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

A number or crystalline solids with polyatomic anions like SO42- or PO43- h ave high-temperature modifications which are both cation conductors and pla stic phases, i.e., their anions exhibit dynamic rotational disorder. During the past decade, there has been a vigorous discussion about a possible cat ion mobility enhancement by dynamic coupling of cation migration and reorie ntation of the polyanions. Proponents of a strong interaction have coined t he term "paddle-wheel" mechanism. We have performed a quasielastic neutron scattering study to investigate rapid anion reorientation in the high-tempe rature phase of sodium ortho-phosphate, alpha-Na3PO4, a plastic-phase fast- ion conductor. Here, the quasielastic scattering is dominated by oxygen sca ttering and is, therefore, almost entirely coherent. In the examined Q rang e, the quasielastic linewidth does not depend on Q, indicating the localize d character of the observed motion. Our experimental results are compared w ith the predictions of four different models for the quasielastic part of t he coherent scattering function for the oxygen atoms, S-q(Q, omega). Accord ing to our results, the reorientational dynamics can be described with a si ngle time constant which is thermally activated with 0.184 eV. Our analysis shows that only three oxygen atoms per anion are participating in the rapi d reorientational motion. (C) 1999 Elsevier Science B.V. All rights reserve d.