Pressure dependence of superconductivity in the ${\mathrm{Na}}_2{\mathrm{Rb}}_{0.5}{\mathrm{Cs}}_{0.5}{\mathrm{C}}_{60}$ fulleride

The pressure dependence of the superconducting transition temperature $T_c$ of the primitive cubic quaternary fulleride ${\mathrm{Na}}_2{\mathrm{Rb}}_{0.5}{\mathrm{Cs}}_{0.5}{\mathrm{C}}_{60}$ ($T_c=8.4\mathrm{}\mathrm{K}$ at ambient pressure) is studied by the ac susceptibility technique to 2.5 kbar. The observed value of ${\mathrm{dT}}_c/dP$ is -1.22(2) K/kbar. Using the compressibility data available for ${\mathrm{Na}}_2{\mathrm{CsC}}_{60},$ we find that the evolution of $T_c$ with interfullerene spacing in ${\mathrm{Na}}_2{\mathrm{Rb}}_{0.5}{\mathrm{Cs}}_{0.5}{\mathrm{C}}_{60}$ mimics that observed for ${\mathrm{Na}}_2{\mathrm{CsC}}_{60}$ and is comparable to the fcc fulleride ${\mathrm{K}}_3{\mathrm{C}}_{60}.$ However, the rate of change, ${\mathrm{dT}}_c/da,$ is about 5–6 times smaller than that reported for the quaternary fullerides ${\mathrm{Na}}_2{\mathrm{Rb}}_{1-x}{\mathrm{Cs}}_x{\mathrm{C}}_{60}$ as a function of x at ambient pressure, implying the presence of a strong alkali-metal specific effect, responsible for the differing effects of chemical and physical pressure on the superconducting properties of $\mathrm{Pa}3\mathrm{}¯$ fullerides.