Ammonium dynamics in the disordered alpha-phase of K sub 1 sub - sub x (NH sub 4) sub x Y (Y = Cl, Br, I). A neutron scattering study

The effect of temperature and concentration on the lattice parameters and amplitude-weighted phonon density of states in mixed salts of ammonium-potassium halides is investigated by neutron powder diffraction and incoherent inelastic neutron scattering. In the disordered alpha-phase (NaCl type) ammonium ions exhibit a fast stochastic reorientation at phonon frequency rates down to ca. 80 K. At 10 K, the incoherent inelastic neutron scattering spectra display four distinct ammonium excitations: two (resonant) modes below and two (localized) above the Debye cut-off energy of potassium halides. High-frequency localized modes correspond to translational and librational vibrations of NH sub 4 ions. These modes are typical for the ordered phases of ammonium halides. The effect of ammonium concentration on localized and resonant modes is studied for the K sub 1 sub - sub x (NH sub 4) sub x I mixed salts. The harmonic excitations of ammonium in a hypothetical low-temperature alpha-phase of NH sub 4 I are approximated to ca. 30, 95, 155 and 250 cm sup - sup 1. In a real low-temperature ordered gamma-phase of NH sub 4 I, translational ammonium vibrations are observed at ca. 140-160 cm sup - sup 1 and librational vibrations at ca.300 cm sup - sup 1