Ammonium dynamics and structural phase transition in Rb1-x(NH4)xI mixed crystals at 20 K

Complete text of publication follows. Crystalline samples of Rb1-x(NH4)xI mixed salts were prepared from aqueous solutions with concentrations of x=0.06, 0.16, 0.40 and 0.66, by slow evaporation. The neutron diffraction (ND) and inelastic incoherent neutron scattering (IINS) spectra were measured simultaneously on the NERA-PR spectrometer at the IBR-2 pulsed reactor at Dubna. The ND spectra showed that crystals form the concentration region 0<x<0.40 were found in the α-phase (NaCl type), while the sample with x=0.66 correspond to the γ-phase of CsCl type. The IINS spectra of the disordered α-phase at 20 K displayed four distinct ammonium excitations: two resonant modes at ca. 2.5 and 10 meV, below the cut-off energy (ca. 11 meV) of phonons of RbI crystal, and two above at ca. 20 and 30 meV. The localised modes correspond to the translational nad torsional vibrations of NH4 groups, which for the ordered γ-phase are observed at ca. 19 and 39 meV, respectively. The resonant modes reflect almost free uniaxial rotations of ammonium in the octahedral cage of Rb ions. The Rb1-x(NH4)xI mixed crystals for concentrations 0<x<0.016 are found in the dynamically orientational disordered state due to the presence of quasielastic neutron scattering (QNS) at 20 K. The static disordered of orientational glass state is found at x=0.40. (author)