Electronic, thermoelectric and optical properties of halide double perovskites: A DFT study using GGA, TB-mBJ, and...

Halide-based double perovskites (HDPs) are an emerging class of inorganic quaternary materials that exhibit a wide range of physical properties, making them promising alternatives to conventional functional materials. In this work, a comprehensive theoretical investigation of three new HDPs, Cs₂RbAsI₆, K₂AlTlI₆, and Cs₂LiSbCl₆, has been carried out using Density Functional Theory (DFT) to unveil their potential characteristics. The full-potential linearized augmented plane-wave (FP-LAPW) method was employed to systematically explore their structural, mechanical, electronic, optical, and thermoelectric properties, thereby providing insight into their possible technological applications. The calculated negative formation energies of these HDPs confirms their thermodynamic stability. The elastic constants were evaluated to determine their mechanical behavior, revealing mechanical stability, ductility, and elastic anisotropy in all compounds. Using the TB-mBJ and HSE06 approximations, the direct bandgaps of Cs₂RbAsI₆ were found to be 2.82 eV and 3.63 eV, respectively, while K₂AlTlI₆ exhibited indirect bandgaps of 2.18 eV and 2.99 eV, and Cs₂LiSbCl₆ showed indirect bandgaps of 3.75 eV and 4.55 eV, respectively. The optical spectra, including absorption coefficient, dielectric response, refractive index, and energy loss function, indicate strong optical activity in the visible and ultraviolet regions, suggesting suitability for photovoltaic and optoelectronic applications. Overall, the promising properties of these HDPs suggest that they are attractive candidates for future experimental investigation and renewable-energy device applications.