First‐Principles Investigation of Structural, Elastic, Electronic, Optical, and Thermodynamic Properties of Lead‐Free Double Perovskites Rb ₂ AuInX ₆

Lead‐free double perovskites are promising candidates for environmentally benign optoelectronic technologies. In this work, the structural, mechanical, electronic, optical, and thermodynamic properties of cubic Rb ₂ AuInX ₆ (X = F, Cl, and Br) were investigated using first‐principles calculations within density functional theory (DFT) using the WIEN2k program package. All compounds adopt the Fmm double‐perovskite structure and satisfy the mechanical stability criteria for cubic crystals. Halide substitution strongly tunes their functional response: Rb ₂ AuInF ₆ exhibits the highest stiffness, widest bandgap, and strongest ultraviolet optical response. In contrast, Rb ₂ AuInCl ₆ and Rb ₂ AuInBr ₆ show reduced bandgaps and enhanced absorption and optical conductivity at lower photon energies. These characteristics suggest their prospective application in visible‐light photodetectors, optical sensors, and various light‐harvesting optoelectronic devices; particularly, Rb ₂ AuInF ₆ renders it a viable contender for UV photodetectors, optical filters, and insulating layers. The thermodynamic analysis further reveals increasing lattice softness and thermal expansion from F to Br, providing an additional route for tuning material behavior through halide chemistry. Overall, the results identify Rb ₂ AuInX ₆ double perovskites as a compositionally tunable lead‐free platform for ultraviolet and visible optoelectronic applications.