Soft X-ray Absorption and Photoemission Spectroscopy Study of Cobalt-Based Thermoelectric Oxides: Ca₃Co₄O₉hbox{Ca}_3hbox{Co}_4hbox{O}_9, Ca₃Co₂O₆hbox{Ca}_3hbox{Co}_2hbox{O}_6, and Bi₂Sr₂Co₂O_yhbox{Bi}_{2}hbox{Sr}_{2}hbox{Co}_2hbox{O}_{y}

摘要

The electronic structures of Co-based potential thermoelectric (TE) oxides, including Ca₃Co₄O₉hbox{Ca}_3hbox{Co}_4hbox{O}_9 and Bi₂Sr₂Co₂O_yhbox{Bi}_{2}hbox{Sr}_{2}hbox{Co}_2hbox{O}_{y} (y = 8 + δ) single crystals and polycrystalline Ca₃Co₂O₆hbox{Ca}_3hbox{Co}_2hbox{O}_6, have been investigated by employing soft x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). Co 2p XAS measurements show that Co ions are nearly trivalent (Co3+hbox{Co}^{3+}) in all of these Co-based TE oxides with a small mixture of Co4+hbox{Co}^{4+} ions in Bi₂Sr₂Co₂O_yhbox{Bi}_{2}hbox{Sr}_{2}hbox{Co}_2hbox{O}_{y}. Valence-band PES and O 1s XAS measurements show that the occupied Co 3d states are located at the top of the valence bands and that the lowest unoccupied states have the primarily Co 3d character, respectively. These findings suggest the importance of the Co 3d electronic structures in determining TE properties of these Co-based oxides.