Thermoelectric Performance of Non-Stoichiometric Permingeatite Cu3+mSbSe4

Non-stoichiometric permingeatites Cu_3+mSbSe₄ (-0.04 ≤ m ≤ -0.02) were synthesized, and their thermoelectric properties were examined depending on the Cu deficiency. Phase analysis by X-ray diffraction revealed no detection of secondary phases. Due to Cu deficiency, the lattice parameters of tetragonal permingeatite decreased compared to the stoichiometric permingeatite, resulting in a = 0.5654-0.5654 nm and c = 1.1253-1.1254 nm, with a decrease in the c/a ratio in the range of 1.9901-1.9903. Electrical conductivity exhibited typical semiconductor behavior of increasing conductivity with temperature, and above 423 K, the electrical conductivity of all samples exceeded that of stoichiometric permingeatite; Cu_2.96SbSe₄ exhibited a maximum of 9.8 × 10³ Sm^-1 at 623 K. The Seebeck coefficient decreased due to Cu deficiency, showing p-type semiconductor behavior similar to stoichiometric permingeatite, with majority carriers being holes. Thermal conductivity showed negative temperature dependence, and both electronic and lattice thermal conductivities increased due to Cu deficiency. Despite the decrease in the Seebeck coefficient due to Cu deficiency, the electrical conductivity increased, resulting in an increase in the power factor (especially a great increase at high temperatures), with Cu_2.97SbSe₄ exhibiting the highest value of 0.72 mWm^-1K^-2 at 573 K. As the carrier concentration increased due to Cu deficiency, the thermal conductivity increased, but the increase in power factor was significant, with Cu_2.98SbSe₄ recording a maximum dimensionless figure-of-merit of 0.50 at 523 K. This value was approximately 28% higher than that (0.39) of stoichiometric Cu₃SbSe₄.