A. Ben Salah

Structure refinements by the Rietveld method of partially substituted hydroxyapatite: Ca9Na0.5(PO4)4.5(CO3)1.5(OH)2

Abstract The structural location and effect due to partially substitution of Na + and CO 3 2− to Ca ++ and PO 4 3− , respectively, in hydroxyapatite, Ca 10 (PO 4 ) 6 OH 2 (HAP), synthesised in aqueous solution have been investigated by X-ray powder pattern fitting methods. The compound crystallises in the hexagonal system (space group P63/m ) with the following cell: a = b =9.3892(4) A, c =6.9019(3) A. Using the Rietveld method, the refinement of occupancy factors shows that the sodium is located preferentially in the calcium site II of the apatite structure. Vacancies in both calcium sites are found and the OH site is fully occupied. No carbonate ion is found in the apatite channel, leading to a B type carbonate apatite. The carbonate ions are located on two faces of the PO 4 tetrahedron. A n extra shift of the OH − ion outside of the metal II triangle is attributed to the replacement of calcium by sodium and a vacancy. The corresponding new substitution mechanism is compared to already known fundamental mechanisms.

Studies by thermally stimulated current (TSC) of hydroxy- and fluoro-carbonated apatites containing sodium ions

Carbonated hydroxyapatites (HAPCO3Na) and fluoroapatites (FAPCO3Na) containing sodium ions have been precipitated by the hydrothermal method. The effect of carbonate and sodium substituted for phosphate and calcium respectively on the dipolar mobility of the OH- and F- ions located inside the apatitic channels of those samples has been studied by thermally stimulated current (TSC). In both apatites two relaxation modes, around -100 and + 50 °C, have been observed. In the HAPCO3Na sample, the relaxation mode fine structure reveals the existence of two cooperative phenomena with compensation temperatures in the vicinity of the hydroxyapatite monoclinic-hexagonal transition. After preheating of samples at 400 °C, the presence of cooperative movements is confirmed by the observation of a compensation phenomenon with a compensation temperature equal to 214 °C. An x-ray diffraction study is in agreement with this attribution. As for the FAPCO3Na sample, the fine structure of the lower temperature relaxation mode only reveals a compensation phenomenon at 5 °C attributed to water molecule reorientations inside apatitic channels.

Structural investigation of the phase transitions of [N(CH3)4]2HgCl4

Abstract The crystal structures of [N(CH3)4]2HgCl4 between 191 K and 350 K have been studied by single crystal X-ray diffraction. Below Tc1 = 277 K the structure is monoclinic P21/a with lattice parameters a = 12.320 (7), b = 9.034 (4), c = 15.402 (4) Å, and β = 89.64 (1)° at T = 191 K. Above Tc1 the structure is orthorhombic Pnma with lattice parameters 12.419 (7), b = 9.068 (4), and c = 15.684 (2) Å at room temperature, and with lattice parameters a = 12.421 (9), b = 9.125 (1), and c = 15.777(4) Å at T = 350 K. At room temperature each TMA cation disorderly occupies two orientations (TMA is tetramethylammonium). At low temperature the structure is fully ordered, with additional shifts and rotations of all ions, in accordance with the results of Asahi et al., J. Phys. Soc. Jpn. 60 (1991) 921. Refinements using restrictions of the non-crystallographic tetrahedral point group for the TMA cations and refinements without restrictions are used to show that TMA has tetrahedral symmetry at all temperatures. The HgCl4 anions are distorted, but the distortions are independent of the temperature. The phase transition at Tc1 is described as an order/disorder transition accompanied by major rotations and shifts of the ions. New evidence for a previously reported phase transition at Tc2 = 320 K (Zangar et al. Phase Transitions 12 (1988) 141) is obtained from NMR. This transition is shown to be a non symmetry breaking structural transition, that correspond to a gradual loss of the orientational order of the TMA cations.

