Changes in Local Structure during the Metal to Insulator Phase Transition of Cu₁₂Sb₄S₁₃

Tetrahedrites such as Cu₁₂Sb₄S₁₃ are prime candidates for thermoelectric devices as they can exhibit a high figure of merit (ZT), and further are naturally abundant. Currently, researchers look to dope these compounds with a transition metal (M) in search of a compound, Cu_12-xM_xSb₄S₁₃, with an increased electrical conductivity while remaining thermally glass-like. The pure tetrahedrite exhibits a metal to insulator transition at T~85K, and doping generally supresses this transition. In an effort to understand the physical mechanism that leads to the peculiar properties of this class of materials, we have studied the changes in local structure of Cu₁₂Sb₄S₁₃ as a function of temperature by analyzing the Extended X-ray Absorption Fine-Structure (EXAFS). Using EXAFS, we probe the Sb and Cu K-edge and find large discontinuities in bond-length and σ² of neighboring atomic shells at the transition temperature.