Preparation and Thermoelectric Performance of Non-Stoichiometric Skinnerite

Non-stoichiometric skinnerite Cu_3+mSbS₃ (-0.04 ≤ m ≤ 0.04) was synthesized via mechanical alloying and hot pressing. A phase analysis, microstructural characterization, and thermoelectric property evaluation were conducted to investigate the effects of Cu deficiency and excess. An X-ray diffraction of the mechanically alloyed powders confirmed the formation of cubic skinnerite, while the hot-pressed Cu-rich samples contained a secondary phase, identified as cubic tetrahedrite (Cu₁₂Sb₄S₁₃). The lattice constant decreased within the range of 1.0341-1.0347 nm for the non-stoichiometric compositions. The microstructural analysis revealed a skinnerite matrix with tetrahedrite inclusions in the Cu-excess samples. The differential scanning calorimetry showed a single endothermic peak at 876 K for the stoichiometric skinnerite, corresponding to its melting point, whereas the non-stoichiometric samples exhibited additional phase transitions at 814-818 K, and a melting reaction at 873-874 K. The electrical conductivity increased with the temperature, indicating non-degenerate semiconductor behavior. Between 323 and 423 K, the electrical conductivity varied depending on the Cu deficiency or excess, but above 423 K, all the non-stoichiometric samples exhibited higher electrical conductivity than the stoichiometric skinnerite. A positive Seebeck coefficient confirmed p-type conduction in all the samples, while Cu deficiency led to a decrease in the Seebeck coefficient but enhanced the power factor, due to increased electrical conductivity. The Cu_2.98SbS₃ sample exhibited the highest power factor of 0.85 mWm^-1K^-2 at 623 K. Although Cu deficiency resulted in increased thermal conductivity due to a higher carrier concentration, the significant enhancement in the power factor led to a maximum dimensionless figure of merit (ZT) of 0.59 at 623 K for Cu_2.98SbS₃, surpassing the ZT of 0.51 for the stoichiometric Cu₃SbS₃.