Anion rotation and cation transport in the rotor phase alpha-sodium orthophosphate: Paddle-wheel mechanism redefined in view of new experimental results

Witschas M, Eckert H, Wilmer D, Banhatti RD, Funke K, Fitter J, Lechner RE, Korus G, Jansen M

Abstract

The high-temperature phase of sodium ortho-phosphate, alpha-Na3PO4, is characterized by a dynamic rotational disorder of its polyatomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, there has been considerable controversy about the question of whether both kinds of motion are dynamically coupled. To resolve this issue we have probed anionic and cationic motion individually over a wide range of experimental time scales. Coherent quasielastic neutron scattering as well as temperature-dependent O-17 NMR lineshape and relaxation spectroscopy serve to characterize the rotational motion of the anions, whereas the cation motion is probed by high-frequency conductivity and Na-23 NMR relaxation measurements. On the picosecond timescale, the combined interpretation of the neutron scattering and electrical conductivity data suggests strong dynamic coupling between the rotation of the phosphate groups about one of the four threefold P-O axes and the spatial fluctuations of nearby sodium ions. On more extended timescales, the NMR data indicate an additional, slower process, corresponding to dynamic jump reorientations of the C-3 axis of rotation. This process appears to be coupled to the translational Na+ transport dynamics as suggested by a strong correspondence between the O-17 and Na-23 NMR relaxation characteristics and the electrical conductivities in the de plateau region. The Na+ transport process can be viewed as highly correlated, not unlike the chain mechanism observed in AgBr.