Anion Rotation and Cation Transport in the Rotor Phase α -Sodium Orthophosphate: Paddle-Wheel Mechanism Redefined in View of New Experimental Results

Abstract

The high-temperature phase of sodium ortho-phosphate, A-Na3PO4, is characterized by a dynamic rotational disorder of its polyatomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, there has been considerable controversy about the question of whether both kinds of motion are dynamically coupled. To resolve this issue we have probed anionic and cationic motion individually over a wide range of experimental time scales. Coherent quasielastic neutron scattering as well as temperature-dependent 17 O NMR lineshape and relaxation spectroscopy serve to characterize the rotational motion of the anions, whereas the cation motion is probed by high-frequency conductivity and 23 Na NMR relaxation measurements. On the picosecond timescale, the combined interpretation of the neutron scattering and electrical conductivity data suggests strong dynamic coupling between the rotation of the phosphate groups about one of the four threefold P-O axes and the spatial fluctuations of nearby sodium ions. On more extended timescales, the NMR data indicate an additional, slower process, * Corresponding authors.

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.…”

Cation Mobility and Anion Reorientation in Sodium Trifluoromethyl Sulfonate

“…Two recent works produced strong evidence for the existence of the paddle-wheel mechanism to be active in the high-temperature modifications of Na 3 PO 4 and LiSO 3 CF 3 . In HÀNa 3 PO 4 , the similar correlation times for the PO 4 3À reorientation about the C 2 axis, as determined by 17 O spin lattice relaxation (T 1 ) NMR spectroscopy, and for the sodium transport, as determined with 23 Na T 1 NMR spectroscopy and conductivity measurements, [13] indeed suggested a strong coupling between these two motions. In b-LiSO 3 CF 3 , the identical activation energies for the triflate anion reorientation ( 17 O NMR spectroscopy) and lithium transport ( 7 Li NMR spectroscopy and conductivity measurements) [14] indicated the same effect to be active.…”

“…A third possible motion of the anion is the isotropic reorientation around the center of gravity of the anion. This motion can be studied via its effect on the 17 O NMR line shape or by the 13 C chemical shift anisotropy (CSA) tensor of the CF 3 group. Since the sodium cations are coordinated by the oxygen atoms of the SO 3 groups of the triflate anion, the analysis of the SO 3 reorientation holds the most promise with respect to a study of a possible coupling of the anion and cation motion.…”

Solid State Ionics: from Michael Faraday to green energy—the European dimension

“…On the basis of time-resolved experimental results for the localized cationic and anionic movements, the paddle-wheel mechanism could later be redefined for α -Na 3 PO 4 , see e.g. Wilmer et al (Germany and France) [312] and Witschas et al (Germany) [313]. …”