Pseudobinary Approach to the Discovery and Design of Copper-Based Sulfides

Copper-based sulfide semiconductors with the structure based on a sphalerite-like network can provide environmentally sound solutions to thermoelectric and photovoltaic energy conversion at a low cost. In this study, we report the synthesis and characterization of a copper-based sulfide, Cu₃₀Ti₆Sb₂S₃₂, discovered within the pseudobinary Cu₃SbS₄-Cu₄TiS₄ system. This new compound crystallizes in a colusite-like cubic structure, which is characterized by “interstitial” Ti atoms forming tetrahedral-octahedral [TiS₄]Cu₆ complexes linearly arranged within the Cu-S corner-sharing matrix. The [TiS₄]Cu₆ unit arrangement distinguishes the crystal structures of Cu₂₆Ti₂Sb₆S₃₂ (isolated units, 0D linkage), Cu₃₀Ti₆Sb₂S₃₂ (chain of linked units, 1D), and Cu₄TiS₄ (plane of linked units, 2D). We emphasize the semiconducting nature and the low thermal conductivity. The electronic structure and vibrational properties are investigated by the combination of experiments and first-principles calculations. We demonstrate that a design approach based on the pseudobinary composition, here, with the generic formula (Cu₃SbS₄)_1-x(Cu₄TiS₄)_x, is a fruitful direction for the discovery of new copper-based sulfides with tunable transport properties.