DFT investigation on thermoelectric, electronic, optoelectronic, elastic, and structural properties of sodium-based halide double perovskites Rb2NaSbZ6 (Z = Cl, Br, and I)

Sodium-based halide double perovskites (HDPs) present a promising alternative to Pb-based perovskites for safe solar and thermal energy conversion devices due to their high durability and non-toxic elements. This study examines the electronic, thermoelectric, elastic, optoelectronic, and structural properties of Rb₂NaSbZ₆ (Z = I, Br, Cl) double perovskite compounds using density functional theory (DFT). These compounds, characterized by a cubic structure, show increasing structural parameters as halogens are substituted from chlorine to iodine. Structural stability is confirmed by evaluating the enthalpy of formation, Gibbs free energy, Born and Huang criteria, and tolerance factor. Pugh's ratio indicates the ductile nature of the compounds. All investigated halide compounds exhibit an indirect band gap ranging from 3.9 to 2.34 eV, with valence and conduction band extrema located at different symmetry points, and a higher effective mass of holes is noted. This study also analyzes the refractive index, optical loss, light absorption, and polarization across the energy range of 0–10 eV. The spectral characteristics indicate that these HDPs are suitable for optoelectronic and photovoltaic devices due to their absorption in the visible and near-ultraviolet spectra. High figures of merit (0.45–0.6) derived from power factor and thermal conductivity calculations suggest the potential of these compositions as thermoelectric devices. These findings provide a comprehensive understanding of these materials, facilitating their further deployment.