Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of NaxCoO2 from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 + x leading to a nearly full Co t2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the eg bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from ag symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh2O4. Like NaxCoO2 it shows a large crystal field gap between narrow t2g and eg manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.

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