Analysis of the Structural, Electronic, and Thermoelectric Properties of Lead–free Cs2AgBiX6 (X = Br, Cl) Double Perovskites for Non–Toxic Technologies

Abstract The double perovskite compounds Cs2AgBiX6 (X = Br, Cl) are promising lead–free alternatives to conventional halide perovskites. In this study, we employ first–principles calculations based on Density Functional Theory within the WIEN2k framework to investigate their structural, electronic, optical, and thermoelectric properties. The structural properties were analyzed through geometry optimization and stability assessment to ensure the most energetically favorable atomic configuration. Crystallographic parameters and structural stability factors were evaluated to understand their influence on mechanical robustness and phase stability. The electronic properties were studied using both the Generalized Gradient Approximation and the Modified Becke–Johnson approximation to compute the electronic structure and bandgap values. The results indicate that both compounds exhibit an indirect bandgap, with the Modified Becke–Johnson approximation providing more accurate bandgap estimations compared to the Generalized Gradient Approximation, making it more consistent with experimental values. Optical analysis revealed that Cs2AgBiCl6 exhibits a stronger optical response, making it a promising material for optoelectronic applications. Additionally, thermoelectric investigations showed that Cs2AgBiBr6 has a higher figure of merit, indicating better potential for thermoelectric applications.