Acta Crystallographica Section C

Crystal Structure Communications

Volume 69, Part 4 (April 2013)

inorganic compounds

Acta Cryst. (2013). C69, 324-329    [ ]

MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data

A. D. Fortes, I. G. Wood and M. J. Gutmann

Abstract: Hexaaqua­magnesium(II) sulfate penta­hydrate, [Mg(H2O)6]SO4·5H2O, and hexa­aqua­magnesium(II) chromate(II) penta­hydrate, [Mg(H2O)6][CrO4]·5H2O, are isomorphous, being composed of hexa­aqua­magnesium(II) octa­hedra, [Mg(H2O)6]2+, and sulfate (chromate) tetra­hedral oxyanions, SO42- (CrO42-), linked by hydrogen bonds. There are two symmetry-inequivalent centrosymmetric octa­hedra: M1 at (0, 0, 0) donates hydrogen bonds directly to the tetra­hedral oxyanion, T1, at (0.405, 0.320, 0.201), whereas the M2 octa­hedron at (0, 0, {1 \over 2}) is linked to the oxyanion via five inter­stitial water mol­ecules. Substitution of CrVI for SVI leads to a substantial expansion of T1, since the Cr-O bond is approximately 12% longer than the S-O bond. This expansion is propagated through the hydrogen-bonded framework to produce a 3.3% increase in unit-cell volume; the greatest part of this chemically induced strain is manifested along the b* direction. The hydrogen bonds in the chromate compound mitigate ~20% of the expected strain due to the larger oxyanion, becoming shorter (i.e. stronger) and more linear than in the sulfate analogue. The bifurcated hydrogen bond donated by one of the inter­stitial water mol­ecules is significantly more symmetrical in the chromate analogue.

Formula: [Mg(H2O)6](SO4)·5H2O and [Mg(H2O)6](CrO4)·5H2O

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Structure factor file (CIF format) (412.2 kbytes)
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Contains datablock I

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Structure factor file (CIF format) (240.0 kbytes)
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Contains datablock II


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