Crystal structure and characterization of a novel acentric imidazolium analog

A novel nonlinear optical crystal [ C 3 N 2 H 5 + ] [ Br - ] has been synthesized and characterized by a single-crystal X-ray diffraction, calorimetric, dilatometric and dielectric measurements. Above 200 K the crystal structure is polar, with symmetry R3. It consists of a highly disordered imidazolium cations and discrete Br - anions. The compound reveals a first order phase transition at 199/201 K (cooling/heating cycle) which leads to the monoclinic centrosymmetric space group P 2 1 / m . The exceptionally large increment of dielectric permittivity accompaning the structure transformation is related to the freezing of cation motions and a significant rebuilding of the anionic substructure.