Ground State Properties and Energy-Related Applications of Cs2CaXCl6 (X = Si, Ge) Double Halide Perovskites: A Novel DFT Study

This investigation applies first-principles DFT methods to examine the fundamental structural, electronic, optical, thermoelectric and mechanical traits of Cs 2CaXCl 6 (X = Si, Ge) compounds, aiming to uncover their potential utility in advanced energy systems. The electron-ion interactions use the Wu–Cohen Generalized Gradient Approximation (WC-GGA) and the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential. All investigated materials are structurally and thermodynamically stable, supported by their negative formation energies and Goldsmith's criteria. Moreover, Cs 2CaSiCl 6 and Cs 2CaGeCl 6 compounds exhibit wide band gaps in the ultraviolet region, ranging from 3.147 to 4.498 eV, depending on the choice of functional (GGA-PBE or TB-mBJ). A comprehensive analysis of the optical parameters, such as complex refractive index, absorption coefficient, and reflectivity, was performed under photon excitation from 0 to 14 eV. These results highlight the potential of Cs 2CaSiCl 6 and Cs 2CaGeCl 6 for future optoelectronic and photovoltaic applications. The thermoelectric properties of Cs 2CaXCl 6 (X = Si, Ge) double halide perovskites were computed using the BoltzTraP code, indicating a remarkable combination of high electrical conductivity and low lattice thermal conductivity, which results in a significant figure of merit and highlights their viability in thermoelectric technologies. The elastic properties of Cs 2CaXCl 6 (X = Si, Ge) confirm their mechanical stability and ductile nature, while highlighting the pronounced anisotropy induced by Ge substitution. These results confirm that the double halide perovskites are promising candidates for multifunctional applications, including nano-electronics, optoelectronics, thermoelectrics, and photovoltaics.