Inelastic neutron scattering study of the superionic solid solutions Ba1-xLaxF2+x

Inelastic neutron scattering spectra of Ba1-xLaxF2+x single crystals have b een measured at temperatures from 450 to 1000 degrees C. A noticeable resul t at x = 0.16 consists in a low-energy inelastic mode (E = 0.8 THz) which i s observed at a specific wavevector only and takes the form of a quasielast ic signal for other wavevectors. This is explained by the assumption that t he mode comprises mobile ions and defect clusters; the characteristic wavev ector corresponds to the short-distance correlation length between the clus ters. For x = 0.31, this mode seems overdamped, which we explain by enhance d interactions between the clusters. (C) 1999 Elsevier Science B.V. All rig hts reserved.