Halide-dependent modulation of electronic and optical properties in K ₂ SbAuX ₆ (X=F, Cl) lead-free perovskites

Lead-free double perovskites are attracting attention as sustainable alternatives to toxic lead-based halides for next-generation optoelectronics. In this work, we employed density functional theory (DFT) with PBE-GGA using the CASTEP framework to investigate the structural, electronic, optical and mechanical properties of K 2 SbAuF 6 and K 2 SbAuCl 6 . Both compounds exhibit stable cubic structures with tolerance factors of 0.92 (F) and 0.878 (Cl). The calculated band gaps are 0.655[Formula: see text]eV for K 2 SbAuF 6 and 0.20[Formula: see text]eV for K 2 SbAuCl 6 , confirming semiconducting behavior with tunable electronic properties. Optical analyses reveal strong absorption across the ultraviolet–visible (UV–Vis) region; K 2 SbAuCl 6 displays superior optical conductivity (3.60[Formula: see text][Formula: see text]cm[Formula: see text] at 7.45–10.65[Formula: see text]eV), while K 2 SbAuF 6 shows enhanced response in the high-energy UV range with conductivity up to 4.44[Formula: see text][Formula: see text]cm[Formula: see text] at 19.09–24.45[Formula: see text]eV. Mechanical evaluations confirm elastic stability, with K 2 SbAuF 6 exhibiting greater ductility ([Formula: see text]) and anisotropy ([Formula: see text]), whereas K 2 SbAuCl 6 demonstrates higher stiffness ([Formula: see text][Formula: see text]GPa) and shear resistance. These results highlight halide substitution as an effective strategy to tailor optoelectronic properties, positioning K 2 SbAuX 6 (X[Formula: see text]F, Cl) as promising lead-free candidates for photovoltaic, photonic and spintronic applications.