Soft X-ray Absorption and Photoemission Spectroscopy Study of Cobalt-Based Thermoelectric Oxides: Ca3Co4O9, Ca3Co2O6, and Bi2Sr2Co2Oy

Abstract

The electronic structures of Co-based potential thermoelectric (TE) oxides, including Ca3Co4O9 and Bi(2)Sr(2)Co(2)Oy (y = 8 + delta) single crystals and polycrystalline Ca3Co2O6, 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+) in all of these Co-based TE oxides with a small mixture of Co4+ ions in Bi2Sr2Co2Oy. 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.