Anion reorientation in an ion conducting plastic crystal – coherent quasielastic neutron scattering from sodium ortho-phosphate

Abstract

A number of crystalline solids with polyatomic anions like SO 4 2− or PO 4 3− have high-temperature modifications which are both cation conductors and plastic phases, i.e., their anions exhibit dynamic rotational disorder. During the past decade, there has been a vigorous discussion about a possible cation mobility enhancement by dynamic coupling of cation migration and reorientation of the polyanions. Proponents of a strong interaction have coined the term “paddle–wheel” mechanism. We have performed a quasielastic neutron scattering study to investigate rapid anion reorientation in the high-temperature phase of sodium ortho-phosphate, α-Na 3PO 4, a plastic-phase fast-ion conductor. Here, the quasielastic scattering is dominated by oxygen scattering and is, therefore, almost entirely coherent. In the examined Q range, the quasielastic linewidth does not depend on Q, indicating the localized character of the observed motion. Our experimental results are compared with the predictions of four different models for the quasielastic part of the coherent scattering function for the oxygen atoms, S q (Q, ω) . According to our results, the reorientational dynamics can be described with a single time constant which is thermally activated with 0.184 eV. Our analysis shows that only three oxygen atoms per anion are participating in the rapid reorientational motion.