Optical conductivity of layered calcium cobaltate Ca₃Co₄O₉

We report the optical properties of layered calcium cobaltate, Ca₃Co₄O₉, which is regarded as a promising candidate for use as a thermoelectric material. The optical conductivity shows three broad peaks related to the inter-band transition below 4 eV, which are quite similar to those in the spectra of Na_xCoO₂. This similarity implies that the CoO₂ layer, which is an essential unit for both Ca₃Co₄O₉ and Na_xCoO₂, is dominant in the energy band structure below 4 eV. In addition, we estimate the effective carrier number per Co site and find similarity between the CoO₂ layers of Ca₃Co₄O₉ and Na_0.75CoO₂, which is consistent with the similarity in their Seebeck coefficients. To discuss the contribution of the rocksalt-type Ca₂CoO₃ layer in Ca₃Co₄O₉, we propose the concept of optical sheet conductivity in the layered materials and estimate its value in the Ca₂CoO₃ layer. A comparison with the spin-polarized band calculation of the LDA  +  Hubbard U formalism with U  =  5 eV suggests that the Ca₂CoO₃ layer has the inter-band transition of 2.6 eV in the spin-down band structure. Evaluation of the valences of Co 3d orbitals indicates the existence of charge transfer from the Ca₂CoO₃ layer to the CoO₂ layer and mixing of Co³⁺ and Co⁴⁺ in the CoO₂ layer, which may be the origin of the large thermoelectric effect. (paper)