A novel nonlinear optical crystal [C3N2H5+][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 P2(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. (C) 2010 Elsevier B. V. All rights reserved.