This research aims to analyse the physical properties of lead-free double perovskites \({\text{A}}_{2} {\text{GaScI}}_{6}\) (where A = Li, Rb, Cs) using first-principles method. The structural stability of the considered material is confirmed through geometry optimization process which includes formation energy, octahedral tilting factor, tolerance factor and analysis of elastic parameters. The electronic properties are evaluated using both generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) and Tran and Blaha modified Becke-Johnson (TB-mBJ) methods through analysis of the density of states and band structures to obtain accurate energy band gaps. Subsequently, the optical parameters, including the absorption coefficient, loss function, optical conductivity, reflectivity, dielectric function, and refractive index, are calculated and analyzed. The results show that all double perovskites exhibit a high absorption coefficient in the visible and UV regions. The calculated mechanical parameters indicated that studied materials are elastically stable, compressible, anisotropic and show high melting temperature. Thermoelectric parameters indicate that electrical and thermal conductivities, power factor, and seebeck coefficient all increase with temperature. The calculated ZT at 300 K is very close to unity and positive seebeck coefficient, indicating p-type behaviour. The calculated band gaps along with optical, thermoelectric and mechanical parameters envisaged that these materials are very suitable for optoelectronic and thermoelectric devices for green energy harvesting.
