A set of quasi-elastic neutron scattering experiments on solid solutions of sodium orthophosphate and sodium sulfate was reported. These materials were not only fast-ion conductors in their high-temperature cubic phases but also exhibited considerable dynamic reorientation disorder of their tetrahedral anions. High-resolution back-scattering experiments were used to examine Na diffusion in xNa2SO4·(1−x)Na3PO4, which was found to be dominated by thermally activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64eV for x = 0.0 to 0.30eV for x = 0.5. Time-of-flight scattering proved to be mainly sensitive to O scattering and thus to anion reorientation. This thermally activated motion exhibited activation energies between 0.184eV (x = 0.0) and 0.052eV (x = 0.5). Analysis of the quasi-elastic intensities as a function of scattering vector Q gave clear evidence of the involvement of cations in the anion motion.

Correlated Motion of Anions and Cations in Fast Cation Conducting Rotor Phases. D.Wilmer, H.Feldmann, R.E.Lechner, J.Combet: Solid State Ionics, 2004, 175[1-4], 463-6