Structural studies of Li 5ReO 6, Li 4NpO 5 and Li 5NpO 6 by neutron and X-ray powder diffraction

Abstract

This paper reports on the preparation of Li 5ReO 6, Li 4NpO 5 and Li 5NpO 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 5ReO 6 and Li 4NpO 5 from the neutron data. The neutron powder diffraction refinement for Li 5ReO 6 confirms a recent single-crystal X-ray diffraction study: monoclinic, space group C2/ m. This structure is retained by Li 5ReO 6 between 298 K and 10 K. The neutron powder diffraction refinement for Li 4NpO 5 confirms earlier X-ray powder diffraction results: tetragonal, space group I4/ m, a=6.698 A ̊ , c=4.415 A ̊ . The lithium and oxygen positional parameters of Li 4NpO 5 have been refined: coordination about Np is an extended octahedron, with each Np ion surrounded equatorially by four oxygen ions at 2.00 Å and axially by two oxygen ions at 2.21 Å. Li 5NpO 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.