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-raydiffraction intensities were collected at room temperature using MoK␣ radiation and a CCD-based area detector. The autunitetype sheet has the composition [(UO 2)(XO 4)]-, X = P or As, and involves the sharing of equatorial vertices of uranyl square bipyramids with phosphate or arsenate tetrahedra. The interlayer region contains H 2 O 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

[Hg5O2(OH)4][(UO2)2(AsO4)2]: A complex mercury(II) uranyl arsenate

“…2 shows the interactions of the UO 2þ 2 cations and Hg(1) that provides one method for interconnecting the layers. Previous reports show that the apical (uranyl) vertices of the uranyl bipyramids can be shared with polyhedra containing higher-valence cations, even though this would over-bond the oxygen position [13]. However, long uranyl oxo interactions with interlayer cations are well represented [13].…”

“…Previous reports show that the apical (uranyl) vertices of the uranyl bipyramids can be shared with polyhedra containing higher-valence cations, even though this would over-bond the oxygen position [13]. However, long uranyl oxo interactions with interlayer cations are well represented [13]. The apical (uranyl) oxygen O(9) provides the m 2 oxygen atom that bridges to Hg (1).…”

“…Of late there has been interest in expanding structural diversity and physico-chemical properties of uranyl phosphates and arsenates through the incorporation of main group elements and transition metals [12][13][14]. One possibility is that the second metal center could display mixed-valence on a stoichiometric level, but this has yet to be observed in compounds in this class.…”

XPS spectra of uranyl minerals and synthetic uranyl compounds. II: The O 1s spectrum

“…Structural studies on different hydrated minerals of the autunite group show that many of the interstitial (H 2 O) groups, not bonded to interstitial cations, are more likely to be released in air (Locock and Burns, 2003a,b;Locock et al, 2004). On the basis of this observation, one would expect that:…”

“…Surface-atmosphere interactions, such as hydration and dehydration are important for the stability of environmentally relevant uranyl minerals such as minerals of the schoepite-and autunite group (Finch et al, 1996(Finch et al, , 1998Locock and Burns, 2003a,b;Locock et al, 2004). Surface-water and surface-atmosphere interactions involve transformation, incorporation or release of different types of O species.…”