856, 3864, 3872, and 3880 cm�1

are calculated in the computed Raman and infrared spectrumrespectively, whereas a wide massif of peaks is observedbetween 3200 and 3700 cm�1 in the experimental infrared spec-trum. At first glance the presence of several peaks in the calcu-lated spectrum seems to be surprising since only one O–Hstretching force constant was used in the simulations. Thisresult is an artefact of the finite-temperature MD calculation.The frequency of the OH stretching vibrations is much higherthan the frequencies of other vibrations in the system and theO–H vibrations behave as normal modes of the system. Thus,no energy exchange occurs between the vibration and othervibrations in the system. This means that the vibrationalamplitude and thus the frequency of the mode can be biasedby the initial conditions (the seed of initial velocities). Theuse of many MD runs starting with different distributions ofinitial velocities avoids this artefact.33

The experimental and calculated Raman and IR spectra inthe region below 1200 cm�1 characteristic for the frameworkvibrations are presented in Figs. 2 and 3. The calculatedRaman spectrum was normalized to the most intense peak at678 cm�1. The figures show that each important feature inthe experimental spectra has its counterpart in the computedones. The intensities of the experimental IR spectrum are notwell reproduced below 500 cm�1. It is worthy of note thatthe experimental infrared and Raman spectra were recordedwith a natural sample also containing aluminium atoms in dis-ordered positions, while the computed spectra are due to thedynamics of an ideal framework with periodic boundary con-ditions implying no defects or impurities. Thus, some differ-ences between the experimental and the calculated spectracan be expected. Further, in the vermiculite samples withlarge interlamellar spacing, the librational motion of watermolecules is expected to absorb around 400–500 cm�1, whichexplains the discrepancy between the calculated and measuredspectra. Finally, the calculation of the IR spectra was carriedout by using a simple point-charge model to obtain the dipolemoment of the system, whereas the Raman spectra were

Table 3 Force-field parameters Kij/mdyn A�1, /mdyn A rad�2, /mdyn

rad�1, and s0i /A, /deg used in MD calculations for MgO6 octahedra

and Mg–O–H(Si) linkages [see eqn. (1)]

Internal coordinate Kij si0

MgO6 octahedrona

O–Mg–O 0.98 90.0

(MgO)/(MgO) 0.00

(OMgO)/OMgO)b �0.08

(OMgO)/(OMgO)c �0.25

(MgO)/(OMgO)b 0.08

(MgO)/(OMgO)c �0.08

Mg–O–H(Si) linkage

Mg–O 1.40 2.06

O–H 8.20 0.96

Mg–O–Mg 0.12 109.47

Mg–O–H 0.12 109.47

(MgO)/(MgO) 0.10

(MgO)/(OH) 0.10

(MgOH)/(MgOMg) �0.11

(MgOSi)/(MgOSi) �0.11

(MgO)/(MgOH)b 0.13

(MgO)/(MgOH)c �0.13

(OH)/(MgOH) 0.13

(OH)/(MgOMg) 1.40

a For MgO stretching force constant see the corresponding entry in

the Mg–O–Mg linkages. b Interaction force constant between internal

coordinates with a common bond. c Interaction force constant

between internal coordinates with a common atom. Fig. 2 Experimental (a) and calculated (b) Raman spectrum of ver-miculite in the framework region.

1960 Phys. Chem. Chem. Phys., 2002, 4, 1957–1963

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online

computed by a more sophisticated model with the set of elec-tro-optical parameters (Table 1). In the latter case a betteragreement between the peak intensities in the experimentaland theoretical spectra was obtained which implies that thequality of the computed IR spectra could be improved usinga similar intensity model.Comparison of the literature data on the Raman spectra of

talc23,39 with the experimental spectrum of vermiculite23 showsvery similar peak positions, whereas significantly higher rela-tive intensities of the peaks at 187 and 678 cm�1 were mea-sured for talc structure. The calculated spectrum correspondsbetter to the vermiculite structure, therefore validating thestructural model used in this work.In order to obtain a reliable attribution of the observed

