Pressure dependence of superconductivity in simple-cubicNa2CsC60

The pressure dependence of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ of simple-cubic ${\mathrm{Na}}_2$ ${\mathrm{CsC}}_{60}$ is studied by an ac susceptibility method with a Swenson-type piston-cylinder apparatus. The observed value of ${\mathit{dT}}_{\mathit{c}}$/dP is -12.5±0.2 K/GPa, significantly larger than the values of -7.8 and -9.7 K/GPa reported earlier for ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$, respectively. Using the compressibility available (that of ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$), we find that the evolution of ${\mathit{T}}_{\mathit{c}}$ with interfullerene spacing in ${\mathrm{Na}}_2$ ${\mathrm{CsC}}_{60}$ is significantly different from that established for merohedrally disordered face-centered-cubic superconductors ${\mathit{M}}_3$ ${\mathrm{C}}_{60}$ (M=alkali metal), but in excellent agreement with the ambient pressure data for the simple cubic superconductors ${\mathrm{Na}}_2$ ${\mathrm{RbC}}_{60}$ and ${\mathrm{Na}}_2$ ${\mathrm{CsC}}_{60}$. The present results provide an additional dimension to the existing features of fulleride superconductors, establishing the importance of the relative orientation of ${\mathrm{C}}_{60}^{3\mathrm{-}}$ ions in affecting the electronic and conducting properties of these materials.