Exploring pressure-induced phase transitions in imidazolium lead bromide hybrids

Abstract

Pressure-driven structural and phonon responses in the series of analogous IM_iPbBr_i+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 a ferroelectric phase can be induced by external pressure. Across all compounds, the increased 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.