Neutron-scattering investigation of ammonium dynamics in (NH 4) 2− xRb xSO 4 mixed salts

Abstract

Inelastic incoherent neutron-scattering (IINS) investigations of the mixed salts (NH 4) 2− x Rb x SO 4 for the whole concentration range, 0.0 ⩽ x ⩽ 2.0, have been performed on the NERA spectrometer at the IBR-2 pulsed reactor in Dubna. The IINS spectra show that ammonium ions undergo fast reorientations even in the ferroelectric phase; however, for (NH 4) 2SO 4, the rotational barrier slightly decreases at the phase transition. In the mixed salts, this effect vanishes with increasing rubidium concentration and above x > 0.4, the stochastic dynamics of the ammonium ions do not depend on the ferroelectric transition. Upon cooling to low temperatures, the ammonium ions form a substitutional protonic glass, within x ranging from approx. 0.4 to 1.2. At high rubidium concentrations, the IINS spectra display the localized dynamics of NH 4 (II), which occupy type II crystallographic position.