bands to specific modes, we discuss the calculated Raman spec-tra in connection with the power spectra computed for eachkind of atom and with the spectra of the symmetry coordinatesdefined by eqns. (2)–(11). The derived assignment of the bandsin the spectra is summarized in Table 4. The spectra of Si, Mgand O atoms are shown in Fig. 4. One can see that the powerspectrum of the oxygen atoms indicates an involvement ofthese atoms in most of the vibrations in the range 100–1200 cm�1. In the region 700–1200 cm�1 the power spectrumof Mg atoms has no significant intensity, in contrast to thespectra of silicon and oxygen atoms. Peaks at 1075, 921 and737 cm�1 can clearly be attributed to Si–O bonds since Siand O atoms are the only ones for which the peaks are presentin this region and have a high intensity (Fig. 4). As there aretwo types of Si–O bonds in the vermiculite structure, we usespectra of the symmetry coordinates defined by eqns. (2)–(5)for the assignment of peaks (Fig. 5). The peak at 921 cm�1

can be attributed unambiguously to the stretching vibrationof the Si–OII bond and the two peaks at 1075 and 737 cm�1

are due to the degenerate stretching coordinates T3 and T 03

of the Si(OI)3 pyramid. The peak calculated at 1033 cm�1

belongs to the symmetric stretching T2 coordinate of the pyr-amid. The small shift between experimental and calculatedpeak positions might be attributed to differences in the chemi-cal composition of the experimental samples and simulatedstructure of the clay.The power spectra (Fig. 4) show that the Mg atoms are

involved in the vibrations with frequencies in the region 750–500 cm�1. Analysis of the spectrum in terms of the S symmetrycoordinates (Fig. 6) allows us to assign some bands to theMg–O bond stretching vibrations. Thus, the most intense peakat 678 cm�1 in the experimental and calculated Raman spectrais due to the degenerate Mg–OII symmetry coordinates S3

and S03.

Below 500 cm�1 the experimental Raman spectrum is char-acterized by two strong lines at 354 and 187 cm�1. On the basisof the symmetry coordinate analysis the experimental Raman

peak at 354 cm�1 can be assigned to the S4 and S5 coordinatesof the OII–Mg–OII structural units. The high intensity of thelatter peak is well reproduced in the calculated spectra at227 cm�1. This band can easily be attributed to theMg–OIII–Mg bending mode by using the angle symmetry-adapted coordinates, as done for the bond stretchingcoordinates of the SiO4 and MgO6 units. In addition, whenin Raman spectrum calculations the electro-optical parametersfor Mg–O bonds were set to zero, the intensity of the band at227 cm�1 has drastically decreased, revealing a large participa-tion of the Mg–O bonds in the mode.

Table 4 Observed and calculated positions/cm�1 of bands in the spec-

tra of vermiculite and proposed assignment (n and d denote bond

stretching and angle deformation, respectively)

Infrared Raman

AssignmentExp. Calc. Exp. Calc.

128 113 d151 d

183 187 227 d Mg–O–Mg

240 d OI–Si–OI

278 278 d Si–OI–Si

310 309 d Si–OI–Si

354 354 d OIII–Mg–OIII+ n Mg–O

367 370 370 d Mg–OIII–H

395 395 d OIII–Mg–OIII

429 411 411 d458 435 434 448 d OI–Si–OI

456 d476 d

508 488 492 d OIII–Mg–OIII

528 532 529 d O–Mg–O

546 552 546 n Mg–OII (B)

622 634 643 n Mg–OII (A)

651 660 n Mg–OIII sym

676 672 678 678 n Mg–OII (E)

700 n Si–OI (A)

723 741 737 n Si–OI (E)

812

957 921 960 921 n Si–OII (A)

995 1033 1033 n Si–OI (A)

1095 1075 1083 1075 n Si–OI (E)

1643

3207 3856 n OIII–H

3390 3864 3860 n OIII–H

3561 3872 3868 n OIII–H

3711 3880 n OIII–H

Fig. 4 Calculated power spectra of silicon, magnesium, and oxygenatoms.

