P-31 and Na-23 solid-state NMR studies of cation dynamics in HT-sodium orthophosphate and the solid solutions (Na2SO4)(x)-(Na3PO4)(1-x)

The high-temperature phases of sodium orthophosphate, HT-Na3PO4, and of the solid solutions (Na2SO4)(x)-(Na3PO4)(1-x) are characterized by their plast ic crystalline state with dynamically disordered PO43- and SO42- anions and a remarkably high cation conductivity. Since HT-Na3PO4 possesses a fully o ccupied cation sublattice (no vacancies), it has been proposed that cation transport and anion reorientations are dynamically coupled ("paddle-wheel m echanism"). However, no direct evidence for this coupling has been reported . In the present study, the validity of this mechanism is investigated on t he basis of Na-23 and P-31 nuclear magnetic resonance (NMR) experiments. Te mperature-dependent measurements of the static P-31 linewidth indicate that in the solid solutions with 0.04 less than or equal to x less than or equa l to 0.25 the acceleration of sodium ionic mobility is closely correlated w ith the acceleration of phosphate rotational motion, associated with a seco nd-order phase transition near 400 K. Temperature-dependent measurements of the 23Na longitudinal and transverse relaxation times have been analyzed u sing the theory of quadrupolar relaxation under nonextreme narrowing condit ions. Consistent with theoretical predictions sizeable dynamic frequency sh ifts are detected. All of the data are consistently analyzed quantitatively in terms of two distinct motional processes. A low-temperature process, wh ose relaxation strength is independent of sample composition, is clearly ac celerated by the onset of fast anion rotation occurring at the second-order phase transition temperature. In addition, a high-temperature process, whi ch is almost absent in HT-Na3PO4 but whose importance increases with increa sing sulfate content, signifies vacancy hopping. This dependence on composi tion is easily understood because the substitution of PO43- by SO42- genera tes cation vacancies. The activation energies of both processes are near 0. 45 eV, and the corresponding timescales grow increasingly similar with incr easing 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.