A first-principles study on the structural, optoelectronic, thermoelectric, elastic and thermodynamic functionality of lead-free direct bandgap double perovskites Rb 2 LiIrX 6 (X=Cl/Br)

This study systematically investigates the optoelectronic, mechanical, transport, and thermodynamic properties of Pb-free halide-based double perovskites (Rb 2 LiIrX 6 , where X[Formula: see text]Cl, Br) for energy conversion. Using density functional theory (DFT) within the full-potential linearized augmented plane-wave (FP-LAPW) framework implemented in the Wien2k code, structural optimizations were performed through the PBEsol generalized gradient approximation (GGA). Energy calculations yield values of [Formula: see text] eV (Cl) and [Formula: see text] eV (Br) for the cubic phase, thereby confirming its thermodynamic stability. Electronic band structures obtained with the modified Becke–Johnson (mBJ) potential reveal direct bandgaps of 2.25 eV (Cl) and 1.69 eV (Br); these findings are further validated by HSE06 hybrid functional calculations, establishing the semiconducting nature of both compositions. Mechanical robustness is confirmed through elastic constants and phonon dispersion analyses, demonstrating resilience against structural deformations. Optical properties display high absorption coefficients, higher static dielectric constants and low reflectivity, highlighting the material’s potential for photovoltaic and photodetector applications. Moreover, low lattice thermal conductivity and thermoelectric figure-of-merit (ZT) values approaching 1 suggest additional potential for thermoelectric energy harvesting. The high Debye temperatures and negative Gibbs free energies over a wide temperature range indicate the thermal stability of these compounds under operational conditions.