Pressure dependence of the superconducting transition temperature in nominalRb0.5Cs2.5C60

We report measurements of the superconducting transition temperature in a compound with a nominal composition ${\mathrm{Cs}}_{2.5}$ ${\mathrm{Rb}}_{0.5}$ ${\mathrm{C}}_{60}$ under hydrostatic pressure of up to 15.5 kbars. We observe two transitions, at temperatures ${\mathit{T}}^{\mathrm{*}}$ and ${\mathit{T}}_{\mathit{c}}$ in this pelletized sample. We identify the lower temperature transition ${\mathit{T}}^{\mathrm{*}}$ with the onset of intergranular shielding currents. The upper transition temperature ${\mathit{T}}_{\mathit{c}}$(P) falls on a universal ${\mathit{T}}_{\mathit{c}}$(P) curve, defined from similar data obtained previously on ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, after we shift the pressure axis for our data by -2.6 kbars. We identify ${\mathit{T}}_{\mathit{c}}$ as a superconducting transition in ${\mathrm{Rb}}_2$ ${\mathrm{CsC}}_{60}$ on the basis of the compressibility analysis and published values of the lattice parameters for alkali-doped ${\mathrm{C}}_{60}$, as well as ${\mathit{T}}_{\mathit{c}}$(P=0) and (${\mathit{dT}}_{\mathit{c}}$/dP${)}_{\mathit{P}=0\mathrm{}}$.