Anion Rotation and Cation Diffusion in Low-Temperature Sodium Orthophosphate:  Results from Solid-State NMR

Abstract

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 17O 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 23Na and 31P 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 17O and 23Na NMR line shape and nutation analyses. The combined experimental evidence suggests strong dynamic coupling between anion and cation motion in low-temperature (LT-) Na3PO4.

Water molecular reorientation in ice and tetrahydrofuran clathrate hydrate from lineshape analysis of 17O spin-echo NMR spectra

“…Besides the problem of water molecular reorientations, there is a wide interest in the fundamental spectroscopy, that is, to observe and understand the evolution of spectral patterns for half-integer nuclei from a static pattern to one where the quadrupolar coupling is completely averaged. [38][39][40][41][42][43] Because of the ubiquitous presence of water in its various phases, we expect considerable relevance of these results to water-containing systems.…”

Simulations of molecular dynamics in solid-state NMR spectra of spin-1 nuclei including effects of CSA- and EFG-terms up to second order

“…For half-integer quadrupolar nuclei the effect of dynamics may be probed by analysis of the central transition [45][46][47][48][49]. This transition is not influenced by the first-order quadrupolar interaction but only by the secondorder quadrupolar interaction.…”

Re‐Entrant Phase Transition of the Crystalline Ion Conductor Ag7P3S11.

“…The potential role of anionic reorientations in triggering such phase transitions has been discussed for a number of ionically conducting rotor phases [34]. For Na 3 PO 4 such motions have recently been characterized in detail using temperature dependent 17 O NMR lineshape analysis [35]. Unfortunately, an analogous study of the present compound is precluded by the highly unfavorable NMR properties of the 33 S isotope.…”