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Published online by Cambridge University Press: 01 February 2011
Results of simple computer simulations and model calculations for ion conducting rotor phases are compared to quasi-elastic neutron scattering data from solid solutions of sodium orthophosphate and sodium sulphate, xNa2SO4 · (1 — x)Na3PO4. These materials are not only sodium fast-ion conductors in their high-temperature cubic phases but also show considerable dynamic reorientation disorder of their tetrahedral anions.
At an elastic energy resolution of about 100 μeV, neutron spectrometry monitored oxygen scattering due to anion reorientation which occurs on the picosecond time scale. This thermally activated process exhibits activation energies between 0.184 eV (x = 0. 0) and 0.052 eV (x = 0.5). Analysis of the quasielastic intensities as a function of scattering vector Q gives clear evidence of the involvement of cations in the anion reorientation.
Increasing the elastic resolution to about 1 μeV FWHM (thereby shifting the dynamic window to the nanosecond scale) allowed to examine sodium diffusion in x Na2SO4 · (1 — x)Na3PO4. This process consists predominantly of thermally-activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64 eV for x = 0.0 to 0.30 eV for x = 0.5.