Phase transitions of thermoelectric FexCu12−xSb4Se13

Cu–Sb–Se ternary chalcogenide compounds have garnered interest as potential thermoelectric materials because of their low cost and environmental benefits. Among these, Cu₁₂Sb₄Se₁₃ (hakite), Cu₃SbSe₄ (permingeatite), Cu₃SbSe₃ (bytizite), and CuSbSe₂ (pribramite) stand out as promising candidates because of their narrow band gaps and low thermal conductivity. However, Cu₁₂Sb₄Se₁₃ is unstable in its pure form because of a charge imbalance in its structure, where all copper atoms exist in the monovalent state (Cu⁺). This instability restricts its potential for thermoelectric applications. In this study, we sought to stabilize Cu₁₂Sb₄Se₁₃ by partially substituting Fe²⁺ ions for Cu⁺ ions, aiming to compensate for the charge imbalance and synthesize Fe_xCu_12−xSb₄Se₁₃ (0.5 ≤ x ≤ 2). X-ray diffraction analysis revealed that adding Fe led to the formation of bytizite and CuFeSe₂ (eskebornite), with the pribramite phase emerging as the Fe content increased. Thermal analysis using differential scanning calorimetry revealed that for samples with Fe content between x = 0.5 and 1.5, bytizite and permingeatite phases coexisted. In the Fe₂Cu₁₀Sb₄Se₁₃ sample, three phases were identified: bytizite, permingeatite, and pribramite. This suggests that synthesizing hakite through charge compensation with Fe was not feasible, as Fe substitution resulted in multiple phase formations rather than stabilizing the hakite phase. In terms of thermoelectric properties, the FeCu₁₁Sb₄Se₁₃ sample exhibited a power factor of 80 μWm⁻¹K⁻² and a maximum figure of merit (ZT) of 0.14 at 623 K.