DFT study of double perovskites Cs2AgBiX6 (X = Cl, Br): An alternative of hybrid perovskites

The double perovskites are promising materials for renewable energy to fulfil the challenges of energy requirements in the world. Therefore, in this article, the optical, thermoelectric, thermodynamic, and mechanical properties of Cs2AgBiX6 (X ​= ​Cl, Br) are addressed comprehensively by the DFT approach. The stabilities regarding structure, and thermodynamics are ensured by findings of tolerance factor, and formation energy. The mechanical stability and ductile nature has been confirmed from elastic constants, Passion's, and Pugh's rules. The hardness, Debye temperature, longitudinal and transverse velocities of sounds are also explained for thermodynamic behavior understanding. The band gap for Cs2AgBiCl6 is 2.76 ​eV, which reduces to 1.9 ​eV by changing Cl with Br due to pd-hybridization of anions and cations. The optical characteristics are elaborated by dielectric constants, absorption, refractive index, and reflectivity. The absorption in visible and ultraviolet regions make them significantly important for solar cells. The ultralow lattice thermal conductivity and high figure of merit have been computed due to large Seebeck coefficient and electrical conductivity.