Lead‐Free Cs2(Cd/Sn)BeCl6 Halide Double Perovskites: A Comprehensive First‐Principles Study of Structural, Optoelectronic, and Thermoelectric Properties for Sustainable Energy Applications

Ab initio simulations based on density functional theory are employed to investigate the structural, electronic, mechanical, optical, and thermoelectric properties of Cs 2 ABeCl 6 (A = Cd, Sn) halide double perovskites. The results demonstrate that both compounds crystallize in the stable cubic Fm‐3m symmetry. Structural and thermodynamic stability are verified through the Goldschmidt tolerance factor ( t G ), octahedral factor ( μ ), modified tolerance factor ( τ ), formation energies (Δ E f ), and Born–Huang criteria. Mechanical property evaluations suggest ductility and significant anisotropy in both compounds. First‐principles calculations using the generalized gradient approximation indicate a direct bandgap of 1.98 eV for Cs 2 CdBeCl 6 and an indirect bandgap of 2.58 eV for Cs 2 SnBeCl 6 . Optical investigations reveal significant absorption, refractive index, extinction coefficient, energy loss spectra, and reflectivity features in the energy range of 0–13 eV, indicating strong interactions within the ultraviolet region. Thermoelectric properties are evaluated using the BoltzTraP code, revealing low thermal conductivities and figure of merit values close to 1 at room temperature, which demonstrates excellent thermoelectric performance. The synergy of semiconducting behavior, strong UV absorption, and high thermoelectric efficiency positions Cs 2 CdBeCl 6 and Cs 2 SnBeCl 6 as promising candidates for optoelectronic and energy applications. This work presents the first theoretical investigation of beryllium‐containing double perovskites, combining beryllium's unique mechanical stability with optoelectronic functionality.