Effect of indium substitution on the thermoelectric properties of orthorhombic Cu₄SnS₄

We report the thermoelectric properties of Cu₄In _x Sn_1-x S₄ (x = 0-0.02), which undergoes a first-order structural phase transition at ∼230 K. Substitution of In³⁺ for Sn⁴⁺ suppresses the phase transition temperature (T _t). Indium substitution reduces the electrical resistivity, and degenerate conduction by the orthorhombic phase is observed. The Seebeck coefficient increases over the whole temperature range and a maximum value occurs in the monoclinic phase as a result of indium substitution. Thermal conductivity decreases as x increases, which enhances the dimensionless figure of merit, ZT. We therefore expect optimization of the chemical composition of indium-doped Cu₄SnS₄ to result in an even larger ZT value.