Structural Dynamics of Temperature- and Pressure-Induced Phase Transitions in Hybrid Imidazolium Lead Chlorides

We report the synthesis and comprehensive characterization of two previously unreported imidazolium lead chlorides, IM 2 Pb 3 Cl 8 ·0.5H 2 O and IM 3 PbCl 5, establishing chloride analogues as a platform to interrogate how inorganic-framework dimensionality and hydrogen-bond topology control structure, dynamics, and functional response in hybrid lead halides. Single-crystal X-ray diffraction reveals that IM 3 PbCl 5 consists of one-dimensional corner-sharing [PbCl 5 ] n 3– chains, whereas IM 2 Pb 3 Cl 8 ·0.5H 2 O adopts a three-dimensional porous framework constructed from edge- and face-sharing PbCl 8 polyhedra hosting IM + cations and water molecules. Variable-temperature calorimetry, diffraction, and Raman spectroscopy show that IM 3 PbCl 5 undergoes a sequence of first-order phase transitions driven by progressive ordering of the imidazolium sublattice coupled to distortions of the inorganic chains and reorganization of N–H···Cl contacts; hydrostatic compression induces three additional transitions at elevated pressures. In contrast, IM 2 Pb 3 Cl 8 ·0.5H 2 O shows no thermal anomalies in the investigated range and exhibits smaller pressure-induced shifts of selected phonon modes. Dielectric spectroscopy indicates thermally activated imidazolium reorientations and ionic transport at elevated temperatures, while optical measurements indicate wide band gaps and intense broadband emission in the spectral range from Vis to NIR.