Lead-free quaternary chalcogenide NaZrCuS3 for efficient thermoelectric energy conversion: insights from first-principles and Boltzmann transport

A comprehensive first-principles investigation is performed to evaluate the thermoelectric potential of the lead-free quaternary chalcogenide NaZrCuS 3 using density functional theory combined with semiclassical Boltzmann transport formalism. Structural and elastic analyses confirm that NaZrCuS 3 crystallizes in a stable orthorhombic phase with strong lattice rigidity and high thermal stability. Electronic structure calculations reveal a direct band-gap semiconductor with a moderate gap of ~ 0.92 eV, which is favorable for suppressing bipolar conduction at elevated temperatures. Orbital-resolved density of states indicates that hole transport is dominated by Cu–S hybridized states, while electron transport is mainly governed by Zr–S states, leading to pronounced anisotropic transport behavior. Thermoelectric transport analysis shows large Seebeck coefficients and high-power factors, particularly for n -type doping. Notably, within the constant relaxation time approximation and the semi-empirical Slack model for lattice thermal conductivity, a power factor of 17.17 \(× \) 10 −3 Wm −1 K −2 and a maximum predicted figure of merit of ZT ≈ 0.78 are obtained at 1000 K along the a -axis, indicating promising high-temperature thermoelectric performance within the present theoretical framework. Although the lattice thermal conductivity is relatively high at room temperature, it decreases significantly at elevated temperatures, enhancing overall efficiency. These results suggest NaZrCuS 3 as a promising environmentally benign thermoelectric candidate for high-temperature waste-heat recovery and provide a theoretical foundation for future experimental validation and performance optimization.