Superior thermoelectric properties of ternary chalcogenides CsAg5Q3 (Q = Te, Se) predicted using first-principles calculations

Abstract

Tailoring novel thermoelectric materials (TEMs) with a high efficiency is challenging due to the difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose ternary chalcogenides CsAg₅Q₃ (Q = Te, Se) as promising TEMs based on first-principles calculations of their thermoelectric properties. Using lattice dynamics calculations within self-consistent phonon theory, we predict their ultralow lattice thermal conductivities below 0.27 W m⁻¹ K⁻¹, revealing the strong lattice anharmonicity and rattling vibrations of Ag atoms as the main origination. By using the mBJ exchange–correlation functional, we calculate the electronic structures with the direct band gaps in good agreement with experiments, and evaluate the charge carrier lifetime as a function of temperature within the deformation potential theory. Our calculations to solve Boltzmann transport equations demonstrate high thermopower factors of 2.5 mW m⁻¹ K⁻² upon p-type doping at 300 K, comparable to the conventional dichalcogenide thermoelectric GeTe. With these ultralow thermal conductivities and high thermopower factors, we determine a relatively high thermoelectric figure of merit ZT along the z-axis, finding the maximum value of ZT_z to be 2.5 at 700 K for CsAg₅Se₃ by optimizing the hole concentration. Our computational results highlight the great potentiality of CsAg₅Q₃ (Q = Te, Se) for high-performance thermoelectric devices operating at room temperature.