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

[C3N2H5]6[Bi4Br18][C3N2H5]6[Bi4Br18] 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, C 2/m . A crystal structure consists of disordered imidazolium cations and ordered discrete tetramers of [Bi4Br18]6-[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→IIIII→III at 227 K and III→IVIII→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]6[Bi4Br18][C3N2H5]6[Bi4Br18] showed that the ν(N–H)ν(N–H), δδ(ring) and δ(C–H)δ(C–H) modes of the imidazolium cations appeared to be very sensitive to the IV→IIIIV→III phase transition. 1H NMR measurements confirmed a key role of the imidazolium cations in the phase transitions mechanisms at low temperatures.