Computational design of Na2LiXF6 (X = Al, Ga, In, Tl) alkali halide perovskites for emerging optoelectronic technologies

Abstract

This research employs density functional theory (DFT) within the GGA-PBE framework to investigate the structural, electronic, mechanical, and optical characteristics of lead-free fluoride-based double perovskites Na₂LiXF₆ (X = Al, Ga, In, Tl). All compounds are found to crystallize in a stable cubic Fm$\bar{3}$m structure, with Goldschmidt tolerance factors confirming their structural integrity. The materials exhibit direct band gaps at the Γ-point, which decrease progressively from 6.83 eV (for Al) to 3.32 eV (for Tl), indicating potential suitability for UV to near-visible optoelectronic applications. The calculated elastic constants verify mechanical stability, showing an increasing trend in ductility with heavier atomic masses. Optical evaluations demonstrate strong transparency in the UV region, distinct dielectric responses, and absorption and reflectivity patterns consistent with band gap variation. Overall, Na₂LiXF₆ compounds emerge as promising lead-free candidates for efficient optoelectronic device applications.