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J. Phys.: Condens. Matter 14 (2002) 1­4

PII: S0953-8984(02)37959-1

C60 fullerene and its molecular complexes under axial and shear deformation
N G Spitsina1 ,MV Motyakin2 ,I V Bashkin3 and K P Meletov
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Institute of Problems of Chemical Physics RAS, 142432 Chernogolovka, Russia Institute of Chemical Physics RAS, Kosygin 4, 117977 Moscow, Russia Institute of Solid State Physics RAS, 142432 Chernogolovka, Russia

Received 11 June 2002 Published Online at stacks.iop.org/JPhysCM/14/1 Abstract We have studied the pristine C60 and its molecular complexes with the organic donors bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF or ET) and tetramethyltetraselenafulvalene (TMTSF) by means of ESR and Raman spectroscopy at high pressure. The important changes in the ESR signal of C60 were observed under axial pressure combined with shear deformation. It is shown that the treatment at a anisotropic pressure of 4 GPa results in a reduction in the symmetry of the C60 molecule and the formation of radicals. Treatment of the molecular complex of (ET)2 ·C60 at a pressure of 4.5 GPa and a temperature of 150 Cleads to the formation of C60 dimers. The Raman spectra of the molecular complex C60 ·TMTSF·2(CS2 )were measured in situ at ambient temperature and pressures up to 9.5 GPa. The pressure behaviour of the Raman peaks reveals singularity at 5.0 ± 0.5 GPa related to the softening and splitting of some of the phonon modes. The residual softening of the Ag (2) mode is the same as in the case of KC60 , which may be an indication that the transition has a charge-transfer character, resulting in the formation of the C-1 60 anionic state charge-transfer complex.

1. Introduction The investigation of the physical and chemical properties of fullerene C60 and its derivatives at ambient and high pressure has been the main focus of research since its discovery [1­5]. The quasi-spherical structure of the C60 molecule, sp2 -like hybridization of molecular bonds and weak van der Waals intermolecular interaction result, in particular, in high stability towards isotropic deformation at high pressure. The phase transitions that occur under high pressure have contributed drastically to the interest in studying their origin and mechanism [2]. The important property of C60 is that its derivatives of the formula MC60 , where M is K, Rb,Cs, Ca etc, are conductors and reveal superconductivity at temperatures up to 39 K [3]. To date such organic compounds have been studied by varying their parameters using `chemical doping' and applying high pressure. Recently, researchers from the Bell Lab (Batlogg, Shon,
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Figure 1. The intensity of the ESR signal as a function of the shear deformation application time (at axial stresses of 2.0 and 2.5 GPa).

and Kloc) have demonstrated spectacular results by creating new class of low-dimensional carrier systems with ultra-high mobility, in metal-oxide semiconductor(MOS)-system on the surface of organic molecular crystals (pentacene, tetracene, C60 ,etc). Due to the low dielectric permittivity and high effective masses of carriers in these systems, much higher r s values can be achieved than in the Si­MOS system. Moreover, with the application of the gate voltage to the MOS structures on C60 , a superconductivity was observed with an electrically tunable critical temperature up to 52 K (recently increased to 117 K in a C60 intercalated CHBr3 compound) [6, 7]. An alternative way of designing conducting fullerene-based materials is to form anionic C60 complexes with organic donors. The synthesis of chemical compounds of C60 and the study of their structure, electronic and optical properties, as well as the influence of pressure on the donor­acceptor interaction in complexes is an important area of research. 2. Experimental procedures and results The ESR spectra of C60 , measured at room temperature and an axial pressure up to 4 GPa combined with shear deformation, are shown in figure 1. Shear deformation at a mean rate of 1.2 â 10-3 rad s-1 wasfound to result in a dramatic increase in ESR absorption. The integral intensity increases proportionally to the application time of the axial stress, whereas the width of the ESR signal decreases with an increase in the application time of the shear deformation. High pressure modifies both the crystal and electronic structures of C60 and results in the appearance of uncoupled electrons related to broken chemical bonds. This process is accompanied by relative change in the number of -and -bonds, the appearance of local and linear defects, excitation of electrons in the conductivity band and lowering of the symmetry of C60 molecule. The ESR absorption may be assigned to the formed radicals. The formation of new chemical bonds is the final stage of the process and the `non-interacting' fullerene cages at low pressure become `interacting' at high pressures. To support this idea, we treated various molecular complexes of C60 at high pressure and studied the transformed materials by means of in situ Raman and IR spectroscopy. We synthesized molecular complexes of C60 with twodifferent TTF-type organic donors, which differ in the crystal structure and intermolecular interaction between the molecules [8, 9]. In the complex of (ET)2 ·C60 (ET, tetrathiafulvalene) the molecules of ET and C60 are packed in two types of layers: those formed by ET and those form by C60 molecules [8]. Both types of layers are parallel to the (bc) plane of the crystal and uniformly alternate along a axis. X-ray analysis revealed that each C60 molecule is sandwiched between a pair of largely


C60 fullerene and its molecular complexes under axial and shear deformation
C60-TMTSF 9.5 GPa

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Figure 2. Raman spectra of C60 ·TMTSF·2(CS2 ) at room temperature and various pressures. The value marked by ` ' is recorded upon the pressure release.

