Polarization-Dependent Soft X-ray Absorption Spectroscopy Study of Layered Thermoelectric Cobalt Oxide: Bi2-xPbxSr2Co2O8+δ

Abstract

The electronic structures of Bi2xPbxSr2Co2O8+ (x = 0, 0.4, 0.6) thermoelectric single crystals have been investigated by using polarization-dependent soft X-ray absorption spectroscopy (XAS). The Co ions are found to be in the Co3+-Co4+ mixed-valence states with a low-spin conguration, and the lowest unoccupied states are found to have mainly an out-of-plane Co a1g character. With increasing x in Bi2xPbxSr2Co2O8+, all the peaks in the O 1s XAS spectra become broadened, suggesting that the improved thermoelectric property in Bi2xPbxSr2Co2O8+ for x > 0 is due to the enhanced delocalized character of Co a1g a1g hole carriers.