Structural studies of Li5ReO6, Li4NpO5 and Li5NpO6 by neutron and X-ray powder diffraction

Abstract

This paper reports on the preparation of Li5ReOr, Li4NpO5 and LisNpO6 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 Li5ReO6 and Li,NpO5 from the neutron data. The neutron powder diffraction refinement for LisReO6 confirms a recent single-crystal X-ray diffraction study: monoclinic, space group C2/m. This structure is retained by LisReO6 between 298 K and 10 K. The neutron powder diffraction refinement for Li4NpO5 confirms earlier X-ray powder diffraction results: tetragonal, space group I4/rn, a = 6.698/~, c = 4.415 /~. The lithium and oxygen positional parameters of Li4NpO5 have been refined: coordination about Np is an extended octahedron, with each Np ion surrounded equatorially by four oxygen ions at 2.00 ~ and axially by two oxygen ions at 2.21 ~. Li5NpO6 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.

Stabilization of O–O Bonds by d0 Cations in Li4+xNi1–xWO6 (0 ≤ x ≤ 0.25) Rock Salt Oxides as the Origin of Large Voltage Hysteresis

“…Turning to Li 4.1 Ni 0.9 WO 6 , initial peak fitting was performed on laboratory PXRD data based on the reported Li 4 NiWO 6 cell ( a = 5.090(3) Å, b = 8.810(4) Å, c = 5.079(1) Å, β = 109.60(5)°) in the C 2/ m space group, which accounted for all of the Bragg reflections and gave a satisfactory Le Bail fit. With an increased Li content, Li 4.1 Ni 0.9 WO 6 moves further away from the Li 3 Ni 2 TaO 6 stoichiometry and adopts a monoclinic structure with four distinct cation sites, similar to the archetypal layered Li 5 ReO 6 rock salt superstructure . The WO 6 octahedra in Li 4.1 Ni 0.9 WO 6 are again isolated from each other, with similar W–W distances to Li 4 NiWO 6 (Figure ) that also correspond to the third cation shell at T 3 = ( a , a /2, a /2) in order to minimize electrostatic repulsions but in a motif distinct from that found in Li 4 NiWO 6 .…”

Synthesis and crystal structure characterisation of sodium neptunate compounds

“…The unit cell parameters are reported in Table 1. The reaction product appears isostructural with the monoclinic Li 5 NpO 6 compound reported by Morss [14]. For this composition, neptunium is expected to be heptavalent.…”

“…They also presented results on the chemical and structural properties of alkali-transuranates containing pentavalent transuranium elements [11]: Li 3 AnO 4 (An = Np, Pu, Am), Na 3 AnO 4 (An = Np, Pu, Am) and Li 7 AnO 6 (An = Np, Pu, Am). The synthesis and characterization of the structural, thermodynamic, magnetic and spectroscopic properties of these ternary and other complex actinide mixed oxides were later reviewed by Keller et al in 1972 [12] and Morss in 1982 [13] and 1994 [14]. In the latter work, Morss pointed out a need for reexamination of the sodium uranates and neptunates and the lack of structural and thermodynamic characterization.…”

Synthesis and crystal structure investigations of ternary oxides in the Na–Pu–O system

“…In a later work [27] Li 5 NpO 6 was prepared by mixing highly reactive Li 2 O 2 and low-fired NpO 2 powders (molar ratio 3:1), at 400-410°C in oxygen atmosphere by the scheme: 2:5Li 2 O 2 ðsÞ þ NpO 2 ðsÞ ! Li 5 NpO 6 ðsÞ þ 0:5O 2 ðgÞ…”