MONOVALENT CATIONS IN STRUCTURES OF THE META-AUTUNITE GROUP

Abstract

Compounds of the meta-autunite group containing monovalent cations (Li, Na, K, Rb, Cs, Ag, TI) have been synthesized by diffusion in gels or by hydrothermal methods, and their crystal structures determined. Single-crystal X-ray-diffraction intensity data were collected at room temperature using MoKalpha radiation and a CCD-based area detector. These compounds contain the autunite-type sheet with composition [(UO2)(XO4)](-), X = P or As, which involves the sharing of equatorial vertices of uranyl square bipyramids with tetrahedra. The interlayer region contains cations and H2O groups, and 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. The smallest monovalent cation, Li, occurs in tetrahedral coordination between fourfold squares of hydrogen-bonded H2O groups. Despite a wide range in ionic radius, Na, K, Rb, Ag and TI randomly substitute for H2O groups in the interlayer, in the same fashion as their ammonium and oxonium analogues. The large Cs cation adopts independent crystallographic sites in the interlayer. The size difference between Cs and the other monovalent cations probably prevents their direct substitution, and may limit the extent of solid solution. With the exception of the Rb and Cs compounds, chemically corresponding uranyl phosphates and uranyl arsenates are isostructural. The structural similarity of Rb[(UO2)(AsO4)](H2O)(3) with metazeunerite, Cu[(UO2)(AsO4)](2)(H2O)(8), may indicate a mechanism of solid solution for monovalent and divalent interlayer cations in the meta-autunite group.