Study of optoelectronic, thermoelectric, mechanical, and thermodynamic aspects of double perovskites Cs₂AgAlX₆ (X = Br, I) for energy applications

Double perovskites are emerging candidates for optoelectronic and thermoelectric applications. Therefore, the present article addresses the thermoelectric, mechanical, thermodynamic, and optical characteristics of Cs₂AgAlX₆ (X = Br, I) using DFT via Wien2k and BoltzTraP codes. The tolerance factor (0.95, 0.93), formation energy (−2.15 eV, −1.95 eV), Born mechanical criteria, and phonon dispersion band structures are calculated to confirm the structural, thermodynamic, mechanical, and dynamical stabilities. The Poisson's ratio, Pugh's ratio, melting temperature, hardness, and anisotropic thermal conductivity have been discussed comprehensively. The band gap has been tuned from 2.70 eV to 1.14 eV by changing the Br with I, which ensures the shifting of the absorption band from ultraviolet to the visible spectrum. The transition of electrons between 4p/5p states of Br/I and 4d states of Ag changes the band gaps and physical aspects of the studied DPs. The optical characteristics are elaborated by optical loss, reflectivity, dispersion, and refraction. Thermoelectric performance is calculated from the figure of merit (0.56, 0.70) at 300 K, which signifies large values at room temperature. Thermoelectric parameters are discussed through thermal conductivity, the Seebeck effect, and electrical conductivity.