Bulk modulus of C-60 studied by single-crystal neutron diffraction

The compressibility of solid C-60 has been investigated by single-crystal n eutron diffraction at pressures up to 5 kbar and temperatures between 70 an d 300 K. Ar and He were used as pressure-transmitting mediums at temperatur es above and below 200 K, respectively. In this way, a large temperature re gion could be explored while avoiding intercalation of gaseous species. At room temperature, a very sharp fcc-to-sc transition was observed proving fu lly hydrostatic loading conditions. Measurements slightly above the glass t ransition showed that the reduction of the bulk modulus due to the pressure -induced reorienration of the C-60 molecules (as monitored by the diffracti on intensity) may be as large as 30%. Measurements below the glass transiti on showed that the bulk modulus of the pentagon phase is about 10% larger t han that of the hexagon phase. [S0163-1829(99)01415-0].