Structural Phase Transition and the Effect of Iodine on Phase Stability in Rb ₃ Bi ₂ Br ₉ Perovskite-Related Halides with 2D Dimensionality

Rubidium-based halide perovskites Rb₃Bi₂Br_9-<i>x</i>I_<i>x</i> (<i>x</i> = 0, 3) represent a lead-free, low-dimensional alternative within the A₃B₂X₉ family, offering promising optoelectronic properties. This work reports the successful synthesis of Rb₃Bi₂Br₉ and Rb₃Bi₂Br₆I₃ via mechanochemical ball milling, yielding highly crystalline products. Structural characterization of Rb₃Bi₂Br₉ halide, using synchrotron X-ray and neutron powder diffraction across a broad temperature range (295-656 K), revealed a reversible phase transition from a low-symmetry monoclinic (space group: <i>P</i>2₁/<i>c</i>) to a high-symmetry trigonal (space group: <i>P</i>3̅̅<i>m</i>1) phase at ∼450 K (both with a 2D dimensionality, concerning the connection of [BiBr₆] octahedra). Thermal expansion coefficients, derived from unit-cell evolution, showed discontinuity across the structural phase transition. Symmetry-adapted distortion mode analysis identified octahedral tilting and rigid [BiBr₆] framework rotations as the primary contributors to the monoclinic distortion, with minor and moderate contributions from stretching, bending, and Rb atoms translations. Optical characterization at room conditions (monoclinic phase) unveiled bandgaps of ∼2.70 and ∼2.21 eV for Rb₃Bi₂Br₉ and Rb₃Bi₂Br₆I₃, respectively. Density functional theory (DFT) calculations corroborated the direct bandgap nature and electronic structure, highlighting the dominant Br and Bi orbital contributions. These results demonstrate the structural richness and optical tunability of Rb₃Bi₂Br_9-<i>x</i>I_<i>x</i>, establishing them as robust candidates for stable, lead-free optoelectronic applications.