High-Pressure Studies of the Rotor-Stator Compound C 60 -Cubane
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0987-PP03-04
High-Pressure Studies of the Rotor-Stator Compound C60-Cubane Bertil Sundqvist1, Agnieszka Iwasiewicz-Wabnig1, Eva Kovats2, and Sandor Pekker2 1 Department of Physics, Umea University, Umea, S-90187, Sweden 2 Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, Budapest, H-1525, Hungary
ABSTRACT Insertion of cubane (C8H8) into the octahedral voids of the C60 lattice leads to the formation of an interesting rotor-stator compound which can be converted into a C60 co-polymer by heating. We have treated a number of C60⋅C8H8 samples for up to 3 h each in the range 380875 K under pressures up to 2 GPa. The resulting materials were investigated by Raman spectroscopy and X-ray diffraction. Depending on treatment conditions, at least five different structural phases can be found. In addition to the four structural phases observed at atmospheric pressure and different temperatures we find that a new polymeric state is created at pressures above 1 GPa, and we tentatively identify its structure as pseudo-orthorhombic. The cubicorthorhombic phase transition line is found to have a slope of 295 K GPa-1, much larger than the slope of the fcc-sc line in pure C60.
INTRODUCTION Pekker et al. [1] recently showed that insertion of cubane, C8H8, into the octahedral voids of fullerene lattices leads to the formation of a family of novel, interesting rotor-stator compounds such as C60⋅C8H8. For geometrical reasons the cubane molecules are locked into static positions while the fullerenes rotate freely due to the very weak intermolecular interactions. The rotational motion persists down to 140 K [1,2]. On heating to above 470 K the cubane molecules react with the C60, resulting in the formation of a disordered fullerene copolymer [1,3]. However, although X-ray diffraction (XRD) shows a very strong disorder, the outward appearance of C60⋅C8H8 crystals does not change [1]. On further heating to above 680 K the cubane breaks down, releasing hydrocarbons [1], and a final product rich in carbon and probably consisting of an amorphous network based on cross-linked fullerene cages is formed. However, again little change is observed in the outward appearance of crystals. We have now investigated the structural stability of C60⋅C8H8 under pressures up to 2 GPa at temperatures between 400 and 875 K in order to find the pressure-temperature phase diagram. As will be shown below, we have identified a new, probably pseudo-orthorhombic polymer phase created above about 1 GPa, and we have found a very high value for the slope of the cubic-orthorhombic phase line. In contrast to pure C60 the polymer structures are determined by the structures of the original lattices rather than by the structure of the molecules.
EXPERIMENTAL DETAILS Small C60⋅C8H8 crystals with a 1:1 stoichiometry were obtained by slow evaporation of a solution of C60 and cubane in toluene. Samples of 3-10 mg each were treated at various high temperatures from 400 to 875 K, at pressures from zero (vacuum) to 2 GPa, then stud
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