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

Abstract

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 Co 4+ with ∼ 30% concentration and Co 3+ 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.

Oxygen Nonstoichiometry in (Ca2CoO3)0.62(CoO2): A Combined Experimental and Computational Study

“…We adopted a computational methodology similar to that successfully used before by Rebola et al: 19 Spin-polarized periodic DFT calculations were performed using the VASP package. 23 The plane wave basis set was limited to a 600 eV energy cutoff for valence electrons while the atomic cores were included via the projector augmented wave (PAW) method.…”

“…Previous attempts to model the electronic structure of CCO have led to results contradicting experimental evidence or have been shown to be sensitive toward the chosen unit cell. 19,20 Recently, we have presented a defect chemical model to describe the oxygen nonstoichiometry in CCO, which is based on the assumption that oxygen vacancies are formed only next to Co 2+ sites within the central Co−O layer in the rock salt subsystem. 21 In this paper, we report our results of a combined experimental and computational study of oxygen nonstoichiometry in CCO.…”

Manipulating Thermal Conductivity by Interfacial Modification of Misfit-Layered Cobaltites Ca3Co4O9

“…1. The simple model of Ca 3 Co 4 O 9 reported by Rébora et al, 8 in which the misfit ratio b RS /b CoO2 is approximated to 3/5, was used in this study. Focusing on the misfitlayered structure and its phonon thermal conductivity, the Sr 3 Co 4 O 9 and Ba 3 Co 4 O 9 models were simply constructed by complete substitution of Sr and Ba for the Ca site in the Ca 3 Co 4 O 9 model, although they are thermodynamically unstable and other phases appear at this composition, for example Sr 6 Co 5 O 15 .…”

Thermoelectric properties of the misfit cobaltate Ca3Co4O9

“…Rébola et al [29] have investigated the atomic and electronic structures employing density functional theory (DFT) and its extension (DFT+U). Both methods lead to good agreement of the structural parameters with experiment.…”

“…The b lattice parameters differ so that a rational approximation with a minimal lattice mismatch has to be chosen. Asahi et al [27] have used b 1 /b 2 = 3/2 which corresponds to a mismatch of 7%, whereas the 5/3 ratio employed by Rébola et al [29] reduces the mismatch to 3%. Contradicting experiment [6,7] to experimental data.…”