Soft X-ray Absorption and Photoemission Spectroscopy Study of Cobalt-Based Thermoelectric Oxides: Ca_(3)Co_(4)O_(9), Ca_(3)Co_(2)O_(6), and Bi_(2)Sr_(2)Co_(2)O_(y)

摘要

The electronic structures of Co-based potential thermoelectric (TE) oxides, including Ca_(3)Co_(4)O_(9) and Bi_(2)Sr_(2)Co_(2)O_(y) (y velence 8 + delta) single crystals and polycrystalline Ca_(3)Co_(2)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 (Co~(3+)) in all of these Co-based TE oxides with a small mixture of Co~(4+) ions in Bi_(2)Sr_(2)Co_(2)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.