IngentaConnect A neutron diffraction and NMR study of the P-T phase diagram of R...

Authors: W aogon sicki J.; Lewicki S.; Kozlenko D.P.; Kozak A.; Nawrocik W.; Kichanov S.E.; Savenko B.N.; Shchedrina T.

Source: Journal of Physics: Condensed Matter, Volume 16, Number 23, 16 June 2004 , pp. 3889-3900(12)

Publisher: Institute of Physics Publishing

Abstract:

The structure of Rb_1-x(NH₄)_xI (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 T₁ have been performed for polycrystalline samples of Rb_1-x(NH₄)_xI (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 two-fold C₂ and three-fold C₃ 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 Rb_1-x(NH₄)_xI (x = 0.29,0.77) mixed crystals. The phase diagram of Rb_0.23(NH₄)_0.77I has similar features to that of NH₄I. Four different phases were found to exist in the temperature and pressure range studied: a disordered agr

phase with an NaCl-type cubic structure, a disordered bgr

phase with a CsCl-type cubic structure, a ggr

phase with a tetragonal structure and antiparallel ordering of ammonium ions, and a dgr

phase with a CsCl-type cubic structure and parallel order of ammonium ions. The phase diagram of Rb_0.71(NH₄)_0.29I contains only agr

and

phases, and is similar to that of RbI.

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

A neutron diffraction and NMR study of the P–T phase diagram of Rb_1-x(NH₄)_xI mixed crystals (x = 0.29,0.77)