Electronic structure of single-crystalline thermoelectric Bi2-xPbxSr2Co2Oy (x=0,0.6) from photoemission and x-ray absorption

Abstract

The electronic structures of Bi2-xPbxSr2Co2Oy (x=0,0.6) thermoelectric single crystals have been investigated by employing photoemission spectroscopy (PES) and polarization-dependent soft x-ray absorption spectroscopy. Co ions are found to be in the Co3+-Co4+ mixed-valent states with the low-spin configuration. The valence-band PES study reveals that the Co 3d states are located at the top of the valence bands, corresponding to the occupied electron configuration of e(g)(pi 4)a(1g)(1+x) (x < 1). Both the topmost occupied states and the lowest unoccupied states have the primarily out-of-plane Co a(1g) character, suggesting that carriers have the Co a(1g) hole character. Angle resolved photoemission spectroscopy measurements show that the lowest binding-energy states are shifted toward E-F with increasing x in Bi2-xPbxSr2Co2Oy, indicating that the carrier states are shifted toward E-F. This study suggests that the thermoelectric property of Bi2-xPbxSr2Co2Oy is improved for x > 0 mainly by the reduced resistivity.