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This paper reports on the preparation of Li 5 ReO 6 , Li 4 NpO 5 and Li 5 NpO 6 by solid-state reactions, the results of X-ray and neutron powder diffraction measurements on all three compounds, and the determination of the structures of Li 5 ReO 6 and Li 4 NpO 5 from the neutron data. The neutron powder diffraction refinement for Li 5 ReO 6 confirms a recent single-crystal X-ray diffraction study: monoclinic, space group C 2/ m . This structure is retained by Li 5 ReO 6 between 298 K and 10 K. The neutron powder diffraction refinement for Li 4 NpO 5 confirms earlier X-ray powder diffraction results: tetragonal, space group I 4/ m , a=6.698 A , c=4.415 A . The lithium and oxygen positional parameters of Li 4 NpO 5 have been refined: coordination about Np is an extended octahedron, with each Np ion surrounded equatorially by four oxygen ions at 2.00 A and axially by two oxygen ions at 2.21 A. Li 5 NpO 6 shows an X-ray powder pattern consistent with the original report, but the neutron diffraction data set could not be refined in terms of the originally proposed space group or any other plausible model structure. |
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