Bestimmung der Kristallstruktur von T-Na₃PO₄ mit Röntgen- und Neutronenpulvertechniken

Abstract

The crystal structure of the low temperature modification of Na 3 P0 4 (T < 601 K) has been determined and refined by means of Rietveld's full-profile technique using X-ray and neutron powder diffraction data. T-Na 3 P0 4 is tetragonal with space group PA2 x c. The cell dimensions are a = 1080.84(6) pm, and c = 681.78(5) pm. At the phase transition the basic arrangement of the Na-and P-sites as compared to H-Na 3 P0 4 is not changed, and can be derived from the Li 3 Bi-and Na 3 AlF 6 -types of structure. In contrast to H-Na 3 P0 4 the P0 4 -groups are completely ordered with respect to their orientations. EinleitungNa 3 P0 4 gehört zu den am längsten bekannten anorganischen Salzen. Gleichwohl blieben Temperaturpolymorphie und Kristallstrukturen bis in die jüngste Vergangenheit umstritten bzw. unbekannt. So wurde von verschiedenen Autoren über die Existenz von bis zu vier verschiedenen Modifikationen berichtet, wobei weder bezüglich der Umwandlungstemperaturen noch der aus Pulverdaten abgeleiteten Kristallsysteme Übereinstimmung erzielt werden konnte (Palazzi und Remy, 1971; Kizilyalli und Welch, 1976;Kolsi, 1976).Eine wesentliche Ursache für die widersprüchlichen Ergebnisse dürfte in dem unterschiedlichen Reinheitsgrad der aus wäßrigen Lösungen hergestellten Präparate liegen. Die bisher reinsten Proben wurden auf festkörperchemischem Wege aus Na 2 0 und Na 4 P 2 0 7 erhalten (Wiench und Jansen, 1980). So dargestelltes Na 3 P0 4 zeigt nur noch eine reversible,

Dipolar Coupling Information in Multispin Systems: Application of a Compensated REDOR NMR Approach to Inorganic Phosphates

“…Na 3 PO 4 was prepared by solid-state reaction of Na 2 CO 3 and Na 4 P 2 O 7 using a previously reported procedure (21). The samples were characterized by standard 27 Al and 31 P MAS-NMR spectroscopy, reproducing the re- sults reported in the literature (20,22,23 27 Al, 23 Na, and 31 P, respectively, on a Bruker DSX 500 spectrometer, equipped with a 4-mm Bruker double resonance probe.…”

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

“…Sample Preparation and Characterization. Powder samples of pure Na 3 PO 4 were obtained by solid-state reaction of Na 2 -CO 3 and Na 4 P 2 O 7 at 800 °C for 48 h. 14 Sample purity was confirmed by X-ray diffraction. Samples were kept dry during all stages of handling and measurement.…”

“…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. There are five distinct sites for the sodium cations, which have different occupancy factors, and whose individual line shapes can be resolved by 23 Na high-resolution solid-state NMR.…”

31P and 23Na Solid-State NMR Studies of Cation Dynamics in HT-Sodium Orthophosphate and the Solid Solutions (Na2SO4)x−(Na3PO4)1-x†

“…The small discrepancy is attributed to a contribution arising from the chemical shift anisotropy, since it is known that in low-temperature sodium orthophosphate the PO 4 3anions show slight deviations from their ideal tetrahedral symmetry. 28 As the temperature is increased, motional narrowing effects become clearly observable in the static 31 P NMR spectra: the linewidth is reduced and the spectra attain partial Lorentzian character. Finally, in the cubic phase formed above the phase transition temperature, a constant Gaussian linewidth of 850 Hz is measured.…”

“…In LT-Na 3 PO 4 and in the solid solutions, however, a small contribution may be expected because the cubic local symmetry is broken in these materials. 28 As Na diffusion is thermally activated, the linewidth is gradually diminished as the correlation time τ c characterizing the cation motion becomes comparable to ∆ -1 . For τ c ∆ , 1, the contribution (b) in eq 1 is completely averaged away, and the static 31 P lineshape is now dominated by the homonuclear contribution (a) between the stationary 31 P spins.…”