Phase transition and high temperature thermoelectric properties of copper selenide Cu_2-xSe (0 ≤ x ≤ 0.25)

With good electrical properties and an inherently complex crystal structure, Cu_2 - xSe is a potential ``phonon glass electron crystal'' thermoelectric material that has previously not attracted much interest. In this study, Cu_2 - xSe (0 \le x \le 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2 - xSe were investigated in the temperature range of 300~K--750~K. The results of X-ray diffraction at room temperature show that Cu_2 - xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x \le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 \le x \le 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu_2Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.