Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of Na_xCoO₂ 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 t_2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e_g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a_g 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 Na_xCoO₂ it shows a large crystal field gap between narrow t_2g and e_g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.