NaxCoO2 in the x\0 regime: Coupling of structure and correlation effects

The study of the strength of correlations in NaxCoO2 is extended to the x=0 end of the phase diagram, where Mott insulating behavior has been widely anticipated. The inclusion of correlation as modeled by the LDA +U approach leads to a Mott transition in the ag subband if U Uc=2.5 eV. Thus, U smaller than Uc is required to model the metallic, nonmagnetic CoO2 compound reported by Tarascon and co-workers. The orbital-selective Mott transition of the ag state, which is essentially degenerate with the eg states, occurs because of the slightly wider bandwidth of the ag bands. The metal-insulator transition is found to be strongly coupled to the CouO bond length, due to associated changes in the t2g bandwidth, but the largest effects occur only at a reduced oxygen height that lies below the equilibrium position.