Thermoelectric properties of olivine-type sulfides Tm2XS4 (Tm = Mn, Fe, X = Si, Ge)

Polycrystalline olivine-type sulfides, Mn2SiS4, Mn2GeS4, Fe2SiS4, and Fe2GeS4, are prepared by solid-state reactions and spark plasma sintering. Among these sulfides, only the electrical conductivities and Seebeck coefficients of Fe2SiS4 and Fe2GeS4 are measurable, owing to their narrower energy gaps compared with those of Mn2SiS4 and Mn2GeS4. Although Fe2SiS4 and Fe2GeS4 exhibit low electrical conductivities, their Seebeck coefficients and thermal conductivities are relatively high and low, respectively, demonstrating their potential as good thermoelectric materials. The highest zT value, 0.008, is obtained for Fe2GeS4 at 700 K, which can be enhanced by hole carrier doping and heavy elemental partial substitution.