Novel layered uranyl arsenates, Ag6[(UO2)2(As2O7)(As4O13)] and A6I[(UO2)2(AsO4)2(As2O7)] (AI-Ag and Na): first observation of a linear As4O136- anion and structure type evolution

Three new layered uranyl arsenates with mixed arsenate anions have been prepared by high-temperature solid-state reactions: Ag-6[(UO2)(2)(As2O7)(As4O13)](1), Ag-6[(UO2)(2)(AsO4)(2)(As2O7)] (2), Na-6[(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.