Ab Initio Investigation of Lead-Free A 2 AlAgX 6 (A = Cs, Rb; X = F, Cl, Br) Double Perovskites for Photocatalytic and Optoelectronic Applications

The growing demand for sustainable energy technologies has accelerated the search for highefficiency, low-cost, and environmentally benign photovoltaic materials. In this work, a comprehensive first-principles density functional theory (DFT) investigation is performed on leadfree double halide perovskites A 2 AlAgX 6 (A = Cs, Rb; X = F, Cl, Br) to evaluate their structural, mechanical, electronic, optical, thermodynamic, and photocatalytic properties. All six compounds are predicted to be thermodynamically stable, crystallizing in a cubic structure, as evidenced by negative formation energies and favorable Goldschmidt tolerance factors. Phonon dispersion calculations confirm the dynamic stability of most compositions at 0 K, while imaginary modes in Rb-based chloride and bromide variants suggest possible stabilization at elevated temperatures. Mechanical analysis reveals that all compounds satisfy the Born stability criteria and exhibit ductile behavior with moderate stiffness, indicating mechanical robustness for optoelectronic applications. Fluoride-based A 2 AlAgF 6 compounds show superior thermal stability, higher melting temperatures, and large Debye temperatures, whereas chloride and bromide variants demonstrate strong visible-to-ultraviolet light absorption, making them promising solar absorber materials. Electronic structure calculations identify all compounds as direct band gap semiconductors, with HSE06 band gaps ranging from 2.32 to 5.57 eV, significantly improving upon GGA-PBE underestimations. Optical absorption onsets closely follow the electronic band gaps, enabling applications spanning photovoltaics to deep-UV photodetectors. Furthermore, favorable band edge alignments indicate strong potential for photocatalytic water splitting. Overall, this study provides a theoretical roadmap for designing next-generation lead-free perovskite-based optoelectronic devices.