Mössbauer spectroscopy studies of molecular magnets (invited)William Michael Reiff Citation: Journal of Applied Physics 63, 2957 (1988); doi: 10.1063/1.340915 View online: http://dx.doi.org/10.1063/1.340915 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/63/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic and Mössbauer spectroscopy studies of nanocrystalline iron oxide aerogels J. Appl. Phys. 99, 08N711 (2006); 10.1063/1.2176894 Magnetic and Mössbauer studies of singlecrystal Fe16N2 and FeN martensite films epitaxially grown bymolecular beam epitaxy (invited) J. Appl. Phys. 76, 6637 (1994); 10.1063/1.358157 Magnetic and conversion electron Mössbauer spectroscopy studies in Fe/Ta multilayers J. Appl. Phys. 73, 6438 (1993); 10.1063/1.352625 A Mössbauer study of fine iron particles (invited) J. Appl. Phys. 63, 4100 (1988); 10.1063/1.340508 Study of magnetism in fine particles of ferric hydroxysulfate by Mössbauer spectroscopy J. Appl. Phys. 54, 307 (1983); 10.1063/1.331702

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MOssbauer spectroscopy studies of molecular magnets (invited) William Michael Reiff Department of Chemistry, Northeastern University, Boston, Massachusetts 02115

A variety of new organc-molecular magnets have been characterized in our and colleagues' laboratories in recent years using classic susceptibility measurements. In this effort, iron-57 Mossbauer spectroscopy has proven a particularly useful additional tool in elucidating charge transfer and interesting, unprecedented magnetic behavior for magnets based on electron donation from decamethylferrocene to polycyanide electron acceptors, e.g., tetracyanoethylene, tetracyano-quino-dimethane, hexacyanobutadiene. Examples of Mossbauer spectra of such systems are presented with emphasis on Ca) slow paramagnetic relaxation at low spin ferric sites as a probe for spin on the polycyanide electron acceptor units; (b) large orbital contributions to internal hyperfine fields concommitant with highly anisotropic magnetic behavior; and (c) onset of multiple hyperfine pattern spectra signifying complex magnetostructural transformations.

INTRODUCTiON

Hyperfine splitting as a result of slow relaxation is rare for low spin iron (III) . This is a consequence of the normally allowed nature of transitions among the components of the ground Kramer's doublet of typical spin doublet ferric spe­cies. Thus for instance with K 3 Fe(CN)6 (Ref. 1) both ap­plication of external magnetic fields and isomorphous dilu­tion in a diamagnetic host (increasing spin-spin relaxation time) in addition to low temperature (increasing spin lattice relaxation time) are a necessity to realize resolved Zeeman splitting effects. Nevertheless, we have discovered that the s = 1/2 decamethylferrocenium (DMFC) cation is an im­portant exception to the preceding observations. This spe­cies is formed in a relatively self-dilute form [shortest Fe(HI)-Fe(HI) distances ;C 8 AJ in a number of new poly­meric charge transfer salts resulting when the s = 0 per methylated ferrocene is reacted with any of a variety of neu­tral polycyanide electron acceptor species (AD) according to the reaction:

I II III

D+ D+ D+ A -- A 2- A 2 --

A - A- f)+ A- D-+- f)+ Some A 2--

D+ D+ D+ A -- f)+ D+ spin A -- A- D+ A- A 2-- A 2- possibilities: D+ lJr lJ + A D+ D+

D+ D+

In this work examples of nonoccurrence and occurrence of charge transfer, slow relaxation, and low-temperature or­dering (ferromagnetism and antiferromagnetism) for struc­ture type I are now discussed.

CHARGE TRANSFER: FERROCENE VERSUS DECAMETHYlFERROCENE WITH TCNE

For the systems considered herein, the ultimate obser­vation of extended, cooperative magnetic behavior crucially

DMFCO+ AD -+ DMFC+ + A-(s-~O) (s=o) (S'= 1/2) (s= 1/2)

where, e.g., AD can be any oftetracyano-ethylene (TeNE), tetracyano-quinodimethane (TCNQ), hexacyano-buta­diene [C4 (CN)6], among other more complex polycyan­ides. (See Fig. 1 for acronyms and corresponding structure schematics. )

We observe slow relaxation and magnetic hyperfine splitting in zero external field at unprecedentedly high tem­peratures ( - 50 K) for the following three structure arche­types and in some instances extended cooperative magnetic order: (I) paranel chains of alternating cations and anions; (II) paranel chains of cations and anions; and (III) parallel chains composed of two D + cations alternating with single A2 - anions as shown below. The observed hyperfine effects are believed in part to be the result of small Zeeman spliuings of D ~ cations arising from local dipolar fields and/or ex­change originating from spin on the An -- species.

