Siwar Walha

Crystal structure, phase transitions and dielectric properties of the mixed compound [Cs0.92(NH4)0.08]2HgBr4

Abstract The crystal structure of [Cs 0.92 (NH 4 ) 0.08 ] 2 HgBr 4 was determined by three-dimensional X-ray diffraction analysis. The space group is Pnma with a = 10.210(2), b = 7.928(1), c = 13.883(1)A 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 NH 4 + 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 NH 4 + cations. Transport properties in this material appear to be due to a H + ion hopping mechanism.

The solid solution (Fe0.81Al0.19)(H2PO4)3 with a strong hydrogen bond

Single crystals of the solid solution iron aluminium tris(dihydrogenphosphate), (Fe(0.81)Al(0.19))(H(2)PO(4))(3), have been prepared under hydrothermal conditions. The compound is a new monoclinic variety (gamma-form) of iron aluminium phosphate (Fe,Al)(H(2)PO(4))(3). The structure is based on a two-dimensional framework of distorted corner-sharing MO(6) (M = Fe, Al) polyhedra sharing corners with PO(4) tetrahedra. Strong hydrogen bonds between the OH groups of the H(2)PO(4) tetrahedra and the O atoms help to consolidate the crystal structure.

Potassium cerium(III) bis(sulfate) monohydrate, KCe(SO4)2.H2O.

The crystal structure of potassium cerium(III) bis(sulfate) monohydrate, KCe(SO4)2.H2O, is built up from irregular independent SO4 tetrahedra, CeO9 polyhedra in the form of distorted tricapped trigonal prisms and K+ ions. Hydrogen bonding between the free water molecule and sulfate groups supplement the ionic bonds characteristic of the rest of the structure.