Interplay between Anion Rotation and Cation Transport in the Plastic High-Temperature Phase of Sodium Ortho-Phosphate

Abstract

The high-temperature phase of sodium ortho-phosphate, α-Na3PO4, belongs to the class of ion conducting plastic crystals, i.e., it is characterized by a dynamic rotational disorder of its poly-atomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, the possibility, nature, and importance of a dynamic interplay between the two kinds of motion have been a subject of continued controversy. Proponents of a strong interplay coined the expression “paddle-wheel mechanism”. In our present contribution we report, for the first time, on the results of dynamic experiments probing the elementary steps of anionic and cationic motion individually. The techniques utilized in this study are coherent quasielastic neutron scattering and high-frequency conductivity spectroscopy, respectively. The data are complemented by an ab-initio molecular-dynamics simulation. Our results provide a view of the movement of anions and cations and of correlations between them. Strong dynamic coupling is detected between the octahedrally coordinated sodium ions and nearby oxygen ions. For translational sodium-ion transport, a chain mechanism appears to be operative.

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

“…This question arises because the X-ray diffraction experiments reveal a particularly short OϪNa distance along the cubic crystal axes involving those sodium ions that are octahedrally surrounded by phosphate groups. Indeed, preliminary Car-Parrinello Molecular-Dynamics calculations do suggest that the mean square displacements of these octahedral cations are somewhat higher than those of the Na ϩ ions in the tetrahedral interstices [54]. On the other hand, the NMR data show no clear evidence for this effect.…”

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

“…Most recently, a strong case in favor of the paddle-wheel mechanism has been presented for the high-temperature phase of sodium orthophosphate, HT-Na 3 PO 4 . From a combination of NMR, inelastic neutron scattering, and electrical conductivity measurements it could be shown that anion rotation and cation transport appear to be highly correlated in this material over a wide range of time and temperature scales. , …”

“…From a combination of NMR, inelastic neutron scattering, and electrical conductivity measurements it could be shown that anion rotation and cation transport appear to be highly correlated in this material over a wide range of time and temperature scales. 12,13 The present study is devoted to the low-temperature phase of Na 3 PO 4 which is thermodynamically stable at temperatures below 600 K. The crystal structure of this compound was solved from X-ray and neutron powder diffraction data in the space group P4 h21c. 14 The unit cell contains eight equivalent phosphate tetrahedra whose orientations are perfectly ordered, as illustrated in Figure 1.…”

Crystal Structure and Ionic Conductivity of Three Polymorphic Phases of Rubidium Trifluoromethyl Sulfonate, RbSO3CF3

“…Some salts with complex anions (e.g., Li 2 SO 4 , Na 3 PO 4 − ) show phase transitions into a dynamically disordered high-temperature phase (rotor phase) which can be accompanied by a drastic increase in the respective cationic conductivity, caused by the ‘volume effect', , as well as by the ‘paddle-wheel mechanism' . This feature makes the alkali salts of the trifluoromethyl sulfonic acid an interesting class of rotor phases according to the various degrees of rotational freedom of the triflate anion compared to, e.g., the phosphate anion.…”