The structures of becquerelite and Sr-exchanged becquerelite

Abstract

The crystal structures of becquerelite, Ca[(UO2)6O4(OH)6](H2O)8, and Sr-exchanged becquerelite obtained by ion exchange, Sr1.27[(UO2)3O3.54(OH)1.46](H2O)3, have been refined using diffraction data collected with MoKa X-rays and a CCD-based detector. The structure of becquerelite, orthorhombic, space group Pn21a, a = 13.8527(5), b = 12.3929(4), c = 14.9297(5) Å, V = 2563.2(1) Å3, has been refined on the basis of F2 for 4875 unique reflections to R1 = 3.39%, calculated using 4581 unique observed reflections (|F| ≥ 4σF), and a goodness-of-fit (S) of 1.04. Sr-exchanged becquerelite was obtained by placing single crystals of synthetic becquerelite in 2.5 M SrCl2 solution for 60 h at 160 °C. The structure of Sr-exchanged becquerelite is trigonal, space group P3, a = 7.020(4), c = 6.992(6) Å, V = 298.4(3) Å3, and has been refined on the basis of F2 for 683 unique reflections to R1 = 4.26%, calculated using the 564 unique observed reflections (|F| ≥ 4σF), and an S of 1.01. The results for becquerelite confirm the cation polyhedra and structural connectivity reported previously, but collection of data for a high-quality crystal using a CCD-based detector has substantially improved the precision of the atom positions. The structure contains a-U3O8-type sheets of uranyl pentagonal bipyramids, with a single symmetrically distinct Ca cation and eight symmetrically distinct H2O groups located in the interlayer. The structure of Sr-exchanged becquerelite also contains α-U3O8-type sheets of uranyl pentagonal bipyramids, although the amount of H in the sheets is lower than for becquerelite. The interlayer contains two symmetrically distinct Sr cations and a single H2O group. Ion exchange of Sr into the interlayer of becquerelite without destruction of the crystals has potential important implications for the mobility of 90Sr in contaminated areas, and for the geologic disposal of nuclear waste.