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In Situ Characterization of Elusive Salt Hydrates-The Crystal Structures of the Heptahydrate and Octahydrate of Sodium Sulfate

Oswald, Iain D. H.; Hamilton, Andrea; Hall, Christopher; Marshall, William G.; Prior, Timothy J.; Pulham, Colin R.


Iain D. H. Oswald

Andrea Hamilton

Christopher Hall

William G. Marshall

Colin R. Pulham


An important intermediate phase in the crystallization of aqueous solutions of sodium sulfate is the highly metastable sodium sulfate heptahydrate (Na2SO4 center dot 7H(2)O). This has been structurally characterized for the first time by in situ single crystal X-ray diffraction. The crystal structure shows that each sodium cation is octahedrally coordinated to water molecules, with a slight distortion due to one of the water molecules being disordered. The hydrated sodium cations are hydrogen-bonded to form a three-dimensional bonded network, which is markedly different from the architecture of one-dimensional bonded chains observed in sodium sulfate decahydrate (mirabilite). This major structural difference explains the reconstructive nature of the transformation observed between the heptahydrate and mirabilite. High-pressure crystallization of a 3.41 mol/kg water aqueous solution of sodium sulfate at 1.54 GPa in a diamond-anvil cell resulted in the formation of a previously unknown sodium sulfate hydrate, which we have determined by single crystal X-ray diffraction methods to be an octahydrate, Na2SO4 center dot 8H(2)O. In this structure the sulfate ions are coordinated directly to sodium ions. This resembles anhydrous sodium sulfate (thenardite) but contrasts with the heptahydrate and decahydrate in which the sodium ions are coordinated exclusively by water molecules. This observation demonstrates how the delicate balance of inter- and intramolecular bonds in the crystal structure can be significantly altered by the application of pressure.


Oswald, I. D. H., Hamilton, A., Hall, C., Marshall, W. G., Prior, T. J., & Pulham, C. R. (2008). In Situ Characterization of Elusive Salt Hydrates-The Crystal Structures of the Heptahydrate and Octahydrate of Sodium Sulfate. Journal of the American Chemical Society, 130(52), 17795-17800.

Acceptance Date Feb 1, 2008
Publication Date Dec 31, 2008
Deposit Date Nov 13, 2014
Publicly Available Date Oct 27, 2022
Journal Journal Of The American Chemical Society
Print ISSN 0002-7863
Electronic ISSN 1520-5126
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 130
Issue 52
Pages 17795-17800
Keywords Neutron powder diffraction; X-ray-diffraction; High-pressure crystallization; Pressure porous materials; Polymorphs; Exploration; Decahydrate dependence behavior
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© 2008 American Chemical Society

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