Crystal structure, phase transitions and dielectric properties of the mixed compound [Cs-0.92(NH4)(0.08)](2)HgBr4

The crystal structure of [Cs-0.92 (NH4)(0.08)](2) HgBr4 was determined by t hree-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 st ructure was refined to R = 0.067. The distribution of atoms can be describe d 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- ioni c bonds and N-H...Br hydrogen bonds ensuring the cohesion of the crystal. D icaesium-ammonium tetrabromomercurate exhibits three phase transitions at T -1 = 237K, T-2 = 244K and T-3 = 513K. These transitions were detected by di fferential scanning calorimetry and analysed by dielectric measurements usi ng the impedance and modulus spectroscopy techniques. The phase change at h igh 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.