STRUCTURAL STUDIES OF LI5REO6 LI4NPO5 AND LI5NPO6 BY NEUTRON AND X-RAY-POWDER DIFFRACTION

This paper reports on the preparation of Li5ReO6, Li4NpO5 and Li5NpO6 by solid-state reactions, the results of X-ray and neutron powder diff raction measurements on all three compounds, and the determination of the structures of Li5ReO6 and Li4NpO5 from the neutron data. The neutr on powder diffraction refinement for Li5ReO6 confirms a recent single- cystal X-ray diffraction study: monoclinic, space group C2/m. This str ucture is retained by Li5ReO6 between 298 K and 10 K. The neutron powd er diffraction refinement for Li4NpO5 confirms earlier X-ray powder di ffraction results: tetragonal, space group I4/m, a = 6.698 angstrom, c = 4.415 angstrom. The lithium and oxygen positional parameters of L4N pO5 have been refined: coordination about Np is an extended octahedron , with each Np ion surrounded equatorially by four oxygen ions at 2.00 angstrom and axially by two oxygen ions at 2.21 angstrom. Li5NpO6 sho ws an X-ray powder pattern consistent with the original report, but th e neutron diffraction data set could not be refined in terms of the or iginally proposed space group or any other plausible model structure.