Effect of He pressure on the superconducting transition temperatures of Na2CsC60 and (NH3)4Na2CsC60

The Na based mixed alkali doped C60 superconductors show anomalous behavior with respect to the ''universal'' superconducting transition temperature Tc vs lattice constant a0 relation followed by most of the fcc A3C60 superconductors. We have measured dtc/dP for Na2CsC60 and (NH3)4Na2CsC60 using solid He as the pressure medium to ∼ 6 kbar finding dTc/dP equal to -0.8±(0.01) K/kbar and -1.0(±0.1)K/kbar for Na2CsC60 and Na2(NH3)4C60 respectively. Our value for Na2CsC60 differs markedly from that obtained by Mizuki et al of about -1.3 K/kbar. However, using N2 or Ar, we obtain values for dTc/dP in substantial agreement with Mizuki et al who used fluorinert to generate their pressure. This work emphasizes the need for compressibility measurements with the same pressure medium in the appropriate temperature range so that meaningful comparisons can be made between various pressure measurements and models which are based on lattic spacing