Electronic structure of Ca3Co4O9 studied by photoemission spectroscopy: Phase separation and charge localization

We report on an electronic structure study of a quasi-two-dimensional Co oxide Ca3Co4O9 with Ca2CoO3 rocksalt layers and CoO2 triangular lattice layers by means of x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), configuration-interaction calculation on a CoO6 cluster model, and unrestricted Hartree-Fock calculation on a multiband d-p model. The Co 2p XPS spectrum shows that the Co valence of the rocksalt layer is similar to that of the triangular lattice layer. The cluster-model analysis of the Co 2p XPS spectrum indicates that the Co sites of the rocksalt and triangular lattice layers commonly have charge-transfer energy Delta of similar to 1.0 eV, d-d Coulomb interaction U of similar to 6.5 eV, and transfer integral (pd sigma) of similar to-2.3 eV. The Co 3d t(2g) peak in the valence-band XPS spectrum remains sharp even above the spin-state transition temperature at similar to 380 K, indicating that the spin-state transition is different from the low-spin to intermediate-spin or high-spin transitions in perovskite LaCoO3. The line shape of the UPS spectrum near the Fermi level can be reproduced by the combination of unrestricted Hartree-Fock results for the charge-ordered insulating (COI) and paramagnetic metallic (PM) states. The analysis shows that the phase separation between the COI and PM states plays important roles in Ca3Co4O9.