s=o

{ 0' } s=1

s= l:s=O

depends on the formation of odd electron paramagnetic ca­tionic and anionic species as a result of charge transfer pro­cesses. The occurrence of complete charge transfer, a one electron oxidation of the diamagnet ferrocene [s = 0, for­many Fe (n) ) to ferrocenium [s = 112, low spin Fe (III)] is immediately obvious in ambient temperature Mossbauer spectra. Thus when unsubstituted ferrocene (Fig. 2) is react­ed with TeNE, a stacked material of structure type I re­sults.2 Early studies of this material suggested charge trans-

2957 J. AppL Phys_ 63 (8). 15 April 1988 0021 0 8979/88/082957 0 05$02.40 @ 1988 American Institute of Physics 2957

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~y'ms for Some PoIY.£y'onide Electron AcceQtors

NC~N

NC~CN

~N N

NC eN

eN eN

CI)yCN CIVeN

0-

reNO

rCNQF"4

reNE

C4(C/>I16

DDQ

FIG. 1. Acronyms and schematics of some important polycyanide electron acceptors.

fer on the basis of optical spectra and small isomer shift changes in Mossbauer spectra.3 However, the Mossbauer spectrum (Fig. 3) is clearly the characteristic quadrupole doublet of unperturbed ferrocene (lJ = 0.42 mm/s, AE = 2.40 mm/s; decamethylferrocene has nearly identical parameters). The temperature dependence (not shown) ex~ hibits linewidth broadening indicating some type of interest­ing order-disorder or other types of structural phase trans­formation but neither significant charge transfer nor ordering-slow paramagnetic relaxation phenomena. Thus, interesting cooperative magnetic phenomena cannot result from ferrocene's type I complex with TeNE. In contrast, permethylation strongly enhances the tendency for ferro­cene to undergo oxidation (decreases its ionization poten­tial) leading to the formation of the decamethylferrocenium cations on reaction with TeNE; this species exhibits an equally characteristic sharp singlet (0 = 0.43 mm/g, r = 0.32 mm/s) such as is observed for unsubstituted ferro~ cenium cations.

[DMFCJ+[DDar

The Mossbauer spectrum of this material of structure type I (Fig. 4) exhibits the slow relaxation mentioned in the

2958

H

FIG. 2. Schematic of mcy­clopentadienyl iron (ferro­cene).

J, Appl. Phys., Vol. 63, No.8, 15 April 1988

zO.oo 0 ~ Il..

~ 2.00 (/) m « ~ 4,00 W I.) 0:: W 293K a.

-2 -1 o 2 3

VELOCITY (mm/s} RELATIVE. TO !RON

FIG. 3. Room-temperature Mossbauer spectrum offerrocene reacted with TCNE, a type I structure system.

introduction. However, the material is nevertheless a para­magnet.4 There is no evidence of magnetic exchange in the same temperature interval (SQUID susceptibility data, X-I vs T, Ho = 30 G, Fig. 5). The Mossbauer spectra are typical of nuclear Zeeman splitting owing to slow relaxation as op­posed to an ordering process, i.e., the magnetic hyperfine splitting occurs gradually over an extended temperature range. In the case of 3D ordering, dHint I dTis typically large near Tcrit and attainment of near saturation value of the in~ ternal field usually occurs over a relatively small tempera­ture interval.

YElOCITYlmm/see) RELATIVE TO IRON

FIG. 4. Temperature dependence of the Messbauer spectrum of [DMFC] + [DDQ]-.

Wi!1lam Michael Reiff 2956

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FIG, 5. Inverse molar sus­ceptibility of [DMFC] + [DDQ]-.

THE DECAMETHYL FERROCENIUM CATION AS A SPIN PROBE

The unexpected observation (at Toften)-4.2 K) ofhy­perone splitting in the Mossbauer spectrum of the [DMFC} + cation (inHo = 0) apparently furnishes a pow­er probe for electron spin on the polycyanide electron accep­tor lattice. This is emphasized in our comparative studies of [DMFCJ + [Id - (A), [DMFCP [PF6 J- (B), [DMFCJ + [BF4 -] (C), and [DMFCl +- [C3 (eN),]-­(D). Salts A, B, and C contain diamagnetic (s = 0) high symmetry "inorganic" anions along with the s = 1/2 DMFC + cations. We find that neither A, B, nor C exhibits magnetic hyperone splitting effects for temperatures as low as 1.5 K save for when spectra are determined in externally applied fields. More to the point, salt (D) has structure type I and the diamagnetic even electron count polycyanide mon­oanion, C3 (CN)S-.5 Likewise, we see no resolved hyperfine effects at low temperatures. There is broadening (Fig. 6), which we speculate is perhaps the result of weak intercation interactions. In this context, we plan to extend our studies of [DMFq + [C3 (CN)s ] - to as low as 0.32 K using a heli­um-3 cryostat. However, it seems clear that strong, resolved Zeeman splitting effects in these materials are crucially relat­ed to the existence of spin on the poly cyanide anion species. It is precisely the Mossbauer spectroscopy of these effects that foreshadowed much of the interesting cooperative mag­netic behavior considered herein.

