No abstract

The high-temperature phase of sodium ortho-phosphate, A-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 17 O 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 23 Na 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 authors.

The high-temperature phases of sodium orthophosphate, HT-Na3PO4, and of the solid solutions (Na2SO4) x −(Na3PO4)1 - x are characterized by their plastic crystalline state with dynamically disordered PO4 3- and SO4 2- anions and a remarkably high cation conductivity. Since HT-Na3PO4 possesses a fully occupied cation sublattice (no vacancies), it has been proposed that cation transport and anion reorientations are dynamically coupled (“paddle-wheel mechanism”). However, no direct evidence for this coupling has been reported. In the present study, the validity of this mechanism is investigated on the basis of 23Na and 31P nuclear magnetic resonance (NMR) experiments. Temperature-dependent measurements of the static 31P linewidth indicate that in the solid solutions with 0.04 ≤ x ≤ 0.25 the acceleration of sodium ionic mobility is closely correlated with the acceleration of phosphate rotational motion, associated with a second-order phase transition near 400 K. Temperature-dependent measurements of the 23Na longitudinal and transverse relaxation times have been analyzed using the theory of quadrupolar relaxation under nonextreme narrowing conditions. Consistent with theoretical predictions sizeable dynamic frequency shifts are detected. All of the data are consistently analyzed quantitatively in terms of two distinct motional processes. A low-temperature process, whose relaxation strength is independent of sample composition, is clearly accelerated by the onset of fast anion rotation occurring at the second-order phase transition temperature. In addition, a high-temperature process, which is almost absent in HT-Na3PO4 but whose importance increases with increasing sulfate content, signifies vacancy hopping. This dependence on composition is easily understood because the substitution of PO4 3- by SO4 2- generates cation vacancies. The activation energies of both processes are near 0.45 eV, and the corresponding timescales grow increasingly similar with increasing sodium sulfate content. Altogether, the results give strong evidence for a dynamic coupling between anionic reorientation and cation diffusion, supporting the concept of a paddle-wheel mechanism.

No abstract

The high-temperature phase of sodium ortho-phosphate, α-Na3PO4, belongs to the class of ion conducting plastic crystals, i.e., it is characterized by a dynamic rotational disorder of its poly-atomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, the possibility, nature, and importance of a dynamic interplay between the two kinds of motion have been a subject of continued controversy. Proponents of a strong interplay coined the expression “paddle-wheel mechanism”. In our present contribution we report, for the first time, on the results of dynamic experiments probing the elementary steps of anionic and cationic motion individually. The techniques utilized in this study are coherent quasielastic neutron scattering and high-frequency conductivity spectroscopy, respectively. The data are complemented by an ab-initio molecular-dynamics simulation. Our results provide a view of the movement of anions and cations and of correlations between them. Strong dynamic coupling is detected between the octahedrally coordinated sodium ions and nearby oxygen ions. For translational sodium-ion transport, a chain mechanism appears to be operative.