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

Y. Wakisaka, S. Hirata, T. Mizokawa, Y. Suzuki, Y. Miyazaki, T. Kajitani

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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.

Original languageEnglish
Article number235107
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number23
Publication statusPublished - 2008 Dec 1


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