-8 4 8

VELOCITY (",m/ .. e) AELA1IVE TO IAON

FIG. 6. MOssbauer spectra of [DMFCJ -; [C,(CN),l- (s = 112,s = 0).

2959 J. Appl. Phys., Vol. 63, No.8, 15 April 1988

ORDERING OF WMFcl+[C.(CN)sJ-: A 3D-FERROMAGNET WITH

t D-INTAACHAIN FERROMAGNETISM

FIG. 7. Temperature dependence of the MOssbauer spectra of [DMFC] + [C. (CN)6 J -.

(DMFC]+[C4(CN)6r AND [DMFC+[TCNE]-

The change of the electron acceptor to hexacyanobuta­diene appears to lead to a 3D-ferromagnetically ordered type I material with appropriately characteristic susceptibility (field-dependent demagnetization effects), and magnetiza­tion (rapid non-Brillouin rise in (T vs H) (not shown), and relatively sudden onset of hyperfine splitting (Fig. 7, Tc = 7.S K).

While the potential for the one electron reduction of TCNE by DMFC is somewhat more negative, [DMFCJ-+ [TCNE] -- can be isolated and has a structure and bulk fer­romagnetic properties similar to those of the hexacyanobu­tadiene compound whose packing structure is shown in Fig. 8. 5 In particular hysteresis and spontaneous zero field mag­netization have been demonstrated for the TCNE analog.

HIGH~FIElO MOSSBAUER SPECTRA

Spin doublet iron(HI) is rare in nature simply because most naturally occurring ligands do not present large

.. [DMFC] [C4(CN)sJ N

(I

N<ll!tC, T ~N C, ...... c .... /c c..... C C

N"" I ...... c C ~N KI N

FIG. 8. Unit cell packing for [DMFC] + [C. (CN)" J -,

William Michael Reiff 2959

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enough ligand fields to induce ferric ion spin pairing. In ad­dition to this and the rarely observed slow relaxation of low spin iron (III) mentioned in the Introduction, cooperative ordering of such compounds is perhaps even more uncom­mon. In addition to those considered herein, one of the few examples that comes to mind is again K 3 Fe(CN)6' He-3,4-dilution Mossbauer spectroscopy study6 shows this to order as a neat (i.e., undiluted) material at 0.129 K with an inter­nal field (Hint of 19.4 T). This value is not far from that expected for an s = 1/2 Fermi contact value, i.e., 11 T /spin. In this light, the "high" ordering temperatures and observed values of Hint (~40 T) for the present molecular magnets are extraordinary with the latter, implying large orbital con­tributions to Hint. Large orbital contributions to magnetic moments generally accompany highly anisotropic single ion (paramagnetic state) and ordered state behavior. One can elegantly probe the latter by the determination of Mossbauer spectra in external fields. Thus for magnetically isotropic single ion ground term based systems [e.g., high-spin Fe(III)-6A J the moments of a ferromagnetically ordered material are relatively easily polarized to the direction of an applied field. On the other hand, anisotropic systems (E or T irreducible representation single ion ground states having orbital angular momentum) will resist such polarization, The Mossbauer spectra will reflect these extremes (even for polycrystalline samples) in terms of a response or lack thereojofthe intensity pattern of the Zeeman split spectrum to an applied field. Specifically, for the case of an isotropic ferromagnet in longitudinally applied fields (Ho II Ey) tran­sitions two and five of a typical six-line Zeeman pattern (i.e., AMI = 0 transitions) will undergo diminution as the do­mains are polarized parallel to Ey, This is the result of the sin2 e dependence of their intensity where e is the angle between Ey and Hint and thus the axis of magnetization for the ordered state. For the ferromagnet a-iron, the moments of a thin foil are fully polarized by Ho ;::::3.0 T (Ref. 7) at which field the intensity of the AMI = 0 transitions has largely vanished. In strong contrast, we find that the molecu­larferromagnet [DMFCJ + [TCNE] -- is highly anisotropic with little or no observable polarization-diminution of the aMI = 0 transitions to as high as Ho = 9 T. A spectrum at Ho = 7.5 T is shown in Fig. 9. This is consistent with the highly anisotropic nature of this magnet. In fact, one sees an unexpected and remarkable increase in amplitude ofthe in­neraMJ = ± 1 transitions relative totheHo = o spectrum. A remarkable additional feature of this spectrum that first becomes evident for 3 T <Ho <4.5 T is the appearance of doublets for the extreme negative and positive velocity AMI = + 1 transitions. Possible implications are that the ap­

plied field is somehow inducing metal ion inequivalence in this "jerromagnet," or more likely that the system is not a simple ferromagnet after all. The applied field spectra are similar to those for certain speromagnetic materials, e.g., the pyrochlore form of FeP 3.

