Ammonium dynamics and structural phase transition in Rb1-x(NH4)(x)I solid solutions at 20 K

The Rb1-x(NH4)(x)I mixed salts for 0 < x < 0.16 were characterized to be in the dynamically disordered state at 20 K due to presence the quasielastic neutron scattering. The orientational glass state (alpha-phase) is found fo r x = 0.40. The ordered gamma-phase tvas observed by neutron diffraction an d incoherent inelastic scattering at x = 0.66. IINS spectra of the disorder ed alpha-phase at 20 K display four distinct ammonium excitations similar t o that in the K1-x(NH4)(x)I system. The low-frequency modes at ca. 2.5 and 10 meV reflect almost free rotations of ammonium in the octahedral cage of Rb ions. The modes at 20 and 30 meV correspond to the translation and libra tion of NH4 groups in crystalline lattice. These modes are observed at ca. 19 and 39 meV, for the ordered gamma-phase, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.