Sb Doping of Metallic CuCr2S4 as a Route to Highly Improved Thermoelectric Properties

We report for the first time the thermoelectric properties of CuCr2-xSbxS4 (0.22 <= x <= 0.5). Although CuCr2S4 has been reported to be a metallic compound, addition of Sb shifts the material toward the semiconductor side. This is confirmed by band structure calculations of CuCr2-xSbxS4 (x = 0, 0.25, 0.5) models. Increasing Sb content enhances the power factor. However, beyond x = 0.3, further Sb addition lowers the electrical conductivity and power factor. A very interesting point is the simultaneous increase of the Seebeck coefficient as well as the electrical conductivity with increasing temperature, which acts like a variable range hopping (VRH) compounds but possesses much better properties than those having VRH. Samples were annealed for 48 h prior to thermoelectric properties measurements to have a reliable dimensionless figure of merit (ZT). An attractive ZT of 0.43 is obtained at similar to 650 degrees C. The attractive thermoelectric properties we discovered by driving a metal compound into a semiconductor make this compound an interesting thermoelectric material especially because of the cheap constituent elements compared to those of typical state-of-the-art thermoelectric materials. Furthermore, this material is stable up to 650 degrees C at least, a relatively high temperature for sulfides. Additionally, we discovered a miscibility gap in this solid solution close to an Sb content of 0.15; although a detailed study dedicated entirely to this miscibility gap would be required, it will encourage the researchers to further explore this system.