Determination of the local structure of CsBi_{4-: X}Pb_xTe₆ (x = 0, 0.5) by X-ray absorption spectroscopy

We have studied the local structure and valence electronic unoccupied states of thermoelectric CsBi₄Te₆ and superconducting CsBi_3.5Pb_0.5Te₆ (T_c ∼ 3 K) by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements. The Bi-L₃ edge EXAFS reveals wide Bi-Te distance distribution for both compounds indicating complex atomic arrangements in the studied system. The mean square relative displacements (MSRDs) of the Bi-Te bond distances appear largely increased in Pb substituted system due to larger overall local disorder, however, one of the Bi-Te bonds shows a reduced disorder. On the other hand, the Bi-L₃ edge XANES is hardly affected by Pb substitution while the Te-L₁ edge XANES reveals increased density of unoccupied Te 5p states. This suggests that the carriers introduced by the Pb substitution in CsBi_4-xPb_xTe₆ preferentially goes on Te sites. Similarly, the Cs-L₃ edge XANES also shows small changes due to Pb-substitution and reduced local disorder indicated by the reduced width of the Cs-L₃ edge white line. We have also shown that the X-ray photoemission spectroscopy (XPS) measurements on various electronic core levels are in a qualitative agreement with the XANES results. These findings are consistent with carrier doping and a reduced disorder in one direction to be likely factors to drive the thermoelectric CsBi₄Te₆ into a bulk superconductor by Pb-substitution in CsBi_4-xPb_xTe₆.