Local structure and anomalous chemical potential shift in the AgBiSe2-2xTex Sx system

Abstract

AgBiSe2 is a potentially important thermoelectric compound in which a partial substitution of Se by S and Te can stabilize the cubic phase at room temperature. The resulting compound AgBiSe2-2xTexSx is found to show small thermal conductivity in the cubic phase with highest thermoelectric performance for x≥0.6. Here the effect of this substitution on the local displacements and the electronic structure of AgBiSe2-2xTexSx has been investigated by x-ray absorption fine structure and x-ray photoelectron spectroscopy in the wide substitution range of x=0.0 to 0.8. The Ag K-edge extended x-ray absorption fine structure measurements reveal substantial change in the local structure across the average structural phase transition from hexagonal to cubic at x∼0.5. The x-ray absorption near edge structure spectra show a gradual change in the local geometry with the substitution. We do not find any evidence of multiple valence states of either Ag or Bi in the x-ray photoelectron spectroscopy (XPS). The core-level XPS spectra show that the cubic phase of AgBiSe2-2xTexSx is characterized by significant relative S deficiency and a possible local order. The core-level binding energies change anomalously with the substitution reflecting the behavior of changing chemical potential. The valence band spectral weight near the Fermi level is found to correlate with the chemical potential and the room-temperature resistivity. The effect of substitution on the local structure and disorder has been discussed in the light of enhanced thermoelectric properties of AgBiSe2-2xTexSx.