Tailoring the thermoelectric and structural properties of Cu-Sn based thiospinel compounds [CuM1+xSn1-xS4 (M = Ti, V, Cr, Co)]

We report here a rich variation of the thermoelectric properties of a series of Cu-Sn based thiospinels of composition CuM1+xSn1-xS4 upon partial substitution at the Sn-site with a variety of transition elements (M = Ti, V, Cr, Co). The optimized synthesis and processing conditions we used enabled us to realize highly densified and homogeneous compounds. The V-series was found to exhibit the lowest performing thermoelectric properties, whereas Co and Ti substituted compounds showed moderate thermoelectric properties with a maximum figure of merit (zT(max)) of similar to 0.02 and similar to 0.07 at 673 K, respectively. In contrast, the Cr substituted compounds exhibited better thermoelectric performance with zT similar to 0.2 at 673 K for the composition CuCr1.2Sn0.8S4. Both p- and n-type compounds were obtained; specifically, the Co and Ti series were found to be n-type, and the Cr series was found to be p-type. Besides the suppressed thermal transport, the attractive thermoelectric properties with the Cr-series can be attributed to the simultaneous increase of the Seebeck coefficient and electrical conductivity with increasing temperature, thus resulting in an improved power factor. Experimental and DFT theoretical calculations also predict a considerable interaction between the carriers and magnetic moments, contributing to a higher effective mass, thus leading to higher thermopower and power factor for the Cr-series. Computed electronic density of states and band structures, in agreement with the experimental findings, envisaged n-type half-metallic character for the CuTi1+xSn1-xS4 paramagnetic compounds, n-type conducting behavior for the CuCo1+xSn1-xS4 compounds, p-type weak half-metallic character for the CuV1+xSn1-xS4 compounds, and p-type semiconducting behavior for the CuCr1+xSn1-xS4 ferromagnetic compounds. Computation of the electronic transport coefficients using the Boltzmann transport equation also suggests a better thermoelectric property, especially the thermopower, for the Cr-series when compared to its Ti/V/Co counterparts.