concave ET molecules and that the rotational motion of C60 is quenched completely. The distances between the centres of the C60 molecules in the (ET)2 ·C60 are 10.03 å (290 K) and 9.923 å (150 K) (the distance between the centres of C60 molecules in the fcc crystal structure is 10.02 å). There is one shortened contact between the C60 molecules in the (ET)2 ·C60 structure: the C ··· Cdistance is 3.30(1) å (the sum of the van der Waals radii of carbon atoms is 3.40 å). Thus the pressure-induced cross-linking of C60 molecules in (ET)2 ·C60 is promising because normally the charge-transfer interaction between the components of this complex is small. In the C60 ·TMTSF·2(CS2 ) (TMTSF, tetramethyltetraselenafulvalene) the molecules of TMTSF, C60 and CS2 are assembled in homomolecular layers of TMTSF molecules and heteromolecular layers formed by C60 and CS2 molecules [9]. Both types of layers are parallel to the (ab ) crystal plane and uniformly alternate along the c-axis. Four fullerene molecules, forming a quasi-square configuration, surround a C60 molecule in a heteromolecular layer and eight CS2 molecules, forming a quasi-square prism, are inscribed in this `fullerene square'. The distance between the centres of C60 molecules is 10.06 å. No shortened contacts are found between the molecules of the C60 ·TMTSF·2(CS2 ) arranged in either homomolecular or heteromolecular layers. Shortenedcontacts occur between the C60 and TMTSF molecules belonging to adjacent layers. In this case, each TMTSF molecule is linked to two C60 molecules from adjacent layers. As a result, the stack of layers is cross-linked by zigzag chains along the c-axis. High-pressure experiments were performed in a `toroid'-type cell and a wedge-type cubic anvil quase-hydrostatic pressure apparatus. Polycrystalline ET2 C60 samples loaded in a highpressure assemblies were treated at 5 GPa and 150 C. After treatment the samples were recovered to ambient conditions and characterized by IR absorption spectroscopy. In situ Raman measurements at high pressure were performed using a Mao­Bell-type gasketed diamond anvil cell. The 4:1 methanol­ethanol mixture was used as a pressuretransmitting medium and the ruby fluorescence technique was used for the pressure calibration. The IR-absorption spectra of pristine C60 , ET and ET2 C60 complex before and after squeezing were measured in the KBr pellet at ambient temperature. The peaks related to ET were not changed by pressurization, while those related to C60 exhibit notable changes: the


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intensities of four main peaks decrease relatively to features in the 550­850 cm-1 region. The IR spectrum of the samples, obtained by the removal of ET after treatment with dicloretane solvent, is similar to that of dimerized C60 [10]. Thus, the dimers or oligomers of C60 can be formed by the pressure treatment. TMTSF donor and C60 acceptor molecules of the Raman spectra of the C60 ·TMTSF·2(CS2 ) complex at ambient temperature and various pressures are illustrated in figure 2. The pressure dependence of almost all modes exhibits a positive response, except for twomodes that have a negative response. The distinct change in the pressure dependence takes place at 5 GPa and this effect is irreversible. This peculiarity is associated with pressure-induced softening of almost all phonon modes, related to the charge transfer between the complex. During the phase transition, a transfer of one electron from TMTSF to the C60 molecule takes place forming a new anionic state C-1 charge-transfer complex. The transformation is 60 irreversible and the residual softening of the Ag (2) mode observed upon the total release of the pressure is in complete analogy to the case of KC60 . 3. Conclusion Changes in the ESR signal of the fullerene C60 induced by pressure and shear deformation were observed. It was shown that an anisotropic pressure of up to 4 GPa through shear deformation transforms reduce the symmetry of C60 molecules, including radical formations. The C60 dimer was synthesized by the high-pressure treatment of the molecular complex of (ET)2 ·C60 . The in situ Raman spectra of the molecular complex C60 ·TMTSF·2(CS2 ) were measured as a function of pressure up to 9.5 GPa at ambient temperature. We indicate that a pressure-induced phase transition has a charge-transfer character, resulting in the formation of the C-1 anionic state charge-transfer complex. 60 Using a high-pressure treatment of different structures of molecular complexes of C60 with organic donors, it is possible to design fullerene-containing compounds with low-symmetry (e.g. polymers and fullerides with complete charge transfer). Acknowledgments This work was supported, in part, by the Russian Foundation for Fundamental Research, grant nos 99-02-17555 and 99-02-17299, and the Russian Research and Development Programme `Fullerenes and Atomic Clusters', grant no 98079. References
[1] Kroto H W, Heath J R, O'Brien S C, Curl R F and Smalley R E 1985 Nature 318 162 [2] Sundqvist B 1999 Adv. Phys. 48 1 [3] Williams J M, Ferraro J R, Thorn R J, Carlson K D, Geiser U, Wang H H, Kini A M and Whangbo M-H 1992 Organic Superconductors (Including Fullerenes). Synthesis, Structure, Properties, and Theory (Englewood Cliffs, NJ: Prentice-Hall) [4] Rao A M et al 1993 Science 259 955 [5] Winter J and Kuzmany H 1992 Solid State Commun. 84 935 [6] Schon J H, Kloc Ch and Batlogg B 2001 Nature 410 189 [7] Schon J H, Kloc Ch, Hwang H Y and Batlogg B 2001 Science 292 252 [8] Konovalikhin S V, D'yachenko O A and Shilov G V 1997 Russ. J. Phys. Chem. 12 2192 [9] Konovalikhin S V, D'yachenko O A, Shilov G V, Spitsina N G and Yagubskii E B 1997 Izv. Akad. Nauk Ser. Khim. 8 1480 [10] Iwasa Y et al 1998 J. Chem. Soc., Chem. Commun. 1411