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

A new family of layered alkali uranyl borates, A(UO)BO (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 2/ (Cs), 2/ (Rb), and 2/ (K). At a low temperature (100 K), Cs(UO)BO becomes isostructural with K(UO)BO as the result of a reversible structure transition by Cs(UO)BO. 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.