DIVALENT TRANSITION METALS AND MAGNESIUM IN STRUCTURES THAT CONTAIN THE AUTUNITE-TYPE SHEET

Abstract

Compounds that contain the autunite-type sheet and divalent cations (Mg, Mn, Fe, Co, Ni) in their interlayers have been synthesized by diffusion in gels or by hydrothermal methods, and their crystal structure determined. Data on single-crystal X-ray diffraction intensities were collected at room temperature using MoKα radiation and a CCD-based area detector. The autunite-type sheet has the composition [(UO2)(XO4)]–, X = P or As, and involves the sharing of equatorial vertices of uranyl square bipyramids with phosphate or arsenate tetrahedra. The interlayer region contains H2O groups and cations in octahedral coordination. The sheets are linked by hydrogen bonding and through bonds from the interlayer cations to oxygen atoms of the sheets. The structural roles of the interlayer cations in determining the symmetries and hydration states observed are discussed. Three different hydration states are observed, and these have characteristic symmetries and basal d-values: dodecahydrates are triclinic (pseudomonoclinic), basal spacing ~11 Å; decahydrates are monoclinic (pseudo-orthorhombic and commonly twinned), basal spacings ~10 Å, and octahydrates are triclinic, basal spacings of ~8.7 Å. Each hydration state corresponds to a different structure-type; the H2O of hydration of these compounds does not vary in the same fashion as the H2O content of zeolites, but rather is required to maintain the integrity of the hydrogen-bonding network.