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The structures of becquerelite and Sr-exchanged becquerelite

Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick, University of Notre Dame, Notre Dame, Indiana 46556-0767, U.S.A.

Correspondence: ^* E-mail: pburns{at}nd.edu

The crystal structures of becquerelite, Ca[(UO₂)₆O₄(OH)₆](H₂O)₈, and Sr-exchanged becquerelite obtained by ion exchange, Sr_1.27[(UO₂)₃O_3.54(OH)_1.46](H₂O)₃, have been refined using diffraction data collected with MoK 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) ų, has been refined on the basis of F² for 4875 unique reflections to R1 = 3.39%, calculated using 4581 unique observed reflections (|F| 4_F), 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) ų, 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| 4_F), 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 -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 -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.

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