Sulfur Vacancy-Driven Band Splitting and Phonon Anharmonicity Enhance the Thermoelectric Performance in n-Type CuFeS₂

Ternary chalcogenides of CuFeS2–x (x = 0.00–0.20) chalcopyrites were synthesized via vacuum melting reaction/uniaxial hot pressing, and their thermoelectrical properties were investigated at temperatures ranging from 315 to 605 K. The crystal structures and microstructures of all samples were examined using powder X-ray diffraction and scanning electron microscopy, respectively. X-ray photoelectron spectroscopy (XPS) was utilized to validate the oxidation states of Cu1+, Fe3+, and S2– in CuFeS2–x. As sulfur vacancy increased, the power factor, S2σ, increased from ∼0.18 mW/mK2 for CuFeS2 to ∼0.20 mW/mK2 for CuFeS1.8 at 605 K due to an increase in the carrier concentration, as evidenced by theoretical calculations using density functional theory (DFT). Additionally, the total thermal conductivity, κtotal, was significantly reduced from ∼2.26 to ∼0.83 W/mK at 605 K for the compositions of CuFeS2 and CuFeS1.8, respectively, owing to the enhanced phonon scattering from the strong acoustic phonon coupling, Umklapp process, and sulfur vacancy-driven low group velocity. Consequently, the sulfur-deficient CuFeS1.8 sample exhibited the highest thermoelectric figure of merit, zT, of 0.14 at 605 K with a notably high hardness of 158 Hv, proving that it is an efficient thermoelectric material for intermediate temperatures.