Pressure dependence of the superconducting transition temperature in nominal Rb0.5Cs2.5C60

We report measurements of the superconducting transition temperature in a compound with a nominal composition Cs2.5Rb0.5C60 under hydrostatic pressure of up to 15.5 kbars. We observe two transitions, at temperatures T* and Tc in this pelletized sample. We identify the lower temperature transition T* with the onset of intergranular shielding currents. The upper transition temperature Tc(P) falls on a universal Tc(P) curve, defined from similar data obtained previously on K3C60 and Rb3C60, after we shift the pressure axis for our data by -2.6 kbars. We identify Tc as a superconducting transition in Rb2CsC60 on the basis of the compressibility analysis and published values of the lattice parameters for alkali-doped C60, as well as Tc(P=0) and (dTc/dP)P=0.