Structures and synthesis of framework Rb and Cs uranyl arsenates and their relationships with their phosphate analogues

Two hydrated uranyl arsenates, Cs2(UO2)[(UO2)(AsO4)]4(H2O)2 (CsUAs) and Rb2(UO2)[(UO2)(AsO4)]4(H2O)4.5 (RbUAs), were synthesized by hydrothermal methods. Intensity data were collected at room temperature using MoKα radiation and a CCD-based area detector. The crystal structure of RbUAs was solved by direct methods, whereas the structure model of the phosphate Cs2(UO2)[(UO2)(PO4)]4(H2O)2 was used for CsUAs; both were refined by full-matrix least-squares techniques on the basis of <f>F2</f> to agreement indices (CsUAs, RbUAs) <f>wR2=0.061,0.041</f>, for all data, and <f>R1=0.032,0.021</f>, calculated for 5098, 4991 unique observed reflections (<f>|Fo|>4σF</f>), respectively. The compound CsUAs is orthorhombic, space group <f>Cmc21</f>, <f>Z=4</f>, <f>a=15.157(2)</f>, <f>b=14.079(2)</f>, <f>c=13.439(2)</f> A˚, <f>V=2867.9(1)</f> A˚3. RbUAs is monoclinic, space group <f>C2/m</f>, <f>Z=4</f>, <f>a=13.4619(4)</f>, <f>b=15.8463(5)</f>, <f>c=14.0068(4)</f> A˚, <f>β=92.311(1)°</f>, <f>V=2985.52(2)</f> A˚3. The structures consist of sheets of arsenate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(AsO4)]−, that are topologically identical to the uranyl silicate sheets in uranophane-beta. These sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with two arsenate tetrahedra on each of two adjacent sheets and whose fifth equatorial vertex is an H2O group, resulting in an open framework with alkali metal cations in the larger cavities of the structures. CsUAs is isostructural with its phosphate analogue, and has two Cs atoms and a H2O group in its structural cavities. RbUAs is not isostructural with its phosphate analogue, although it has a homeotypic framework. Its structural cavities are occupied by three Rb atoms and four H2O groups; one Rb position and three of the interstitial H2O groups are half-occupied. The partial occupancies of these positions probably result from the accommodation of the larger As atoms (relative to P) in the framework and resultant larger cavities. [Copyright &y& Elsevier]