Electrical properties of the mixed compound K2.51(NH4)1.49Hg3Cl10.2H2O

Abstract Crystals of K2.51(NH4)1.49Hg3Cl10.2H2O undergo two phase transitions at T1=395 and T2=424 K. phase I—(395 K)→phase II—(424 K)→phase III These transitions have been detected by differential scanning calorimetry, impedance and modulus spectroscopy techniques. The first transition at T=395 K is related to NH4+ reorientation and H+ diffusion. The conductivity relaxation parameters associated with some K+/NH4+ and H+ conduction have been determined from an analysis of the M″/M″ max spectra measured in a wide temperature range. Transport properties in this material appear to be due to an K+/NH4+ and H+ ions hopping mechanism.

Structural and vibrational studies of mixed potassium-ammonium chloromercurate(II) dihydrates K2.51(NH4)1.49Hg3Cl10.2H2O

Abstract The crystal structure of K 2.51 (NH 4 ) 1.49 Hg 3 Cl 10 .2H 2 0 has been determined by X-Ray single-crystal analysis. The title compound crystallises in the space group Pmcm and has the unit cell dimensions a =4.5110(15)A, b =14.489(2), c =16.074(4) A, Z =2. The refinement converged to R =0.03 8 and WR 2 =0.105. The structure may be described as consisting of alternating layers and chains built up from HgCl 6 octahedra. The ammonium groups (or K + ) and water molecules are located between the chains and layers. IR and RAMAN spectroscopic studies were performed to confirm results of the radiocristallographic method.

Phase transition in the compound NH3(CH2)5NH3TlCl5

Penthylene diammonium pentachlorothallate(III) exhibits a phase transition at 316 K This transition has been characterized by optical birefringence, dielectric measurements differential scanning calorimetry and spectroscopic measurements on polycrystalline samples. The space group and the cell parameters of phase I were determined by X-ray diffraction from single crystals. Phase I has space group P212121, with Z = 4, a = 7.696(3), b = 13.2890(17) and c = 13.503(18) Å. The transition is both displacive and order-disorder involving mainly conformational changes of the organic chain [NH3(CH2)5NH3]2+ coupled with distortion of the TlCl6 octahedra. This behaviour and the crystal dynamics will be discussed and compared with those found in the literature for homologous compounds.

DSC, X-Ray diffraction and Raman studies in the compound K2.51(NH4)1.49Hg3Cl10.2H2O

Abstract The mixed compound potassium–ammonium mercurate exhibits two phase transitions at T1=395 K and T2=424 K: phase I−(395 K)→phase II−(424 K)→phase III These transitions have been detected by differential scanning calorimetry, X-ray diffraction and Raman scattering on polycrystalline samples of K2.51(NH4)1.49Hg3Cl10.2H2O. The room temperature phase has space group Pmcm (a=4.511(4) A, b=14.489(9) A and c=16.074(12) A). Phase II is disordered and exhibits orthorhombic symmetry (a=4.500(1) A, b=17.758(5) A and c=22.895(5) A). Hydrogen bonding, the nature and the degree of structure (dis)order and the mechanisms of the phase transitions are discussed. The NH4+ ions show an important orientation disorder at the high temperatures.

Crystal structure of penthylenediammonium pentachlorothallate(III)

The penthylenediammonium pentachlorothallate(III) salt is orthorhombic with the following unit cell dimensions: a = 7.696(3) Å, b = 13.2890(17) Å, and c = 13.503(18) Å, space group P212121 with Z = 4. The structure was solved by Patterson methods and refined to a final R value of 0.0387 for 1991 observed reflections. The structure consists of penthylenediammonium cations and polynuclear anions in which slightly distorted [TlCl6]3− octahedral sharing two vertices are interconnected into chains. These chains are themselves interconnected by means of the N–H⋅⋅⋅Cl bonds originating from the [NH3(CH2)5NH3]2+ entities forming a three-dimensional network.

Crystal structure of monomethylammonium tribromocadmate(II)

CH3NH3CdBr3 crystallizes in the space group Ccm21, a = 13.693(1) Å, b = 7.906(1) Å, c = 6.886(1) Å, V = 745.46(15) Å3, Z = 4, Dx = 3.42 Mg m−3. The structure of the title compound is one-dimensional consisting of infinite chains made of face sharing CdBr6 octahedra. The methylammonium groups are situated in the space between chains. The cations and anions are joined by hydrogen bonding.

Crystal structure of a new double salt hydrate SrCd2Cl6.8H2O

AbstractSrCd2Cl6⋅8H2O crystallizes in the space group