Ag+dynamics in the superionic and liquid phases of Ag2Se and Ag2Te by coherent quasi-elastic neutron scattering

The atomic motion of Ag+ ions in superionic and liquid Ag2Se and Ag2Te has been investigated by coherent quasi-elastic neutron scattering. The presence of a Q-dependent broadening of the quasielastic peak indicates that the Ag+ ion is mobile in the superionic phase of both materials. This motion takes place within a body-centred cubic sub-lattice of Se2- or Te2- ions respectively. Two Lorentzian components were required in order to fit the quasi-elastic broadening: a narrow peak that is attributed to the diffusion of the cations, and a much broader and less intense component that may be due to a localized motion of these ions. An asymmetry in the quasi-elastic peak indicates the presence of a low-energy excitation as reported for other superionic materials. The widths of the narrower Lorentzian showed the characteristic de Gennes narrowing at Q values close to the maximum in SAgAg(Q). A comparison of the widths of the quasi-elastic peak for the two liquids indicates a similar Ag+ diffusion coefficient but a difference in the mechanism of diffusion. The results for liquid Ag2Te indicate a jump-diffusion mechanism in comparison to those for Ag2Se that follow Fick's law.