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)
The current study systematically investigates the optoelectronic, mechanical, transport, and thermodynamic properties of Pb-free halide-based double perovskites (Rb 2 LiIrX 6 , where X = Cl, Br) for energy conversion. Using density functional theory (DFT) within the fullpotential linearized augmented planewave (FPLAPW) framework implemented in the Wien2k code, structural optimizations were performed through the PBEsol generalized gradient approximation (GGA). Energy calculations yield values of −0.63 eV (Cl) and −0.41 eV (Br) for the cubic phase, confirming thermodynamic stability. Electronic band structures obtained with the modified BeckeJohnson (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, elevated static dielectric constants, low reflectivity, and pronounced photoconductivity, highlighting the material's promise for photovoltaic and photodetector applications. Moreover, low lattice thermal conductivity and thermoelectric figureofmerit (ZT) values approaching unity 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 circumstances.