Novel layered uranyl arsenates, Ag6[(UO2)2(As2O7)(As4O13)] and AI6[(UO2)2(AsO4)2(As2O7)] (AI–Ag and Na): first observation of a linear As4O136− anion and structure type evolution

Abstract

Three new layered uranyl arsenates with mixed arsenate anions have been prepared by high-temperature solid-state reactions: Ag₆[(UO₂)₂(As₂O₇)(As₄O₁₃)] (1), Ag₆[(UO₂)₂(AsO₄)₂(As₂O₇)] (2), Na₆[(UO₂)₂(AsO₄)₂(As₂O₇)] (3). The crystal structures of the synthesized compounds have been solved by direct methods, and found to be based upon 2-D anionic layers with complex topology. The layers have composition [(UO₂)₂(As₂O₇)(As₄O₁₃)]⁶⁻ in 1 and [(UO₂)₂(AsO₄)₂(As₂O₇)]⁶⁻ in 2 and 3. In the structure of 1, UO₇ pentagonal bipyramids share corners with linear As₂O₇⁴⁻ and zigzag-like As₄O₁₃⁶⁻. Compounds 2 and 3 are isostructural and based upon corner-sharing between UO₇ pentagonal bipyramids, AsO₄³⁻ and As₂O₇⁴⁻groups. The A⁺ cations (Ag⁺ in 1 and 2, Na⁺ in 3) are in between the layers. The zigzag-like tetramer [As₄O₁₃]⁶⁻ in the structure of 1 is the first example of a linear tetramer in arsenate chemistry.