Computational study of Rb₂LiWX₆ (X = Cl, Br): Electronic, mechanical, optical, and thermoelectric properties for energy applications

With the aim of developing eco-compatible materials for future energy systems, we presesent a comprehensive first-principles investigation of the structural, mechanical, electronic, optical, and thermoelectric properties of the lead-free double perovskites Rb₂LiWCl₆ and Rb₂LiWBr₆. The optimized crystal geometries and calculated tolerance factors confirm their structural stability. The elastic constants (C₁₁, C₁₂, C₄₄) satisfy Born's mechanical stability criteria, and both compounds exhibit anisotropic mechanical behavior. Electronic structure calculations using the modified Becke–Johnson (mBJ) potential reveal a semiconducting character with direct band gaps of 2.99eV (Cl-based) and 2.43 eV (Br-based). Optical properties, including the dielectric function, absorption coefficients, and reflectivity, are evaluated using the WIEN2k framework, demonstrating strong ultraviolet absorption, moderate refractive indices, and favorable dielectric responses, highlighting their suitability for optoelectronic applications. Thermoelectric properties, calculated via Boltzmann transport theory using the BoltzTraP code, yield promising figures of merit (ZT ≈ 0.76–0.78 at 800 K), attributed to high Seebeck coefficients and low thermal conductivities. These results position Rb₂LiWCl₆ and Rb₂LiWBr₆ as promising multifunctional materials for photovoltaic and thermoelectric applications, especially in the mid-temperature regime.