A neutron diffraction and NMR study of the P-T phase diagram of Rb1-x(NH4)(x)I mixed crystals (x=0.29,0.77)

摘要

The structure of Rb1-x(NH4)(x) I (x = 0.29, 0.77) mixed crystals was studied by means of powder neutron diffraction in the temperature range 20-300 K and at ambient pressure. Measurements of spin-lattice relaxation time T, have been performed for polycrystalline samples of Rb1-x(NH4)(x)I (X = 0.29, 0.77) in the temperature range 95-300 K and pressure range 0-800 MPa by the proton NMR technique using the saturation method. Activation parameters were obtained for a model of complex ammonium ion reorientations around twofold C-2 and three-fold C-3 axes. The activation volume for different phases of the investigated compounds was determined. The experimental data were used for a construction of P-T phase diagram of Rb1-x (NH4)(x)I (x = 0.29, 0.77) mixed crystals. The phase diagram of Rb-0.23(NH4)(0.77)I has similar features to that of NH4I. Four different phases were found to exist in the temperature and pressure range studied: a disordered alpha phase with an NaCl-type cubic structure, a disordered beta phase with a CsCl-type cubic structure, a gamma phase with a tetragonal structure and antiparallel ordering of ammonium ions, and a 3 phase with a CsCl-type cubic structure and parallel order of ammonium ions. The phase diagram of Rb-0.71 (NH4)(0.29)I contains only alpha and beta phases, and is similar to that of RbI.