Electronic structure of Ca3 Co4 O9 studied by photoemission spectroscopy: Phase separation and charge localization

We report on an electronic structure study of a quasi-two-dimensional Co oxide Ca3 Co4 O9 with Ca2 CoO3 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 Co2p 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 Co2p XPS spectrum indicates that the Co sites of the rocksalt and triangular lattice layers commonly have charge-transfer energy Δ of ∼1.0 eV, d-d Coulomb interaction U of ∼6.5 eV, and transfer integral (pdσ) of ∼-2.3 eV. The Co3d t2g peak in the valence-band XPS spectrum remains sharp even above the spin-state transition temperature at ∼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 Ca3 Co4 O9.