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Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al [1], we have attempted to synthesise Li deficient Li x BC ( x = 1, 0.8, 0.6 and 0.4) and look for superconductivity in this system. Our synthesis procedure, following the recipe for MgB 2 , involves reaction of elemental components in a Ta crucible at 900o C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P6 3 /mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, was observed in the temperature range of 300 to 4 K. This is possibly related to the presence of disorder in the B-C stacking; evidence for which is suggested from a study of the vibrational modes of Li x BC through infrared spectroscopy.

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Solid CT0 has been subjected simultaneously to high pressures and temperatures (HPHT), with pressures upto 7.5 GPa and temperatures upto 750°C. X-ray diffraction measurements on the recovered samples indicate that the initial h.c.p. solid C70 transforms to a rhombohedral structure which recovers to an f.c.c. structure on annealing. The associated changes in the intra molecular vibrational modes have been probed through infrared (IR) and Raman measurements. The 1R measurements on these HPHT samples show splitting of some of the pristine modes and occurrence of several new modes. These sharp IR modes in the HPHT treated samples, which are seen to be different from that reported for photopolymerised C70, have been attributed to the formation of C70 dimers. 0 1997 Published by

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To investigate the polymerization of fullerenes, solid C 60 and C 70 have been subjected simultaneously to high pressures and temperatures, with pressures up to 7.5 GPa and temperatures up to 800°C. X-raydiffraction measurements indicate that the fcc solid C 60 transforms to an orthorhombic structure consisting of a polymerized linear chain of C 60 molecules. The associated changes in the intramolecular vibrational modes have been probed through Raman and infrared measurements. In contrast to solid C 60 , under similar conditions, polymerization is not observed in the case of solid C 70 , although there is a structural transition from the initial hcp structure to a rhombohedral structure. This lack of polymerization in C 70 is discussed in terms of the structure of solid C 70 under high pressure and temperature conditions, coupled with the topochemical features of the C 70 molecule.