First-principles study of the atomic and electronic structures of misfit-layered calcium cobaltite (Ca2CoO3)(CoO2)1.62 using rational approximants

The atomic and electronic structures of the misfit-layered thermoelectric oxide material Ca3Co4O9 are investigated using detailed first-principles computations performed within the framework of density functional theory (DFT) and its DFT+U extension to account for electron correlations. The structure of Ca3Co4O9, composed of two incommensurate subsystems—a distorted rocksalt-type Ca2CoO3 layer sandwiched between hexagonal CoO2 layers—is modeled by means of Fibonacci rational approximants with systematically increasing unit cells.We show that good agreement with photoemission and transport experiments can be obtained regarding the contribution of the two subsystems to states near the Fermi level, when electron correlations are taken into account with a Hubbard U. The size of the rational approximant plays a secondary role in the analysis; the relatively “small” structure of composition (Ca2CoO3)6(CoO2)10 represents a good model for investigating the atomic and electronic properties of Ca3Co4O9. Within the DFT+U formalism, the metallic conductivity of Ca3Co4O9 is shown to result from itinerant holes in the hexagonal CoO2 layers, in which the Co atoms are predicted to have a mixed valence of Co4+ with ∼30% concentration and Co3+ with ∼70% concentration, both in low-spin configurations. In most cases, the resulting electronic structures show very good agreement with available data from transport and magnetic measurements.