Abstract
The crystal structure of [Cs-0.92 (NH4)(0.08)](2) HgBr4 was determined by three-dimensional X-ray diffraction analysis. The space group is Pnma with a = 10.210(2), b = 7.929(1), c = 13.883(1)Angstrom and Z = 4 at 293K. The structure was refined to R = 0.067. The distribution of atoms can be described as isolated HgBr(4)(2-)tetrahedra, Cs+ and NH4+ cations. The main feature of this structure is the coexistence of two types of bonds: Cs+ - Br- ionic bonds and N-H...Br hydrogen bonds ensuring the cohesion of the crystal. Dicaesium-ammonium tetrabromomercurate exhibits three phase transitions at T-1 = 237K, T-2 = 244K and T-3 = 513K. These transitions were detected by differential scanning calorimetry and analysed by dielectric measurements using the impedance and modulus spectroscopy techniques. The phase change at high temperature is related with the orientational disorder of NH4+ cations. Transport properties in this material appear to be due to a H+ ion hopping mechanism.