Effect of annealing temperature on the phase transition, band gap and thermoelectric properties of Cu2SnSe3

The effect of annealing temperature on the phase transition of Cu2SnSe3 was investigated in order to study the thermoelectric (TE) properties of the various Cu2SnSe3 phases. The stoichiometric composition of Cu2SnSe3 was synthesized by melt solidification and annealing at various temperatures followed by water quenching. Rietveld refinement was used to calculate the amount of monoclinic and cubic phases for each sample. XRD analyses reveal that the samples annealed at 720 and 820 K have mostly a monoclinic phase along with a small amount of cubic phase. The Cu2SnSe3 annealed at 960 K was mostly cubic. TE properties of the cubic phase Cu2SnSe3 were studied for the first time, and it was found that it has much higher ZT (similar to 0.09) than the monoclinic phase at 600 K. Better TE performance of the cubic phase can be attributed to the smaller band gap (similar to 0.92 eV) compared to that of monoclinic Cu2SnSe3 (similar to 1.0 eV) at room temperature. First principles calculations further confirmed the conductive metallic nature of the cubic phase Cu2SnSe3. The power factor (S-2 sigma) of the cubic phase, 0.24 mW m(-1) K-2, was higher than that of the monoclinic phase, 0.096 mW m(-1) K-2, at 600 K, but the difference between the thermal conductivities of the two phases was very small. Small polymorphic modification with increasing annealing temperature results in compositionally similar but different crystallographic phases, which is one of the possible reasons for the very similar thermal conductivities of the two phases. The electrical conductivity of the cubic phase, which is larger than that of the monoclinic phase, and the similar thermal conductivities of the two phases lead to the higher ZT of the cubic Cu2SnSe3.