Dependence of the superconducting transition temperature and lattice parameter on hydrostatic pressure for Rb3C60

The equation of state (EOS) of polycrystalline Rb3C60 was accurately determined to 1 GPa at 295 K and 14 K using high-pressure neutron diffraction. In a separate experiment on Rb3C60, the variation of the superconducting transition temperature T-c with hydrostatic pressure was also measured. The resulting dependence of T-c on lattice parameter a is almost linear, with initial slope dT(c)/ldlna similar or equal to +520 K, but is less steep than the T-c(a)-dependence obtained at ambient pressure if Rb is substituted with smaller alkali metal cations. T-c(a) for Rb3C60 thus does not follow a universal curve in high pressure and substitution experiments, as previously claimed. Starting with the measured EOS at room temperature, we use a Debye model to estimate the Murnaghan EOS for both NaCl (used here as a pressure calibrant) and Rb(3)C(60)over the entire temperature region below 400 K. Copyright (C) 1996 Published by Elsevier Science Ltd