31P and 23Na Solid-State NMR Studies of Cation Dynamics in HT-Sodium Orthophosphate and the Solid Solutions (Na2SO4)x−(Na3PO4)1-x†

Abstract

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 PO43- and SO42- 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 PO43- by SO42- 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.