Fluoride‐Based Halide Double Perovskites X ₂ BiAuF ₆ (X = K, Rb): A Computational Insight into their Structural Integrity, Optoelectronic Performance, and Thermoelectric Potential

Halide double perovskites (HDPs) are emerging as tunable and eco‐friendly alternatives to lead‐based materials for advanced optoelectronic and energy applications. This study presents a comprehensive first‐principles investigation of the structural, electronic, optical, elastic, and thermoelectric (TE) properties of fluoride‐based HDPs X 2 BiAuF 6 (X = K, Rb), using density functional theory within the full potential linearized augmented plane wave framework and the Tran–Blaha modified Becke–Johnson potential. The calculated Goldschmidt tolerance factor τ G values of 0.95 for Rb 2 BiAuF 6 and 0.92 for K 2 BiAuF 6 confirm a stable cubic structure. Electronic structure calculations using mBJ potential reveal indirect bandgaps of 1.29 eV for K 2 BiAuF 6 and 1.31 eV for Rb 2 BiAuF 6 , indicating their suitability for optoelectronic applications. While HSE06 functional increases the bandgap values to 2.106 and 2.229 eV, respectively. Additionally, optical analysis via the dielectric function reveals strong absorption in the visible and UV regions, along with low reflectance below 0.20. The static refractive index n (0) is found to be 1.73 for K 2 BiAuF 6 and 1.72 for Rb 2 BiAuF 6 . TE properties are evaluated using the BoltzTraP simulation tool, based on electronic structure inputs from first‐principles calculations. Both materials exhibit a maximum thermopower factor of 4.26 × 10 11 Wk −2 m −1 s 1 at 800 K, increasing with temperature. The results demonstrate appreciable electrical conductivity and substantial Seebeck coefficients, emphasizing their potential for integration into next‐generation optoelectronic and TE devices.