Abstract
The atomic and electronic structures of the misfit-layered thermoelectric oxide material Ca${}_{3}$Co${}_{4}$O${}_{9}$ 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 Ca${}_{3}$Co${}_{4}$O${}_{9}$, composed of two incommensurate subsystems---a distorted rocksalt-type Ca${}_{2}$CoO${}_{3}$ layer sandwiched between hexagonal CoO${}_{2}$ 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 (Ca${}_{2}$CoO${}_{3}$)${}_{6}$(CoO${}_{2}$)${}_{10}$ represents a good model for investigating the atomic and electronic properties of Ca${}_{3}$Co${}_{4}$O${}_{9}$. Within the DFT+U formalism, the metallic conductivity of Ca${}_{3}$Co${}_{4}$O${}_{9}$ is shown to result from itinerant holes in the hexagonal CoO${}_{2}$ layers, in which the Co atoms are predicted to have a mixed valence of Co${}^{4+}$ with $\ensuremath{\sim}30%$ concentration and Co${}^{3+}$ with $\ensuremath{\sim}70%$ 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.
Highlights
Transition metal oxides have been the focus of many experimental, theoretical, and computational studies, as they exhibit a wide range of functional properties including colossal magneto-resistance, two-dimensional electron gas,ferroism, and superconductivity, to name a few
We have reported results on and analyses of first principles calculations, performed within the framework of standard density functional theory (DFT) and DFT+U, for misfit-layered CCO modeled by rational approximants with systematically increasing unit cell sizes
When electron correlations are taken into account within a DFT+U formalism, d states derived from Co atoms in the RS subsystem are observed to have very little, if any, contribution to density of states (DOS) at Ef, and the states that give rise to the metallic conductivity of CCO are essentially all derived from Co atoms in the hexagonal CoO2 subsystem, in agreement with results from photoemission data
Summary
Transition metal oxides have been the focus of many experimental, theoretical, and computational studies, as they exhibit a wide range of functional properties including colossal magneto-resistance, two-dimensional electron gas, (multi)-ferroism, and superconductivity, to name a few. CCO stands out as the only layered cobalt oxide system containing one cation with nominally different oxidation states in both subsystems, namely Co2+ in the RS layers and Co4+ in the hexagonal CoO2 layers. This makes CCO an ideal system for studying effects such as charge transfer or orbital ordering both experimentally and theoretically
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