Ammonium dynamics in the disorder a-phase of K1- x (NH4) x Y (Y=Cl, Br, I). a neutron scattering study

Abstract

Temperature and concentration effects on the lattice parameters and the amplitude weighted phonon density of states (AWPDS) in the mixed salts of ammonium-potassium halides were investigated by neutron powder diffraction (NPD) and incoherent inelastic scattering (IINS). In the disorder α-phase (NaCl type), ammonium ions perform a fast stochastic reorientation at phonon frequencies rate, down to ca. 80 K. The IINS spectra at 10 K displayed the four distinct ammonium excitations, two (resonant) modes below and two (localised) above the cut-off frequency of the AWPDS of potassium halides. The high frequency localised modes correspond to the translational and librational vibrations of NH4 ions. These modes are typical for ordered phases of ammonium halides. Ammonium concentration effects on the localised and resonant modes were studied for the K1− x (NH4) x I mixed salts and the harmonic excitations of ammonium in the hypothetical low temperature α-phase of NH4I were approximated to ca. 30, 95, 155 and 250 cm−1. In the real low temperature ordered γ-phase of NH4I, translational ammonium vibrations were observed at ca. 140-160 cm−1 and librational at ca. 300 cm−1.