CoO 2 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. 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 coworkers. The orbital-selective Mott transition of the ag state, which is essentially degenerate with the e′g states, occurs because of the slightly wider bandwidth of the ag bands. The metal-insulator transition is found to be strongly coupled to the Co-O 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.