Applications of Alkali Metal Hydroxide Hydrofluxes to the Synthesis of Single‐Crystal Ternary Actinide Oxides

Abstract

Hydrofluxes are hydrated salts with melting points well below that of the dehydrated salt and boiling points well above that of water,a ffording ar eaction medium, in which mild temperatures and pressuresc an be accessedf or the synthesis of materials. Herein, the use of alkali metal hydroxide hydrofluxes for the synthesis of single crystal a-Na 2 NpO 4 is described, and the single crystal X-ray structure of a-Na 2 NpO 4 ,a long with its X-ray absorption spectra and vibrational spectra,i sr eported. The ability to synthesize complex oxideso ft he actinides, in particular,t ransuranium materials, under mild conditions will serve to advance our ability to explore the structureproperty relationships of the fe lements.

Synthesis of Single Crystal Li2NpO4 and Li4NpO5 from Aqueous Lithium Hydroxide Solutions under Mild Hydrothermal Conditions

“…Based on nuclear site group analysis for Li 4 NpO 5 , which crystallizes in the centrosymmetric I 4/ m space group, we expect to see 10 Raman active modes (4 A g , 4 B g , and 2 E g ), all of which are visible in the Raman spectrum collected using circularly polarized incident light (Figure ). We observe a strong, sharp peak at 700 cm –1 in the Raman spectrum of Li 4 NpO 5 which is initially reminiscent of an actinyl symmetric stretching vibration as was assigned in Li 2 NpO 4 and our earlier studies of Na 2 NpO 4 . However, the neptunium in this structure is found in an anti-yl coordination, having two long and four short Np–O bonds, thus requiring a careful reconsideration for vibrational band assignments.…”

“…We observe a strong, sharp peak at 700 cm −1 in the Raman spectrum of Li 4 NpO 5 which is initially reminiscent of an actinyl symmetric stretching vibration as was assigned in Li 2 NpO 4 and our earlier studies of Na 2 NpO 4 . 6 However, the neptunium in this structure is found in an anti-yl coordination, having two long Polarized Raman Spectroscopy. Polarized Raman spectroscopy was used to help discern the symmetry and origin of the vibrational modes for the peaks in the Raman spectrum of Li 4 NpO 5 .…”

Reactions of Neptunium(V) in Alkali-Metal Hydroxides

“…Recently, we reported the synthesis of single-crystal disodium neptunate(VI), Na 2 NpO 4 , using mild thermal conditions in an alkali-metal hydroflux of sodium hydroxide. 5 The mild reaction conditions and our prior success in this area of chemistry inspired us to investigate similar reactions of neptunium using aqueous solutions of other alkali-metal hydroxides as well as their hydrofluxes as reactive solvents. Alkali-metal hydroxides and hydrofluxes of alkali-metal hydroxides have been used in the preparation of a variety of metal oxides of the transition metals, rare earths, and uranium and thorium.…”

“…Also, it is recognized that the synthesis of actinide compounds from alkaline solutions and the extreme caustic environments of alkali-metal hydrofluxes represents a significant opportunity to synthesize complex oxide and hydroxide materials as single crystals, as demonstrated by us and others. 4,5,10,11,27 Synthetic studies in environments such as these are highly relevant to nuclear waste disposal, environmental remediation of high-level wastes, subsurface transport and solubility, and the development of new waste forms for used nuclear fuel. 28 Herein we report our exploration of the phase space of reactions of neptunium in aqueous KOH and RbOH solutions.…”

“…The synthesis of actinide-containing molecules and materials has expanded considerably over the last several years. Synthetic studies employing less traditional solvents as reaction media including molten salts, reactive fluxes, , alkaline solutions, and hydrofluxes, , have considerably expanded our catalogue of actinide structure types, serving to highlight the richness and complexity of chemistry within the 5f elements. The synthesis of actinide and, in particular, transuranium molecules, compounds, and materials aides studies of the complex electronic structure of the early actinide elements, particularly as it relates to their bonding and magnetic properties. ,− Furthermore, the exploratory synthesis of actinide compounds informs our knowledge of their speciation and structure as it relates to environmental chemistry and remediation, waste management, and efforts aimed at developing sustainable and economical nuclear fuel cycles for energy production.…”