Fig. 3 Experimental (a) and calculated (b) infrared spectrum of ver-miculite in the framework region.

Phys. Chem. Chem. Phys., 2002, 4, 1957–1963 1961

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online

4 Conclusion

In this work the vibrational spectra and structure of vermicu-lite have been investigated by the use of Raman and infraredspectroscopy complemented by quantum chemical calculationsand molecular dynamics simulations. Parameters of thegeneralized valence force field for the six-fold coordinatedmagnesium atoms were derived by ab initio quantumchemical calculations and combined with the force field fortetracoordinated Si atoms in aluminosilicates.20,21 Theobtained force field was applied for MD simulations andyielded a structure and vibrational spectra in good agreementwith the experimental data. On the basis of the calculations,the bands in the experimental spectra are assigned in termsof the symmetry-adapted internal coordinates of the buildingunits of the vermiculite structure. The calculations indicatethat some of the bands observed in the vibrational spectra ofclays can be, in a good approximation, attributed to vibrationsof internal coordinates (or to combinations of such coordi-nates) of the structural subunits of the structure. This featuredifferentiates clay and zeolite structures. For the latter, themodelling studies show the absence of general correlationbetween bands in the vibrational spectra and the presence ofspecific structural elements in the lattice.35 This work confirmsthat theoretical methods are valuable tools to study the

dynamics of complex structures. The developed force field isof sufficient quality to be used in future studies of the structureand dynamics of intercalated species in vermiculite structures.

Acknowledgement

MA gratefully acknowledges a fellowship from the ‘‘CentreNational de la Recherche Scientifique ’’ and from the ‘‘RegionNord–Pas de Calais ’’. This work is part of the ‘‘Programme deRecherche Concerte: Sites et Sols Pollues ’’ supported by the‘‘Region Nord–Pas de Calais ’’ and the ‘‘Fonds Europeen deDeveloppement Economique des Regions ’’ (FEDER). Theauthors wish to thank Dr. J. Laureyns and C. Bodelot for theirhelp in recording the Raman and infrared spectra of vermicu-lite samples.

References

1 P. G. Rouxhet, N. Samudacheata, H. Jacobs and O. Anton, ClayMiner., 1977, 12, 171.

2 C. T. Johnston, S. F. Agnew and D. L. Bish, Clays Clay Miner.,1990, 38, 573.

3 R. L. Frost, P. M. Fredericks and J. R. Bartlett, Spectrochim.Acta, 1993, 49A, 667.

4 R. L. Frost, T. H. Tran and J. Kristof, Vib. Spectrosc., 1997, 13,175.

5 C. T. Johnston, G. Sposito and R. R. Birge, Clays Clay Miner.,1985, 33, 483.

6 A. Wiewiora, T. Wieckowski and A. Sokolowska, Arch. Mineral.,1979, 35, 5.

7 B. J. Teppen, K. Rasmussen, P. M. Bertsch, D. M. Miller andL. Schafer, J. Phys. Chem. B, 1997, 101, 1579.

8 D. Bougeard, K. S. Smirnov and E. Geidel, J. Phys. Chem. B,2000, 104, 9210.

9 G. Sposito, N. T. Skipper, R. Sutton, S. H. Park, A. K. Soper andJ. A. Greathouse, Proc. Natl. Acad. Sci. USA, 1999, 96, 3358.

