Pressure effects in the triangular layered cobaltites Nax Co O2

We have measured transport properties as a function of temperature and pressure up to 30 GPa in the Nax Co O2 system. For the x=0.5 sample, the transition temperature at 53 K increases with pressure, while paradoxically the sample passes from an insulating state to a metallic ground state. Compression on the x=0.75 sample transforms the sample from a metallic state to an insulating state. We discuss our results in terms of interactions between band structure effects and Na+ order.