Sulfur Vacancy-Driven Band Splitting and Phonon Anharmonicity Enhance the Thermoelectric Performance in <i>n</i>-Type CuFeS<sub>2</sub>
Manojkumar Moorthy, Prakash Govindaraj, Rajasekar Parasuraman, Animesh Bhui, Sri Sai Samhitha Gadhavajhala, Bhuvanesh Srinivasan, Kathirvel Venugopal, Suresh Perumal
Abstract
Ternary chalcogenides of CuFeS 2– 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 Cu 1+, Fe 3+, and S 2– in CuFeS 2– x . As sulfur vacancy increased, the power factor, S 2 σ, increased from ∼0.18 mW/mK 2 for CuFeS 2 to ∼0.20 mW/mK 2 for CuFeS 1.8 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, κ total, was significantly reduced from ∼2.26 to ∼0.83 W/mK at 605 K for the compositions of CuFeS 2 and CuFeS 1.8, 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 CuFeS 1.8 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.