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/dlna ≃ +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 Rb₃C₆₀ over the entire temperature region below 400 K.