Ion conducting rotor phases - new insights from quasielastic neutron scattering

Ion conducting rotor phases are high-temperature modifications of simple in organic salts with typically cubic structure. Here the polyatomic tetrahedr al anions like SO42- or PO43- show considerable rotational disorder, while the cations exhibit fast ion conductivity. A long lasting dispute concentra tes on the question if and how the anion reorientational motion influences or enhances the cation transport in these materials. We report new results from quasielastic neutron scattering experiments on several ion conducting rotor phases, viz., Na3PO4, Li2SO3, and LiNaSO4. Sodium ion diffusion in Na 3PO4 has been examined on the backscattering spectrometer IN16. Here the da ta can easily be interpreted on the basis of the Chudley-Elliott model, yie lding reasonable values for jump distance, mean residence times, and diffus ion coefficients. While in Na3PO4 the linewidths due to anion reorientation (studied in an earlier time-of-flight measurement) and cation hopping diff er by roughly three orders of magnitude, the situation in Li2SO4 and LiNaSO 4 is completely different: here both types of motion have a similar time sc ale, and the time-of-flight experiment reveals both diffusional and reorien tational contributions to the quasielastic scattering. (C) 2001 Elsevier Sc ience B.V. All rights reserved.