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

Abstract

Two hydrated uranyl arsenates, Cs 2(UO 2)[(UO 2)(AsO 4)] 4(H 2O) 2 ( CsUAs) and Rb 2(UO 2)[(UO 2)(AsO 4)] 4(H 2O) 4.5 ( RbUAs), were synthesized by hydrothermal methods. Intensity data were collected at room temperature using Mo Kα radiation and a CCD-based area detector. The crystal structure of RbUAs was solved by direct methods, whereas the structure model of the phosphate Cs 2(UO 2)[(UO 2)(PO 4)] 4(H 2O) 2 was used for CsUAs; both were refined by full-matrix least-squares techniques on the basis of F 2 to agreement indices ( CsUAs, RbUAs) w R 2=0.061,0.041, for all data, and R 1=0.032,0.021, calculated for 5098, 4991 unique observed reflections (| F o|>4 σ F ), respectively. The compound CsUAs is orthorhombic, space group Cmc2 1, Z=4, a=15.157(2), b=14.079(2), c=13.439(2) Å, V=2867.9(1) Å 3. RbUAs is monoclinic, space group C2/ m, Z=4, a=13.4619(4), b=15.8463(5), c=14.0068(4) Å, β=92.311(1)°, V=2985.52(2) Å 3. The structures consist of sheets of arsenate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO 2)(AsO 4)] −, 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 H 2O 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 H 2O 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 H 2O groups; one Rb position and three of the interstitial H 2O 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.