Crystal Growth of Alkali Uranyl Borates from Molten Salt Fluxes: Characterization and Ion Exchange Behavior of A₂(UO₂)B₂O₅ (A = Cs, Rb, K)

Abstract

A new family of layered alkali uranyl borates, A2(UO2)­B2O5 (A = Cs, Rb, K), was synthesized as high quality single crystals via high temperature flux growth methods. At room temperature, the compounds are structurally closely related although they crystallize in different monoclinic space groups, specifically P21/c (Cs), C2/m (Rb), and C2/c (K). At a low temperature (100 K), Cs2(UO2)­B2O5 becomes isostructural with K2(UO2)­B2O5 as the result of a reversible structure transition by Cs2(UO2)­B2O5. The title phases represent the first examples of uranyl borates resulting from high temperature flux growth utilizing alkali halide fluxes. The synthesis, structures, and thermal, optical, and ion exchange properties are reported, and modeling of the atomic structure and disorder of the ion exchanged phases is discussed.

Crystallization of A3Ln(BO3)2 (A = Na, K; Ln = Lanthanide) from a Boric Acid Containing Hydroxide Melt: Synthesis and Investigation of Lanthanide Borates as Potential Nuclear Waste Forms

“…To investigate materials that have potential as future nuclear waste forms, we have focused on crystalline materials, including oxides, fluorides, phosphates, and, as described herein, borates. − The investigation of TRU elements is difficult due to their high level of radioactivity, prompting the use of surrogates such as Nd 3+ and Eu 3+ for Am 3+ and Cm 3+ in exploratory chemistry since their oxidation states, ionic radii, and coordination preferences are virtually identical; furthermore, they are commercially available and not radioactive.…”

Investigation of Rare Earth-Containing Double Phosphates of the Type A3Ln(PO4)2 (Ln = Y, La, Pr, Nd, and Sm–Lu) as Potential Nuclear Waste Forms

“…Our research also involves polyanion materials, − and we have explored rare earth double phosphates of the type A 3 Ln­(PO 4 ) 2 as potential nuclear waste form materials. High-level nuclear waste (HLW) contains numerous elements that will persist for tens to hundreds of thousands of years, including most of the transuranic (TRU) elements, which will require highly stable and robust frameworks in which to sequester them. − Compared to uranium and thorium, there is a pronounced dearth of known extended structures for the minor actinides, including americium and curium.…”

Alkali Uranyl Borates: Bond Length, Equatorial Coordination and 5f States

“…Would it be only the result of a steric effect? UO 6 and UO 7 groups also occur within the borate family [16]. Comparison between the relatively simple structures of UO 2 BO 4 and AUO 2 BO 3 (A = alkali ion) shows that despite similar layered structures of uranyl chains, the presence of alkali influences the coordination of the uranyl from UO 8 to UO 7 .…”