Advanced Computational Study on the Lead‐Free Halide Double Perovskites Cs 2 RbYX 6 (X = Cl, Br, I): Insights into Structural, Optical, and Thermal Properties

ABSTRACT The physical properties of halide double perovskites Cs 2 RbYX 6 (X = Cl, Br, I) were investigated using first‐principles calculations within the framework of Density Functional Theory (DFT). The structural optimization was performed using generalized gradient approximation, while the electronic, optical, and thermoelectric properties were analyzed using the Tran–Blaha modified Becke‐Johnson (TB‐mBJ) potential. The elastic constants and related mechanical parameters were computed using the IR‐elast program, confirming the cubic Fm‐3m phase to be mechanically stable and ductile. Thermodynamic properties were evaluated utilizing the quasi‐harmonic approximation (QHA) framework. These compounds have substantial UV light optical absorption with direct bandgaps (6.53 eV for Cs 2 RbYCl 6 to 5.41 eV for Cs 2 RbYBr 6 and 4.07 eV for Cs 2 RbYI 6 ) that decrease from Cl to I. Materials containing Cs 2 RbYX 6 show promise as lead‐free thermoelectric and optoelectronic alternatives due to their positive Seebeck coefficients and rising ZT with temperature.