Halide-Dependent Optical Properties of Rb 2 AuInX 6 (X = F, Cl) Double Perovskites: Insights from DFT and Machine Learning

Halide double perovskites (HDPs) A 2 BB′X 6 are promising lead-free materials for optoelectronic and thermoelectric applications. Here, the structural, electronic, optical, photovoltaic, and thermoelectric properties of cubic Rb 2 AuInF 6 and Rb 2 AuInCl 6 are investigated using density functional theory (DFT) and machine learning (ML). Optimized lattice constants are 9.13 and 10.71 Å, with direct TB-mBJ band gaps of 1.98 and 1.293 eV, respectively. The valence-band edges are dominated by Au-5d and halide-p states, while optical absorption reaches ~10 5 cm -1 . Boltzmann transport calculations predict maximum ZT values of 0.56 and 0.74 at 1200 K, with SLME efficiencies of 23.3% and 33.0%, respectively. An ensemble ML model trained on 33 literature ZT values predicts 0.75 for Rb 2 AuInCl 6 , closely matching the BoltzTraP value (0.74; 1.4% deviation). These results identify Rb 2 AuInCl 6 as a promising lead-free multifunctional material.