NaₓCoO₂ in the x -> 0 Regime: Coupling of Structure and Correlation effects

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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 is no less than 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.