Ion dynamics in solid solutions of sodium phosphate and sodium sulfate

Abstract

Time-of-flight quasielastic neutron scattering experiments have been performed on solid solutions of Na₂SO₄ and Na₃PO₄ in order to examine the reorientational anion motion in the picosecond range. The obtained data were analysed on the basis of the Q-dependent quasielastic intensity. Comparing the predictions of different models for the coherent quasielastic scattering of individual rotating units with the experimental results gives a clear indication that the observed motion is performed by three oxygen atoms which rotate on a circle, while the fourth oxygen atom, at least on the time scale defined by the experiment, remains fixed. The quasielastic intensity in the low-Q regime is well explained by the involvement of cations in the anion reorientation. Quantitative agreement between model predictions and experimental data was obtained under the assumption that only the phosphate (and not the sulfate) ion rotation is observed in our experiment. The results strongly support the view of a dynamic coupling between cations and anions in the Na₂SO₄–Na₃PO₄ system.