Rb2BSiF6(B = Si, Sn, Ge): Newly predicted lead-free fluoride double perovskites for stable and efficient optoelectronics

We report the first comprehensive first-principles investigation of a new family of lead-free fluoride double perovskites, Rb 2 BSiF 6 (B = Si, Sn, Ge), aimed at stable and efficient optoelectronic applications. Using density functional theory within the PBEsol and HSE06 frameworks, we systematically analyze their structural, mechanical, dynamical, thermodynamic, electronic, and optical properties. All three compounds crystallize in the cubic Fm–3m structure and satisfy mechanical and Born-Huang stability criteria, while phonon dispersion calculations confirm dynamical stability with no imaginary modes. HSE06 calculations reveal direct band gaps of 1.28-1.66 eV at the Γ point, accompanied by ultralight hole effective masses and moderate electron masses arising from strong F 2p-B np hybridization. Optical spectra show sharp absorption onsets, high absorption coefficients exceeding 10 4 cm −1 in the visible range, moderate dielectric screening, low reflectivity, and favorable refractive indices. Thermodynamic analysis within the quasi-harmonic approximation further confirms stability over a wide temperature range. These results establish Rb 2 BSiF 6 compounds as promising, environmentally benign candidates for next-generation photovoltaic and optoelectronic devices.