Theoretical investigation of structural and mechanical stability, electronic, optical, and transport behaviour of double halide perovskites K 2 GaBiX 6 (X = Cl, Br, and I) for optoelectronic applications

In this work, double perovskites K2GaBiX6 (X = Cl, Br, and I) are studied using density functional theory (DFT). Physical properties are calculated after confirming the structural stability by geometry optimization, formation energy (Ef), tolerance factor (tg), and elastic constants (Cij). Electronic behavior is determined by calculating band structures and density of states using GGA-PBE and HSE06 approximations and obtained band gaps are 2.02 eV, 1.094 eV, and 0.484 eV for K2GaBiCl6, K2GaBiBr6, and K2GaBiI6 respectively by using GGA-PBE approximation with direct band gap nature and 1.92 eV, 1.54 eV, and 0.80 eV with HSE06 approximation. The optical parameters; dielectric function epsilon(w), refractive index n (w) loss function (w), reflectivity (w), conductivity (1\fs), and absorption coefficient alpha(w) are also calculated and all materials have shown good absorption in Infrared (IR), Visible, and Ultra-voilet (UV) regions. Elastic parameters revealed that K2GaBiCl6 and K2GaBiI6 are brittle while K2GaBiBr6 has ductile nature. Thermoelectric (TE) properties; thermal conductivity (K), electrical conductivity (sigma), Seebeck coefficient (S), and figure of merit (ZT) are also calculated and found that these are promising material for TE applications. The calculated electronic, optical, and transport parameters indicated that these materials are very suitable for optoelectronic and thermoelectric devices.