Results of simple computer simulations and model calculations for ion conducting rotor phases were compared to quasi-elastic neutron scattering data from solid solutions of sodium orthophosphate and sodium sulphate, xNa2SO4(1-x)Na3PO4. These materials were not only Na fast-ion conductors in their high-temperature cubic phases, but also exhibited considerable dynamic reorientational disorder of their tetrahedral anions. At an elastic energy resolution of about 100µeV, neutron spectrometry monitored O scattering due to anion reorientation which occurred on the ps time-scale. This thermally activated process involved activation energies of 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 anion reorientation. Increasing the elastic resolution to about 1µeV full-width at half-maximum (thereby shifting the dynamic window into the ns scale) permitted the examination of Na diffusion in xNa2SO4(1-x)Na3PO4. This process predominantly involved thermally activated jumps between tetrahedrally coordinated sites, with the activation energies ranging from 0.64eV for x = 0.0 to 0.30eV for x = 0.5.

Sodium Ion Conduction in Plastic Phases - Dynamic Coupling of Cations and Anions in the Picosecond Range. D.Wilmer, H.Feldmann, R.E.Lechner, J.Combet: Journal of Materials Research, 2005, 20[8], 1973-8