8 Clearly, more study of this novel field-dependent behavior is needed.

[DMFCr [TCNQ]-

Both of the preceding TCNE and C4 (CN)6- systems exhibit dominant ID-ferromagnetic intrachain effects

2960 J. Appl. Phys., Vol. 63, No, 8, ; 5 April 1968

5_ooi ~

T=4.2K

H=O

I

VELOCITY (mm/sec) RELATIVE TO IRON

FIG. 9. Miissbauer spectrum of [DMFC] + [TCNE] ,- at 4.2 K, Ho = 0 and in a longitudinal applied field, Ho = 7.5 T.

(0;:::: + 30 K) along with 3D-ferromagnetic ground states. We conclude this article with brief consideration of [DMFCJ ,~ [TCNQ] - , likewise a 1D ferromagnet along the D + A - D + A - chains. However, this type I material has a three-dimensional antiferromagnetic ground state ( TN = 2.5 K) that exhibits a low-field (0.15 T) metamagnetic

transition to a "paramagnetic" state (Ref. 9 and references therein). In addition, an unusual magnetostructural trans­formation leading to inequivalent iron sites and accompany­ing the 3D-ordering process at 2.5 K is apparent in the Moss­bauer spectra (Fig. 10). The transformation is highly

~-----, ',--.. '"\ .. -""

V 4.301<

-8 -4 0 4

i20PK,i -8 ~4 i o I J 1 8

VELOCITY <mm I •• c} RELATIVE TO IRON

FIG. 10. Some Mossbauer spectra for [DMFC] r [TCNQ]- in the vicinity of TN (-2.5 K).

William Michael Reiff 2960

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reproducible, funy reversible, and results in an intensity ra­tio for the inequivalent Fe(IH) sites of 2:1 with internal fields of 39.0 and 43.5 T for the more and less intense hyper­fine patterns, respectively. It seems likely that the observed transformation is in some way the result of a soft-phonon mode along the D i A - chain direction. We speculate one possibility. This is the trimerization, ... D + A -- D + A -- D+ A -- ... -> ••• {D + A{D -+ }{A-D + A -} ... which results in 2: I Fe(IH) site inequivalence and distinctly different in­ternal fields at these sites. Further effort is clearly necessary to test this view. In any event, we close by stating that the rich variety of both magnetic and structural behavior appar­ent for this new series of molecular magnets based on deca­methylferrocene win provide us with interesting research challenges for the next severa! years.

ACKNOWLEDGMENTS

This research was supported by the U.S. National Science Foundation Division of Materials Research, Solid State Chemistry Program grant No. DMR 83137100 This

2961 J. Appl. Phys_, Vol. 63, No.8, 15 April 1988

research is part of an ongoing, fruitful collaboration with my colleagues Dr. Joel S. Miller, E.I. DuPont Central Research and Development, and Dr. Arthur J. Epstein, Ohio State University. The authoris also indebted to Dr. J.H. Zhang for many of the measurements discussed herein.

'w. T. Oosterhuis and G. Lang, Phys. Rev. 178.439 (1969). 2E. Adman, M. Rosenblum, S. Sullivan, and T. N. Margulis, J. Am. Chern. Soc. 89, 4540 (1967).

3R. L Co!lin~ and R. Pettit, 1. lnorg. Nue!. Chern. 29,503 (1967). 4J. S. Miller, P. J. Krusic, D. A. Dixon, W. M. Reiff, Z. H. Zhang, E. C. Anderson, and J. Epstein, J. Am. Chem. Soc. 108,4459 (1986).

5J. S. Miller, J. C. Calabrese, H. Rommelmann, S. R. Chittipeddi, J. H. Zhang. W. M. Reiff, and A. J. Epstein, J. Am. Chern. Soc. 100, 769 (1987).

6J. L. Groves, A. J. Becker, L. M. Chirovsky, W. P. Lee, G. W. Wang, and C. S. Wu, Hyperfine Interactions 4, 930 (1978).

7N. Blum, S. Foner, R. B. Frankel et al., Phys. Rev. 181, 863 (1969). By. Calage, M. Zernirli. 1. M. Greneche, F. Varret, R. De Pape, and G. Fercy, J. Solid State Chern. 69,197 (1987).

9J. S. Miller, J. H. Zhang, W. M. Reifi~ D. A. Dixon, L. D. Preston, A. H. Reis, Jr., E. Gebert, M. Extine, J. Troup, A. J. Epstein, and M. D. Ward, J. Phys. Chern. 91, 4344 (1987).

William Michael Reiff 2961

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