Exploring Pressure-Induced Phase Transitions in Imidazolium Lead Bromide Hybrids

Abstract

Pressure‐driven structural and phonon responses in the series of analogous IMiPbBri+2 (i = 1, 2, 3) frameworks have been studied by combining high-pressure single-crystal X-ray diffraction, high-pressure Raman spectroscopy, and temperature-dependent IR/Raman measurements. At 0.3 GPa, IMPbBr3 undergoes a first-order transition from the ambient hexagonal phase to an ordered polar orthorhombic phase, and between 4.2 and 4.7 GPa the symmetry is lowered in a new phase persisting to at least 8.5 GPa. In IM2PbBr4, the triclinic framework remains stable up to ~1.6–2.1 GPa, when an isostructural first-order transition aligns the imidazolium cations; a higher-pressure anomaly near ~6.2 GPa manifested by Raman band broadening is consistent with stress-related disordering, possibly indicating the onset of partial amorphization. IM3PbBr5 exhibits an isostructural ordering at ~0.33 GPa and a second transition near 2.0–2.4 GPa, followed by a reversible stress-related anomaly around 6.2 GPa, demonstrating that the ferroelectric phase can be induced by external pressure. Across all compounds, pressure strengthens N–H···Br hydrogen bonds, suppresses cation dynamics, and induces distortions in PbBr6 octahedra, stabilizing the low-temperature phases. The results identify transition pressures and reversibility limits governed by structural dimensionality across the series.