First-principles investigation of lead-free Rb₃SbX₆ (X = F, Cl, Br, I) mixed-halide double perovskites promising for photovoltaics and scintillation

In this work, density functional theory (DFT) calculations were performed to systematically explore the structural, mechanical, electronic, optical, light yield and thermodynamic properties of a series of environmentally benign, lead-free mixed-halide double perovskites with the general formula Rb₃SbX₆ (X = F, Cl, Br, I). The optimized lattice constants were found to increase from 9.61 Å for Rb₃SbF₆ to 13.01 Å for Rb₃SbI₆. Electronic band structures were obtained using the Tran-Blaha modified Becke-Johnson (TB-mBJ) exchange-correlation functional, revealing a decreasing band gap trend from 5.477 eV (F) to 2.851 eV (I), in accordance with the increasing ionic radius of the halide anions. Density of states (DOS) analysis highlighted the specific orbital contributions to the valence and conduction bands. Optical properties, including the complex dielectric function <i>ε</i>(<i>ω</i>), absorption coefficient <i>α</i>(<i>ω</i>), reflectivity <i>R</i>(<i>ω</i>), and refractive index <i>n</i>(<i>ω</i>), were evaluated, demonstrating strong optical responses across the series. Notably, the estimated ideal light yield for Rb₃SbI₆ indicates its potential for scintillation applications. Thermodynamic stability was assessed through temperature-dependent calculations of the Gibbs free energy, unit cell volume, entropy (<i>S</i>, J mol^-1 K^-1), and bulk modulus (<i>B</i>, GPa). The results consistently exhibit negative Gibbs free energies, thermal expansion with increasing temperature, rising entropy, and a gradual decrease in bulk modulus, confirming good thermal stability. These findings suggest that Rb₃SbX₆ (X = F, Cl, Br, I) double perovskites are promising candidates for use in radiation detection and optoelectronic devices.