# Structural Phase Transition and the Effect of Iodine on Phase Stability in Rb3Bi2Br9 Perovskite-Related Halides with 2D Dimensionality

## TL;DR

This paper studies the structural and optical properties of a lead-free halide material, Rb3Bi2Br9–xIx, and finds it undergoes a phase transition and has tunable optical properties.

## Contribution

The novel contribution is the synthesis and structural characterization of Rb3Bi2Br9 and Rb3Bi2Br6I3, revealing a phase transition and their optoelectronic potential.

## Key findings

- Rb3Bi2Br9 undergoes a reversible phase transition from monoclinic to trigonal at ~450 K.
- Optical bandgaps of ~2.70 eV and ~2.21 eV were observed for Rb3Bi2Br9 and Rb3Bi2Br6I3, respectively.
- DFT calculations confirmed the direct bandgap nature and electronic structure dominated by Br and Bi orbitals.

## Abstract

Rubidium-based halide perovskites Rb3Bi2Br9–x
I
x
(x = 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: P21/c) to a high-symmetry trigonal (space group: P3̅̅m1) 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 Rb3Bi2Br9–x
I
x
, establishing them as robust candidates for stable, lead-free
optoelectronic applications.