Study of physical aspects of lead-free double perovskites Cs2AuXZ6 (X = Al/Ga; Z = Cl/Br) for solar cells and thermoelectric applications

Double perovskites are widely recognized in the scientific community for their non-toxic, low-cost, efficient, and stable energy-harvesting properties. This study uses density functional theory and Boltzmann transport theory to explore the physical properties of Cs2AuXZ6 (X = Al/Ga, Z = Cl/Br). The structural stability of the material is confirmed by calculating Goldschmidt's tolerance factor. Formation energy values are predicted to assess the synthesizability and thermal stability of Cs2AuXZ6. Mechanical properties are analyzed in detail to examine elasticity, ductility, strength, and anisotropy. The obtained elastic constants are further used to calculate thermodynamic parameters for predicting thermal behavior. The electronic properties are evaluated, revealing that Cs2AuAlCl6, Cs2AuAlBr6, and Cs2AuGaCl6 exhibit semiconductor characteristics with band gaps of 1.86 eV, 0.89 eV, and 0.84 eV, respectively, while Cs2AuGaBr6 is found to exhibit metallic behavior. These compounds are promising candidates for optoelectronic applications due to their high absorption coefficients (104 cm⁻1), low reflection (below 0.20), and substantial optical conductivity within the visible spectrum. These materials also show considerable potential for heat energy conversion, exhibiting high power factors and figures of merit (0.72, 0.69, and 0.68) for these double perovskite halides. These findings suggest that these materials are promising for optoelectronic and thermoelectric applications.