NEUTRON-SCATTERING INVESTIGATION OF AMMONIUM DYNAMICS IN (NH4)(2-X)RBXSO4 MIXED SALTS

Inelastic incoherent neutron-scattering (IINS) investigations of the m ixed salts (NH4)(2-x)RbxSO4 for the whole concentration range, 0.0 les s than or equal to x less than or equal to 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 th e ferroelectric phase; however, for (NH4)(2)SO4, the rotational barrie r 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 ammon ium ions form a substitutional protonic glass, within x ranging from a pprox. 0.4 to 1.2. At high rubidium concentrations, the IINS spectra d isplay the localized dynamics of NH4 (II), which occupy type II crysta llographic position. (C) 1998 Elsevier Science B.V. All rights reserve d.