10 F. R. C. Chang, N. T. Skipper and G. Sposito, Langmuir, 1997,13, 2074.

11 E. S. Boek, P. V. Coveney and N. T. Skipper, J. Am. Chem. Soc.,1995, 117, 12 608.

12 C. H. Bridgeman and N. T. Skipper, J. Phys.: Condens. Matter,1997, 9, 4081.

13 J. O. Titiloye and N. T. Skipper, Chem. Phys. Lett., 2000, 329, 23.14 S. Karaborni, B. Smit, W. Heidug, J. Urai and E. Van Oort,

Science, 1996, 271, 1102.15 S. A. Boyd, J.-F. Lee and M. Mortland, Nature (London), 1988,

333, 345.16 G. D. Williams, N. T. Skipper, M. V. Smalley, A. K. Soper and

S. M. King, Faraday Discuss. Chem. Soc., 1996, 104, 295.17 L. S. Kajita, Clays Clay Miner., 1997, 45, 609.18 R. Dhamodharan, J. D. Jeyaprakash, S. Samuel and M. K. Rajes-

wari, J. Appl. Polym. Sci., 2001, 82, 555.19 A. C. D. Newman, in Chemistry of Clay and Clay Minerals,

Mineralogical Society, London, 1987.20 V. A. Ermoshin, K. S. Smirnov and D. Bougeard, Chem. Phys.,

1996, 202, 53.21 V. A. Ermoshin, K. S. Smirnov and D. Bougeard, Chem. Phys.,

1996, 209, 41.22 V. A. Ermoshin, K. S. Smirnov and D. Bougeard, Surf. Sci., 1996,

368, 147.23 A. Wang, J. Freeman and K. E. Kuebler, in Raman Spectroscopic

Characterization of Phyllosilicates, 33rd Lunar and PlanetaryScience Conf., 11–15 March 2002, Houston, TX, Abstract 1374.

24 A. McL. Mathieson and G. F. Walker, Am. Mineral., 1954, 39,231.

25 M. Bartsch, P. Bornhauser, G. Calzaferri and R. Imhof, J. Phys.Chem., 1994, 98, 2817.

26 J. R. Hill and J. Sauer, J. Phys. Chem., 1995, 99, 9536.27 B. J. Teppen, D. M. Miller, S. Q. Newton and L. Schafer, J. Phys.

Chem., 1994, 98, 12 545.28 M. J. Frisch, M. J. Trucks, H. B. Schlegel, P. M. W. Gill, B. G.

Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Peters-son, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham,V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski,B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng,

Fig. 5 Calculated spectra of the stretching symmetry coordinates T[eqns. (2)–(5)] of SiO4 tetrahedra.

Fig. 6 Calculated spectra of the stretching symmetry coordinates S[eqns. (6)–(11)] of MgO6 octahedra.

1962 Phys. Chem. Chem. Phys., 2002, 4, 1957–1963

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online

P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle,R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees,J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez andJ. A. Pople, Gaussian 94, Revision E.2, Gaussian Inc., Pittsburgh,PA, 1995.

29 A. Allouche and J. Pourcin, Spectrochim. Acta, 1993, 49A, 571 ;A. Allouche, Redong, QCPE 628.

30 W. C. Swope, H. C. Anderson, P. H. Berens and K. R. Wilson,J. Chem. Phys., 1982, 76, 637.

31 P. H. Berens and K. R. Wilson, J. Chem. Phys., 1981, 74,4872.

32 K. S. Smirnov and D. Bougeard, J. Raman Spectrosc., 1993, 24,255.

33 P. Bornhauser and D. Bougeard, J. Raman Spectrosc., 2001, 32,279.

34 J. A. Creighton, H. W. Deckman and J. M. Newsam, J. Phys.Chem., 1994, 98, 248.

35 K. S. Smirnov and D. Bougeard, Catal. Today, 2001, 70, 243.36 R. M. Badger, J. Chem. Phys., 1934, 2, 128.37 C. S. Blackwell, J. Phys. Chem., 1979, 83, 3251.38 C. S. Blackwell, J. Phys. Chem., 1979, 83, 3257.39 G. S. Rosasco and J. J. Blaha, Appl. Spectrosc., 1980, 34, 140.

Phys. Chem. Chem. Phys., 2002, 4, 1957–1963 1963

Publ

ishe

d on

15

Apr

il 20

02. D

ownl

oade

d by

Lom

onos

ov M

osco

w S

tate

Uni

vers

ity o

n 18

/12/

2013

15:

20:1

6.

View Article Online