High pressure polymerization of the Li-intercalated fulleride Li3CsC60

The structural features of the superconducting Li3CsC60 fulleride are studied by synchrotron X-ray powder diffraction as a function of pressure and temperature. In contrast with the behavior of Li2CsC60, Li3CsC60 shows a phase transition below room temperature from fcc (space group Fm (3) over bar m) to an orientationally ordered primitive cubic phase (space group Pa (3) over bar), isostructural with the metastable phases of Na(2)A'C-60. Despite the slow cooling protocols adopted, the system shows no other phase transitions even after prolonged standing at 200 K. The pressure dependence of the structure of Li3CsC60 at ambient temperature was followed up to 8.12 GPa. A phase transition occurs at 0.72 GPa to a low symmetry fullerene bridged polymeric phase with short center-to-center interfullerene separations of similar to 9.38 Angstrom. This phase was found to be monoclinic (space group P2(1)/a), isostructural with the low-temperature polymer phase of Na2RbC60 and the high-pressure polymer phase of Na2CsC60. The pressure evolution of the monoclinic lattice constants reveals a highly incompressible, tightly packed, and strongly anisotropic polymeric structure.