Developing Waste Forms for Transuranic Elements: Quaternary Neptunium Fluorides of the Type Na_xMNp₆F₃₀ (M = Ti, V, Cr, Mn, Fe, Co, Ni, Al, Ga)

Abstract

A series of quaternary Np(IV) fluorides was synthesized using a mild hydrothermal synthesis approach. The compositions are all of the type Na x MNp6F30, where M = Ti(III), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Al(III), and Ga(III) and x = 4 for divalent metals, x = 3 for trivalent metals. The compounds all crystallize in the P-3c1 space group and are isotypic with actinide analogues Na x MAn6F30 (An = Ce, U, Th, Pu). Structure data from the neptunium crystals were combined with data from the other actinide analogues to establish the tetravalent, nine-coordinated ionic radii of neptunium (1.030(2) Å), plutonium (1.014(1) Å), and cerium (1.012(2) Å). Radiation damage studies were also carried out on a surrogate material, the cerium analogue Na3AlCe6F30, which indicates that the structure type has low resistance to amorphization. Density functional theory calculations were carried out to compute the band gaps and enthalpies of formation variations among the isotypic cerium and actinide structures to compare the stability of the structures.

Crystal structure and magnetism of actinide oxides: a review

“…For later actinides particularly, issues both of supply and high radioactivity limit the amount of material available, complicating synthesis. For such species, hydrothermal synthesis has been shown to be scalable down to very small sample reaction masses, [25,[204][205][206] using as little as <1 mg of actinide [206]. Recently, we have shown that flux growth can also be scaled down to the mg scale or even 1 mg scale [207,208].…”

Comparative Study on Crystal Structures and Synthetic Techniques of Ternary Hafnium/Zirconium Fluorides

“…For the hydrothermal route using aqueous HF, we established that mild conditions, 160–200 °C and relatively short reaction times, 24 h, followed by slow cooling to room temperature, often yielded high-quality crystals of ternary and quaternary fluorides whose structures could readily be determined. We have used this approach to explore, in general, the preparation of transition metal- and rare earth-containing fluorides and oxyfluorides, including vanadates, niobates, tantalates, tungstates, molybdates, manganates, cobaltates, nickelates, and cerates, as well as numerous actinide-containing fluoride phases containing Th, U, Np, Pu, and Am. ,,− ,− During this process, by experience, we have learned much about how to choose the optimum reaction conditions. In recent work, we explored hafnium fluorides, which yielded a number of hydrated phases, including Cs 2 [M­(H 2 O) 6 ]­[Hf 2 F 12 ] (M = Ni, Co, and Zn) and CuHfF 6 (H 2 O) 4 .…”