Exploring band gap modulation and thermoelectric excellence in X <sub>3</sub> NI <sub>3</sub> (X = Sr, Ba) halide perovskites
Łukasz Szeleszczuk, Katarzyna Mądra-Gackowska, Marcin Gackowski
Abstract
Abstract The growing demand for efficient waste heat recovery has intensified the search for high-performance thermoelectric (TE) materials with a figure of merit (ZT) above 2.5. In this study, first-principles calculations based on DFT combined with Boltzmann transport theory were employed to investigate the structural, electronic, thermal, and thermoelectric properties of synthesized Mg 3 NF 3 and newly predicted halide perovskites X 3 NI 3 (X = Sr, Ba). Structural stability was verified through geometric optimization, negative formation energies, elastic constants, and phonon dispersion analyses. The band gaps corresponding to Mg 3 NF 3 , Sr 3 NI 3 , and Ba 3 NI 3 were estimated to be 6.811, 1.279, and 0.807 eV (HSE06), respectively, which are insulating and semiconducting, respectively. In contrast, Sr 3 NI 3 and Ba 3 NI 3 show remarkably low thermal conductivities (0.43 and 0.45 Wm −1 K −1 , respectively) combined with large Seebeck coefficients (123.89 and 135.10 μV/K), yielding promising ZT values of 2.5 and 2.6 at 1000 K. The combination of mechanical robustness, thermal stability, and excellent thermoelectric performance positions Ba 3 NI 3 and Sr 3 NI 3 as promising candidates for high-temperature thermoelectric applications. In contrast, Mg 3 NF 3 , despite its modest ZT value, exhibits a high Debye temperature and melting point, indicating strong resistance to thermal degradation and suitability for high-temperature environments.