First-principles study of structural, electronic and optical properties of non-toxic RbBaX$_3$ (X = F, Cl, Br, I) perovskites under hydrostatic pressure

We have investigated the structural, mechanical, electronic and optical properties of Rb-based cubic perovskite RbBaX$_3$ (X = F, Cl, Br, I) under hydrostatic pressure, using first-principle density functional theory (DFT). All RbBaX$_3$ perovskites exhibit thermodynamic and mechanical stability at ambient pressure. RbBaF$_3$ remains structurally stable across all examined pressures, while RbBaCl$_3$, RbBaBr$_3$, and RbBaI$_3$ maintain mechanical stability up to 60, 60, and 40 GPa, respectively. These materials are ductile even at elevated pressure. RbBaF$_3$ has a direct bandgap of 4.80 eV while other compositions exhibit indirect band gaps of 4.37, 3.73, and 3.24 eV with halide atoms of Cl, Br, and I, respectively. Under elevated hydrostatic pressure, only RbBaCl$_3$ and RbBaI$_3$ exhibit an indirect-to direct band transition while others preserve their nature of band gap. Our results show that spin-orbit coupling significantly affects only the valance bands of larger-sized halides (Cl, Br, I). With hybrid functional (HSE) correction, the band gaps of these four materials increase to 6.7, 5.6, 4.8 and 4.4 eV, respectively, but the nature of direct/indirect band transition remains unchanged. Orbital-decomposed partial density of states calculation reveals that the halogen p-orbitals dominate the valence band near the Fermi level, while Rb 5s-orbital affects the conduction band minima the most. Investigation of the optical properties reveals wide-band absorption, low electron loss, moderate reflectivity and lower refractive index in the UV to deep-UV range. The strength and range of absorption increases significantly with hydrostatic pressure, suggesting that RbBaX$_3$ perovskites are promising candidates for tunable UV-absorbing optoelectronic devices.