Exploring lead free Rb2AlInX6 halide double perovskites for advanced energy harvesting applications.

Halide double perovskites have recently attracted attention as stable and environmentally benign alternatives to lead based perovskites for optoelectronic and energy applications. However, detailed insights into their stability, electronic structure, and multifunctional properties remain limited. In this study, the physical properties of Rb2AlInX6 (X = Cl, Br) were systematically examined by first-principles calculations. The structural stability of both compounds was confirmed through formation enthalpy, tolerance factor (τ G), octahedral factor (µ), and octahedral misfit (Δµ), all of which fall within the accepted stability ranges. Both Rb2AlInCl6 and Rb2AlInBr6 crystallize in the cubic Fm3m phase with optimized lattice constants of 20.37 and 21.43 bohr, respectively. Electronic structure analysis identifies both Rb2AlInCl6 and Rb2AlInBr6 as semiconducting with calculated bandgaps of 2.85 eV and 1.90 eV, respectively, underscoring their potential for optoelectronic applications. Mechanical stability, verified via Born criteria, was further supported by elastic tensor analysis, demonstrating isotropic and robust mechanical behavior. The Rb2AlInBr6 exhibits moderate absorption extending into the visible region, while the Rb2AlInCl6 primarily absorbs in the near UV. These features suggest potential for optoelectronic or UV photodetection applications. While thermoelectric analysis shows notable power factor values at 800 K, pointing toward possible thermoelectric applications. These findings provide a comprehensive understanding of Rb2AlInX6 halides, offering valuable insights into their multifunctional prospects in next generation optoelectronic and thermoelectric devices.