Role of crystal transformation on the enhanced thermoelectric performance in Mn-doped Cu₂SnS₃

Mohite-type Cu₂SnS₃ exhibits a large degree of manipulation in both electronic and phonon transport properties by transition metal doping. In this work, the effects of Mn doping on the transport properties of Cu₂SnS₃ were investigated. A crystal transition occurred upon doping from cation-ordered monoclinic structure to cation-disordered cubic and tetragonal structures. Such a crystal transition caused a significant increase of DOS effective mass of carriers, m^*, which should be caused by the improved degeneracy of the bands at the Fermi level due to the improved symmetry, and also by the participation (in the upper valence bands) of unfilled d-orbitals of Mn cations that share crystallographically equivalent sites with Cu cations. Meanwhile the lattice thermal conductivity (∼0.4 W m⁻¹K⁻¹ at ∼773 K) was suppressed effectively, which should also be linked to the disorder structure induced by Mn-doping. Benefitting from the increased carrier density and much improved m^*, a remarkable power factor of 0.92 mWm⁻¹K⁻² has been obtained at 723 K. The highest ZT ∼0.68 at 723 K is almost 15 times increased as compared to that of pristine Cu₂SnS₃.