First principles insights into the structural, thermoelectric and optical behaviour of Bi-based half Heusler alloys RhNbBi and IrNbBi

Abstract The urgent demand for sustainable energy conversion technologies has stimulated intensive research on thermoelectric materials capable of directly converting waste heat into electricity. In this work, the Bi-based half-Heusler alloys RhNbBi and IrNbBi are investigated through density functional theory to elucidate the interplay between their structural, electronic, mechanical, phonon, thermoelectric, thermodynamic and optical properties relevant to semiconductor and energy applications. Structural optimization and elastic analyses performed using WIEN2k and Quantum ESPRESSO confirm dynamically and mechanically stable non-magnetic metallic ground states with strong 4d/5d-6p hybridization and ductile behaviour satisfying the Born-Huang criteria. Phonon dispersions free from imaginary frequencies verify dynamical stability, while quasi-harmonic Debye modeling reveals excellent thermal robustness up to 1200 K with smoothly varying heat capacity and entropy. The optical spectra, derived from the complex dielectric function, exhibit large static dielectric constants ε₁(0) =70 for RhNbBi and 90 for IrNbBi, high refractive indices n(0) ≈ 7.8–9.0, and strong absorption extending to 12 eV, consistent with experimentally reported optical data for CoTiSb and YbPtBi analogues, thereby validating the present theoretical predictions and establishing their reliability for Bi-based half-Heusler systems. Transport coefficients, evaluated using BoltzTraP2, indicate positive Seebeck coefficients and p-type conduction, suggesting their potential for thermoelectric energy conversion at elevated temperatures. Overall, this study provides comprehensive first-principles insights into Bi-based 4d/5d half-Heusler frameworks, establishing RhNbBi and IrNbBi as promising thermally robust, optically active and mechanically stable candidates for next-generation thermoelectric and optoelectronic devices.