Unveiling the Potential of Fluorine-Based Double Perovskites A ₂ InGaF ₆ (A=Cs, Na) for UV-Visible Optoelectronics via First-Principles Analysis for Optoelectronic Applications

Recently, fluoride-based double perovskites have emerged as promising lead-free wide-band-gap materials for advanced optoelectronic applications, though their fundamental properties remain underexplored. In this work, density functional theory is employed to systematically investigate the structural, electronic, mechanical, and optical properties of A 2 InGaF 6 (A = Na, Cs). Structural optimization and elastic constant calculations confirm mechanical stability, while A-site substitution significantly influences stiffness and elastic anisotropy. Electronic structure analysis using GGA-PBE, SOC-PBE, and HSE06 functionals reveals indirect wide band gaps, with HSE06 values of 2.70 eV for Na 2 InGaF 6 and 3.55 eV for Cs 2 InGaF 6 . Orbital-resolved density of states indicates that the In–Ga–F framework governs the band edges, whereas alkali-metal cations mainly affect electronic behavior through structural modification. Optical spectra within the independent-particle approximation demonstrate strong near-ultraviolet absorption, moderate dielectric screening, and high-energy plasmonic features in the vacuum-ultraviolet region. These characteristics highlight their suitability for ultraviolet photodetectors and radiation-related photonic devices rather than conventional photovoltaics. Overall, A-site engineering effectively tunes multifunctional properties, positioning A 2 InGaF 6 compounds as attractive candidates for lead-free, ultraviolet-focused optoelectronic platforms and encouraging future experimental validation.