The structures of two sodium uranyl compounds relevant to nuclear waste disposal

Single crystals of two sodium uranyl phases, Na2[(UO2)3O3(OH)2] and Na4(UO2)2(Si4O10)2(H2O)4 (designated NAUOH and NAURSI, respectively), were hydrothermally synthesized. Na uranyl oxyhydroxide (NAUOH) is monoclinic, space group P21/n, a=0.70476(3) nm, b=1.14126(6) nm, c=1.20274(6) nm, β=90.563(1)°, V=0.96733(8) nm3. The structure of NAUOH was solved by direct methods and refined on the basis of F2 for 4027 unique reflections measuring using Mo Kα X-radiation and a CCD-based Smart APEX detector. The agreement index (R1) is 2.7%, calculated using 2955 unique observed reflections (|Fo|⩾4σF), and the goodness-of-fit is 0.68. The structure contains uranyl pentagonal bipyramids that share vertices and edges, forming sheets parallel to (0 1 0). The sheets are linked by double chains of Na polyhedra located in the interlayer. NAUOH is a Na analogue of the mineral compreignacite, and may correspond to the Na uranyl oxide hydrate formed in studies of the oxidative dissolution of spent nuclear fuel. The phase Na4(UO2)2(Si4O10)2(H2O)4 (NAURSI) is the Na analogue of KNa3(UO2)2(Si4O10)2(H2O)4, which was found as an alteration phase on hydrothermally treated actinide-bearing borosilicate waste glass. It is monoclinic, space group C2/m, a=1.2770(1) nm, b=1.3610(1) nm, c=0.82440(8) nm, β=119.248(2)°, V=1.2501(2) nm3. The structure of NAURSI was solved by direct methods and refined on the basis of F2 for 1483 unique reflections measured with Mo Kα X-radiation and a CCD-based Smart 1 K detector. The agreement index (R1) is 1.9%, calculated using 1333 unique observed reflections (|Fo|⩾4σF), and the goodness-of-fit is 1.08. The structure is based on a uranyl silicate framework, with Na cations and H2O groups located in interstitial sites. The structure is closely related to that of KNa3(UO2)2(Si4O10)2(H2O)4.