First-Principles calculations to investigate structural, mechanical, electronic, optical and thermoelectric properties of RuNbAs and RuNbSb Half-Heusler alloys

This study presents a first-principles investigation of the structural, thermodynamic, mechanical, electronic, optical and thermoelectric properties of RuNbZ (Z = As, Sb) half-Heusler (HH) alloys using density functional theory (DFT). Structural optimization, negative formation energies and phonon dispersion calculations with no imaginary frequencies confirm the thermodynamic and dynamical stability of both compounds. The calculated elastic constants satisfy the Born–Huang mechanical stability criteria, indicating that RuNbAs and RuNbSb are mechanically stable and exhibit ductile behavior. Electronic-structure calculations using the generalized gradient approximation (GGA), spin–orbit coupling (SOC) and GGA+U reveal that both compounds are indirect band-gap semiconductors. The calculated GGA band gaps are 0.3427 eV and 0.3588 eV for RuNbAs and RuNbSb, respectively, while inclusion of SOC reduces the band gap of RuNbAs and RuNbSb to 0.3412 eV and 0.3484 eV, which increase to 1.0873 eV and 1.0467 eV, respectively, upon inclusion of the Hubbard U correction. Optical calculations indicate strong absorption and high optical conductivity accompanied by relatively low energy-loss behaviors in the low-energy region, suggesting their potential for optoelectronic applications. The electronic transport properties were evaluated using DFT combined with semi-classical Boltzmann transport theory as implemented in BoltzTraP2, while the lattice thermal conductivity was calculated using phono3py. The calculated thermoelectric figure of merit reaches maximum values of 1.11 for RuNbAs and 1.65 for RuNbSb at 1200 K. These results demonstrate that RuNbAs and RuNbSb possess favorable structural, electronic, optical and thermoelectric properties and are promising candidates for high-temperature thermoelectric and optoelectronic applications.