First-principles study of mixed-halide perovskites RbSrBr3−𝑥I𝑥: Tuning physical properties through halide alloying

Abstract

This study employs first-principles density functional theory using GGA-PBE and hybrid HSE06 functionals to

systematically investigate halide substitution effects on the structural, electronic, elastic, and optical properties

of mixed-halide perovskites RbSrBr3−𝑥I𝑥 (𝑥 = 0 to 3). Progressive replacement of Br− with I− induces a 7.7%

lattice expansion and a phase transition from cubic to tetragonal, followed by a return to the cubic phase at

full iodination. All compositions are thermodynamically, mechanically, and dynamically stable, as confirmed

by negative formation energies, tolerance factors, Born stability criteria, and ab initio molecular dynamics. The

band gap remains indirect, decreasing from 4.94 eV to 4.20 eV with increasing iodine content. Bulk modulus

decreases from 16.9 GPa to 13.1 GPa, indicating softer bonding in I-rich phases. Optical studies reveal strong

compositional dependence of absorption and dielectric properties, making these materials promising candidates

for UV photodetectors, UV-responsive sensors, and radiation-shielding coatings.