Structural characterization, thermal, dielectric, vibrational properties and molecular motions in

C₃N₂H₅]₆Bi₄Br₁₈] has been synthesized and characterized by the X-ray (at 293 and 110 K), calorimetric, dilatometric and dielectric measurements. At room temperature it crystallizes in the monoclinic space group, C2/m. A crystal structure consists of disordered imidazolium cations and ordered discrete tetramers of Bi₄Br₁₈]^6-. This compound reveals a rich polymorphism in a solid state. It undergoes three solid–solid phase transitions: from phase I to II at 426/423 K (heating–cooling), II→III at 227 K and III→IV at 219.5/219 K. A clear dielectric relaxation process is found in the room temperature phase II. Infrared studies of the polycrystalline C₃N₂H₅]₆Bi₄Br₁₈] showed that the ν(N–H), δ(ring) and δ(C–H) modes of the imidazolium cations appeared to be very sensitive to the IV→III phase transition. ¹H NMR measurements confirmed a key role of the imidazolium cations in the phase transitions mechanisms at low temperatures.