Pressure and Temperature Evolution of the Structure of Solid C 70

The pressure dependence of the structure of solid C70 at ambient temperature has been measured to 25 GPa by energy-dispersive X-ray diffraction. The face-centred cubic structure transforms irreversibly to a rhombohedral phase at 0.35 GPa. Crystalline C70 is stable to 18 GPa when a phase change to amorphous carbon occurs. C70 has a slightly smaller compressibility than C60. The temperature dependence of the structure at ambient pressure has also been investigated by neutron diffraction. A transition to a rhombohedral phase occurs near 280 K with the f.c.c. structure coexisting at least to 200 K. The crystallographic symmetry of the lowest-temperature phase appears to be lower than rhombohedral. On warming, the rhombohedral phase is stable to 350 K. Strong static and dynamic disorder dominates the crystal chemistry of solid C70.