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 a nionic 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, t he dynamics of the low-temperature phase have been characterized comprehens ively by complementary NMR methods. Temperature-dependent O-17 NMR line sha pe analyses indicate that the phosphate ions undergo 3-fold rotation on the time scale of milliseconds. There appears to be one preferred axis of rota tion, however. Variable-temperature Na-23 and P-31 NMR spectra reveal furth er that the sodium cations exhibit considerable mobility. Both anionic and cationic motion appear to be jointly thermally activated and are characteri zed by correlation times of comparable magnitude. At temperatures about 70. K below the phase transition, diffuse diffraction peaks observed in X-ray p owder 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 w ithin this temperature range suggest that both the cations and the anions e xhibit strongly accelerated dynamics within these domains. The number of nu clei contributing to these domains are quantified on the basis of O-17 and Na-23 NMR line shape and nutation analyses. The combined experimental evide nce suggests strong dynamic coupling between anion and cation motion in low -temperature (LT-) Na3PO4.