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 PO4 3- and SO4 2- 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 PO4 3- by SO4 2- 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.

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

“…41 In the present study, although the THF hydrate residual secondorder lineshape has averaged to pseudo-isotropic, the material melts before any shifting towards the true isotropic shift has begun.…”

“…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.…”

Coupled anion and cation dynamics of silver orthophosphate in the picosecond range

“…Cation and anion dynamics in cubic Na 3 PO 4 have previously been analyzed by NMR techniques [11,12] and quasielastic neutron scattering [13][14][15][16]. Three dynamic processes could be detected in sodium orthophosphate: a fast anion reorientation around one of the C 3 axes parallel to 〈100〉, a slow reorientation of that C 3 axis and a jump diffusion process of sodium cations via empty lattice sites.…”

Second-order quadrupolar shifts as an NMR probe of fast molecular-scale dynamics in solids

“…As pointed out by Werbelow and London [7], however, it is better described as a quenching of the isotropic second-order quadrupolar shift by rapid tumbling. In solids, in contrast, the effect is less well known, although temperature-dependent isotropic quadrupolar shifts have been observed, especially in single-crystal and static-powder NMR studies of ionic conductors [8][9][10].…”