Compressibility ofC60in the temperature range 150–335 K up to a pressure of 1 GPa

We have measured the volume of nominally 99.9% pure ${\mathrm{C}}_{60}$ as a function of pressure (p) in both the fcc and sc phases up to 1 GPa over the temperature range 150 to 335 K using a highly accurate piston-and-cylinder technique. At zero pressure, the room-temperature bulk modulus B(0) is 6.8 GPa for the fcc phase and about 8.7–9.5 GPa for the sc phase. For the latter phase, B(0) increases strongly with decreasing temperature to 10.4 GPa at 152 K and for both phases B increases rapidly with increasing pressure. The fcc→sc transformation is always smeared over large ranges in p. Surprisingly, above 200 MPa B is lower in the sc phase than in the fcc phase, which we explain as arising from molecular reorientation with increasing pressure. The formation of a mainly ‘‘hexagon’’-oriented, ordered phase above 0.6 GPa at low T gives rise to observable anomalies in B(p). © 1996 The American Physical Society.