Quasi-elastic and low-energy inelastic neutron scattering from fast ion conducting AgBr

An apparently anomalous low-energy mode has been observed in AgBr close to the melting point, T-m, by inelastic neutron scattering using the IN6 time- of-flight spectrometer at ILL. Further measurements using the RITA triple-a xis spectrometer at Ris phi National Laboratory show that in fact the "anom alous" behaviour is an artefact produced by the time-of-flight technique, c oupled with a rapid expansion of the lattice close to melting. The behaviou r of the 200 Bragg peak intensity as T-m is approached and is shown to be c onsistent with the proposal that a second-order transition to a fast ion co nducting phase would occur at T-c = 705 K > T-m, = 701 K, but that melting occurs first. The observed quasi-elastic scattering is in excellent agreeme nt with that expected from the ionic conductivity on the basis of the Nerns t-Einstein relation. (C) 1999 Elsevier Science B.V. All rights reserved.