Electronic structure of single-crystalline thermoelectricBi2−xPbxSr2Co2Oy(x=0,0.6)from photoemission and x-ray absorption

Abstract

The electronic structures of ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$ $(x=0,0.6)$ 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 ${\mathrm{Co}}^{3+}\text{\ensuremath{-}}{\mathrm{Co}}^{4+}$ 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 ${e}_{g}^{\ensuremath{\pi}4}{a}_{1g}^{1+x}$ $(x<1)$. 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 ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$, indicating that the carrier states are shifted toward ${E}_{F}$. This study suggests that the thermoelectric property of ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$ is improved for $x>0$ mainly by the reduced resistivity.