Features of the structure and dynamics of Me1−x (NH4) x SCN (Me = K, Rb) mixed crystals

Abstract

Neutron scattering is used to study the structure and dynamics of Me1 − x(NH4)xSCN (Me = K, Rb) mixed crystals along the concentration section of 0.0 < x < 1.0 at room temperature 10 and 290 K. Phase transitions in Me1 − x (NH4)xSCN mixed crystals are analyzed by neutron powder diffraction. The measured spectra of inelastic incoherent neutron scattering from mixed crystals in a concentration range of 0.0 < x < 1.0 at 10 are transformed into the generalized phonon density of states G(E) in the one-phonon incoherent approximation. Using G(E), we determine the changes in ammonium ion dynamics during phase transitions. Low energy resonance and local translational (two bands) and librational (two bands) modes are observed in the disordered rhombic phase at 10 K. The low energy resonance mode is not found in the ordered monoclinic phase at 10 K, though the local translational mode in the form of two bands and the local librational mode in the form of four bands are present there. The low energy resonance mode appears due to hybridization of the phonon spectrum of the host crystal with rotational tunneling modes of the split librational ground state of the impurity’s molecular ammonium ion.