Computational insights into lead-free K2RbBiX6 (X = F, Cl, Br, I) halide perovskites for next-generation optoelectronic applications

Driven by the need for lead-free, eco-friendly materials in optoelectronics and photovoltaics, this study investigates the structural, electronic, mechanical, and optical properties of K₂RbBiX₆ (X = F, Cl, Br, I) double perovskites using first-principles DFT calculations. All compounds adopt a stable cubic Fm-3m structure, with lattice constants increasing from 9.84 Å (F) to 13.08 Å (I) and tolerance factors between 0.927 and 1.018. HSE06 calculations reveal indirect bandgaps for K₂RbBiF₆ (4.58 eV) and K₂RbBiI₆ (3.20 eV), while K₂RbBiCl₆ and K₂RbBiBr₆ show direct bandgaps of 4.27 eV and 3.81 eV, favorable for light absorption. DOS analysis shows Bi-p and halide p-orbital hybridization near the band edges. All materials meet Born mechanical stability criteria. K₂RbBiF₆ is the stiffest, while K₂RbBiCl₆ balances strength and ductility, with C₁₁ = 37.74 GPa, Pugh's ratio of 2.39, Poisson's ratio of 0.32, and anisotropy of 6.68. Optically, K₂RbBiCl₆ shows the highest static dielectric constant (3.03), strong visible absorption, refractive index of 1.74, and optical conductivity (∼2.30 Ω⁻¹ cm⁻¹). Its direct bandgap, mechanical resilience, and excellent optical response make K₂RbBiCl₆ a promising lead-free candidate for next-generation optoelectronic and photovoltaic devices.