Optical conductivity of layered calcium cobaltate Ca3Co4O9

摘要: We report the optical properties of layered calcium cobaltate, Ca3Co4O9, which is regarded as a promising candidate for use thermoelectric material. The conductivity shows three broad peaks related to inter-band transition below 4 eV, are quite similar those in spectra Na x CoO2. This similarity implies that CoO2 layer, an essential unit both Ca3Co4O9 and CoO2, dominant energy band structure 4 eV. In addition, we estimate effective carrier number per Co site find between layers Na0.75CoO2, consistent with their Seebeck coefficients. To discuss contribution rocksalt-type Ca2CoO3 layer propose concept sheet materials its value layer. A comparison spin-polarized calculation LDA  +  Hubbard U formalism U  =  5 eV suggests has 2.6 eV spin-down structure. Evaluation valences 3d orbitals indicates existence charge transfer from mixing Co(3+) Co(4+) may be origin large effect.