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

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.

High thermoelectric performance in p-type ZnSb upon increasing Zn vacancies: an experimental and theoretical study

“…To reveal the reason behind the significant reduction in the κ latt , the Debye–Callaway model was adopted to fit the κ latt vs. T data and the various scattering contributions were examined using the relationship: 80–85 where θ D , ν , τ T , and z are the Debye temperature, the mean sound speed, the total phonon relaxation time, and the reduced phonon frequency, respectively. From the Matthiessen rule, one can express the τ as, 83,86 τ −1 = τ PD −1 + τ U −1 + τ N −1 + τ B −1 + τ D −1 where τ PD −1 , τ U −1 , τ N −1 , τ B −1 and τ D −1 are scattering relaxation times of point defects, Umklapp-process, nano-inclusions, boundary, and dislocations respectively.…”