Compressibility of M3C60 fullerene superconductors - Relation between Tc and lattice parameter

X-ray diffraction and diamond anvil techniques were used to measure the isothermal compressibility of K3C60 and Rb3C60, the superconducting, binary alkali-metal intercalation compounds of solid buckminsterfullerene. These results, combined with the pressure dependence of the superconducting onset temperature Tc measured by other groups, establish a universal first-order relation between Tc and the lattice parameter a over a broad range, between 13.9 and 14.5 angstroms. A small second-order intercalate-specific effect was observed that appears to rule out the participation of intercalate-fullerene optic modes in the pairing interaction.