Ion conducting rotor phases: new insights from quasielastic neutron scattering

Abstract Ion conducting rotor phases are high-temperature modifications of simple inorganic salts with typically cubic structure. Here the polyatomic tetrahedral anions like SO 4 2− or PO 4 3− show considerable rotational disorder, while the cations exhibit fast ion conductivity. A long lasting dispute concentrates 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., Na 3 PO 4 , Li 2 SO 4 , and LiNaSO 4 . Sodium ion diffusion in Na 3 PO 4 has been examined on the backscattering spectrometer IN16. Here the data can easily be interpreted on the basis of the Chudley–Elliott model, yielding reasonable values for jump distance, mean residence times, and diffusion coefficients. While in Na 3 PO 4 the linewidths due to anion reorientation (studied in an earlier time-of-flight measurement) and cation hopping differ by roughly three orders of magnitude, the situation in Li 2 SO 4 and LiNaSO 4 is completely different: here both types of motion have a similar time scale, and the time-of-flight experiment reveals both diffusional and reorientational contributions to the quasielastic scattering.