Experimental and computational phase boundary mapping of Co₄Sn₆Te₆

Binary Co₄Sb₁₂ skutterudite (also known as CoSb₃) has been extensively studied; however, its mixed-anion counterparts remain largely unexplored in terms of their phase stability and thermoelectric properties. In the search for complex anionic analogs of the binary skutterudite, we begin by investigating the Co₄Sb₁₂-Co₄Sn₆Te₆ pseudo-binary phase diagram. We observe no quaternary skutterudite phases and as such, focus our investigations on the ternary Co₄Sn₆Te₆via experimental phase boundary mapping, transport measurements, and first-principles calculations. Phase boundary mapping using traditional bulk syntheses reveals that the Co₄Sn₆Te₆ exhibits electronic properties ranging from a degenerate p-type behavior to an intrinsic behavior. Under Sn-rich conditions, Hall measurements indicate degenerate p-type carrier concentrations and high hole mobility. The acceptor defect Sn_Te, and donor defects Te_Sn and Co_i are the predominant defects and rationally correspond to regions of high Sn, Te, and Co, respectively. Consideration of the defect energetics indicates that p-type extrinsic doping is plausible; however, Sn_Te is likely a killer defect that limits n-type dopability. We find that the hole carrier concentration in Co₄Sn₆Te₆ can be further optimized by extrinsic p-type doping under Sn-rich growth conditions.