Influence of extreme conditions on the formation and structures of caesium uranium(vi) arsenates. Issue 47 (16th November 2015) - Yu, Na
Abstract : A new family of uranium(vi ) arsenates was obtained under extreme conditions of pressure and temperature. The high pressure phases demonstrate significant differences in chemical compositions and structural properties. Abstract : Four new uranyl arsenates, Cs2 [(UO2 )(As2 O7 )] (1 ), α-Cs[(UO2 )(HAs2 O7 )] (2 ), β-Cs[(UO2 )(HAs2 O7 )] (3 ), Cs[(UO2 )(HAs2 O7 )]·0.17H2 O (4 ), were synthesized by high-temperature/high pressure (HT/HP) reactions at 900 °C and 3 GPa. These phases were subsequently characterised structurally as well as chemically. We demonstrated that compound1 can also be obtained at ambient pressure. Compounds1, 2, and4 are based on two-dimensional (2D) anionic layers with two different topological types. The layers possess a similar composition, [(UO2 )(As2 O7 )] 2− in1 and [(UO2 )(HAs2 O7 )] − in2 and4 . However, the presence of hydrogen in2 and4 leads to a change in coordination modes of the pyroarsenate groups. There are additional 0.17 H2 O molecules per formula unit in4, which cause slight distortions of the layers in4 . All these layers can be simplified to a common net, which is typical of autunite-like layered compounds. Compound3 is a polymorph of compound2, but the structural arrangements in these two are significantly different. The structure of3 is based upon a three-dimensional (3D) framework, in which UO7 is coordinated by arsenate groups in order to form uranyl anion sheets, and UO6 is located within the interlayers. Bond valance analysis proved the presence of OH − groups in compounds2, 3, and4, respectively, and water molecules in4 . The Raman analyses enabled the study of the local environments of the arsenate and the uranyl groups within the investigated phases, respectively. It turned out that the applied HT/HP synthesis method strongly affects the crystal chemistry as well as the observed structural features of all obtained compounds.