Hydrogen bonding in coquimbite nominally FeSubsc

Authors: Juraj Majzlan Tamara Ðorđević Uwe Kolitsch Jürg Schefer

Publish Date: 2010/09/23

Volume: 100, Issue: 3-4, Pages: 241-248

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Abstract

Using singlecrystal Xray diffraction at 293 200 and 100 K and neutron diffraction at 50 K we have refined the positions of all atoms including hydrogen atoms previously undetermined in the structure of coquimbite P bar 31c a = 109242/108822 Å c = 170863 / 171543 Å V = 176583/175925 Å3 at 293 / 50 K respectively The use of neutron diffraction allowed us to determine precise and accurate hydrogen positions The O–H distances in coquimbite at 50 K vary between 098 and 101 Å In addition to H2O molecules coordinated to the Al3+ and Fe3+ ions there are rings of six “free” H2O molecules in the coquimbite structure These rings can be visualized as flattened octahedra with the distance between oxygen and the geometric center of the polyhedron of 246 Å The hydrogenbonding scheme undergoes no changes with decreasing temperature and the unit cell shrinks linearly from 293 to 100 K A review of the available data on coquimbite and its “dimorph” paracoquimbite indicates that paracoquimbite may form in phases closer to the nominal composition of Fe2SO43·9H2O Coquimbite on the other hand has a composition approximating Fe15Al05SO43·9H2O Hence even a “simple” sulfate Fe2x Al x SO43·9H2O may be structurally rather complexThe neutron diffraction was performed at Trics diffractometer at SINQ Paul Scherrer Institute PSI Villigen Switzerland This study was financially supported by the Deutsche Forschungsgemeinschaft grant MA 3927/21 We thank two anonymous reviewers for their constructive criticism and L Nasdala for the editorial handling of the manuscript

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