NaxCoO2 in the x→0 regime:: Coupling of structure and correlation effects -: art. no. 115110

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 a(g) subband if U >= U-c=2.5 eV. Thus, U smaller than U-c is required to model the metallic, nonmagnetic CoO2 compound reported by Tarascon and co-workers. The orbital-selective Mott transition of the a(g) state, which is essentially degenerate with the e(g)(') states, occurs because of the slightly wider bandwidth of the a(g) bands. The metal-insulator transition is found to be strongly coupled to the Co - O bond length, due to associated changes in the t(2g) bandwidth, but the largest effects occur only at a reduced oxygen height that lies below the equilibrium position.