Hydrogen bonding in coquimbite, nominally Fe_2(SO_4)_3·9H_2O, and the relationship between coquimbite and paracoquimbite

摘要

Using single-crystal X-ray 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 (P31c, a=10.924(2)/10.882(2) A, c=17.086(3) / 17.154(3) A, V=1765.8(3)/1759.2(5) A~3, 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 0.98 and 1.01 A. In addition to H_2O molecules coordinated to the Al~(3+)and Fe~(3+)ions, there are rings of six “free” H_2O 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 2.46 A. The hydrogen-bonding 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 Fe_2(SO_4)_3·9H_2O. Coquimbite, on the other hand, has a composition approximating Fe_(1.5)Al_(0.5)(SO_4)_3·9H_2O. Hence, even a “simple” sulfate Fe_(2-x)Al_x(SO_4)_3·9H_2O may be structurally rather complex.