Synthesis, crystal structure, and transport properties of quaternary tetrahedral chalcogenides

Abstract

Quaternary chalcogenides with tetrahedral zinc-blend structure types continue to be of interest for thermoelectrics applications. We report on the synthesis, crystal structure, and high temperature transport properties of Cu_2.1Fe_0.9SnSe₄, Cu_2.2Fe_0.8SnSe₄ and Cu_2.2Zn_0.2Fe_0.6SnSe₄. The identity and compositions for each specimen were established using a combination of Rietveld refinement and elemental analysis and indicate that all compositions are homogeneous with the stannite crystal structure. Excess Cu reduces the electrical resistivity, ρ, by an order of magnitude compared with Cu₂FeSnSe₄ with no significant degradation of the Seebeck coefficient, S. The energy band gaps were estimated from the high temperature S values and indicate that Cu_2.1Fe_0.9SnSe₄ and Cu_2.2Fe_0.8SnSe₄ possess narrow band gaps, 0.18 eV and 0.25 eV, respectively, as compared to most other quaternary chalcogenides. The power factor (PF = S²/ρ) increases with decreasing Fe content. Although Cu_2.2Fe_0.8SnSe₄ possesses a smaller PF than that of Cu_2.2Zn_0.2Fe_0.6SnSe₄, a ZT of 0.45 was obtained at 750 K for Cu_2.2Fe_0.8SnSe₄ due to its low thermal conductivity.