Manipulating the phase transformation temperature to achieve cubic Cu5FeS4−xSex and enhanced thermoelectric performance

Abstract

Cu₅FeS₄, one of the promising Earth-abundant and low-toxicity Cu-based thermoelectric materials, transforms from an orthorhombic structure to a cubic phase when heated, which may impede its applications at elevated temperatures. Herein, we demonstrate that the phase transformation temperature of Cu₅FeS₄ can be suppressed to lower temperatures by replacing S by Se, as demonstrated in a series of Cu₅FeS_4−xSe_x (x = 0, 0.4, 0.8, 1.5, 2.0) compounds. Upon increasing the Se content x from 0 to 2.0, the phase transformation of Cu₅FeS_4−xSe_x takes place at progressively lower temperatures, with Cu₅FeS₂Se₂ having a high cubic structure at room temperature. The Se-substituted Cu₅FeS₄ compounds achieve tremendously improved electrical performance, which is consistent with the calculated higher transport coefficient and weighted mobility. Combined with the still low lattice thermal conductivity, the Se-substituted Cu₅FeS₄ compounds achieve improved zT, for example, an improved peak zT of ∼0.6 at 735 K in Cu₅FeS_3.6Se_0.4, and an average zT of ∼0.42 from 300 to 735 K in Cu₅FeS_2.5Se_1.5 that is about 130% higher than that of Cu₅FeS₄. This study sheds light on enhancement of the thermoelectric performance of Cu₅FeS₄ based materials via manipulation of their phase transformation temperature.