Pressure dependence of superconductivity in the Na2Rb0.5Cs0.5C60 fulleride

The pressure dependence of the superconducting transition temperature T-c o f the primitive cubic quaternary fulleride Na2Rb0.5Cs0.5C60 (T-c = 8.4 K at ambient pressure) is studied by the ac susceptibility technique to 2.5 kba r. The observed value of dT(c/)dP is - 1.22(2) K/kbar. Using the compressib ility data available for Na2CsC6.0, we find that the evolution of T-c with interfullerene spacing in Na2Rb0.5Cs0.5C60 mimics that observed for Na2CsC6 0 and is comparable to the fcc fulleride K3C60. However, the rate of change , dT(c/)da, is about 5-6 times smaller than that reported for the quaternar y fullerides Na2Rb1-xCsxC60 as a function of x at ambient pressure, implyin g the presence of a strong alkali-metal specific effect, responsible for th e differing effects of chemical and physical pressure on the superconductin g properties of Pa (3) over bar fullerides.