Anion rotation and cation diffusion in low-temperature sodium orthophosphate: Results from solid-state NMR

Numerous ionic crystals are known in which both cations and anions possess considerable mobility in the solid state. During the past decade, there has been considerable controversy about the question of whether cationic and anionic motion can be dynamically coupled in such materials. This issue has been studied recently on the plastic crystalline material high-temperature (HT-) Na3PO4, which forms from the low-temperature modification in a first-order phase transition at a temperature near 600 K. In the present study, the dynamics of the low-temperature phase have been characterized comprehensively by complementary NMR methods. Temperature-dependent O-17 NMR line shape analyses indicate that the phosphate ions undergo 3-fold rotation on the time scale of milliseconds. There appears to be one preferred axis of rotation, however. Variable-temperature Na-23 and P-31 NMR spectra reveal further that the sodium cations exhibit considerable mobility. Both anionic and cationic motion appear to be jointly thermally activated and are characterized by correlation times of comparable magnitude. At temperatures about 70.K below the phase transition, diffuse diffraction peaks observed in X-ray powder diffraction data indicate the appearance of local clusters possessing the symmetry of the high-temperature phase. The strongly increased thermal volume expansion coefficient and the observation of excess specific heat within this temperature range suggest that both the cations and the anions exhibit strongly accelerated dynamics within these domains. The number of nuclei contributing to these domains are quantified on the basis of O-17 and Na-23 NMR line shape and nutation analyses. The combined experimental evidence suggests strong dynamic coupling between anion and cation motion in low-temperature (LT-) Na3PO4.