Natrophosphate, Arctic Mineral and Nuclear Waste Phase: Structure Refinements and Chemical Variability

Abstract

The crystal structures of natural (Mt. Koashva, Khibiny alkaline massif, Kola Peninsula, Russian Arctic) and synthetic (obtained from an aqueous solution of sodium phosphate and sodium fluoride (1:1) by evaporation at room temperature (RT)) natrophosphate, Na7(PO4)2F·19H2O, have been investigated using single-crystal X-ray diffraction analysis. Natrophosphate and its synthetic analogue are cubic, Fd-3c, a = 27.6942(3) Å (natrophosphate at RT), a = 27.6241(4) Å (natrophosphate at 100 K), a = 28.1150(12) Å (synthetic analogue at RT), a = 27.9777(7) Å (synthetic analogue at 100 K). The crystal structure is based upon the super-octahedral [Na6(H2O)18F]5+ polycationic complexes consisting of six edge-linked Na6(OH2)5F octahedra sharing one common fluorine vertex. The A site is statistically occupied by Na and H2O with the prevalence of H2O with the refined occupancy factors O:Na equal to 0.53:0.47 for natrophosphate and 0.75:0.25 for its synthetic analogue. The coordination of the A site in synthetic natrophosphate is enlarged compared to the natural sample, which agrees well with its higher occupancy by H2O molecules. The general formula of natrophosphates can be written as Na6+xHxF(PO4)2·(19 + x)H2O, where x = 0–1. The chemical variability of natrophosphate allows to explain the discrepancies in its solubility reported by different authors. The information-based parameters of structural complexity are equal to 3.713 bit/atom and 2109.177 bit/cell that allows to classify natrophosphate as a structurally very complex mineral.

Sodium site occupancy and phosphate speciation in natrophosphate are invariant to changes in NaF and Na3PO4 concentration

“…While Rietveld refinement is a technique to measure site occupancies, many other groups in literature have analyzed natro-phosphate with X-ray diffraction and subsequential refinements. 13,19,32 Despite this, because the X-ray scattering contribution of light atoms, such as H+, are low, the conclusiveness of refinements of XRD data in discriminating HPO 4 2− from PO 4 3− in the natrophosphate salt is limited.…”

“…1 H, 19 respectively. The solid-state MAS NMR spectra of the natrophosphate crystals were acquired with a 2.5 mm HXY probe operating in double resonance mode.…”

“…Multinuclear MAS NMR spectroscopy was conducted to characterize the local chemical environment of H 2 O, F − , PO 4 3− and Na + ions in natrophosphate crystals grown in F-rich or F-poor solution conditions via 1 H, 19 F, 31 P and 23 Na, respectively. The 1 H MAS NMR spectra of the natrophosphate samples is shown in Fig.…”

“…27 Likewise, the composition of natrophosphate has been hypothesized to be variable due to partial replacement of the PO 4 3− groups in the structure with H x PO 4 3−x (x = 1 or 2). 13,19 Substitution of hydrogen into phosphates containing mono and trivalent cations to occurs in other complex phosphates precipitated from acidic solutions, 28 which is explained since hydrogen phosphate and dihydrogen phosphate are the predominant species at low pH conditions. 29 If occurring in natrophosphate crystals, the formation of the H x PO x 3−x would necessitate a reduction in Na + in site 2 to satisfy the balance of the charge, specifically resulting in the postulated stoichiometry of natrophosphate occurring as Na 7−x H x FPO 4 •19H 2 O.…”

“…16 Based on the size of the unit cell of natrophosphate, the complexity of the hydrogen bond network of water, and partial occupancy of sodium, the structure of natrophosphate is quantitively classified 17 as structurally very complex based on information-based parameters. 18,19 0.25), an NaF solution was prepared by the dissolution of 0.8398 g of NaF salt into 20 g of deionized water and an Na 3 PO 4 solution was prepared by the dissolution of 13.1152 g of Na 3 PO 4 •12H 2 O salt in 80 g of deionized water. The solutions were separately heated on a hotplate to a temperature of 70 °C measured by thermometer and then mixed with vigorous stirring.…”