Flux crystal growth: a versatile technique to reveal the crystal chemistry of complex uranium oxides

Abstract

Molten flux crystal growth is a thriving field for the discovery of uranium oxides.

Framework Polymorphism and Modular Crystal Structures of Uranyl Vanadates of Divalent Cations: Synthesis and Characterization of M(UO2)(V2O7) (M = Ca, Sr) and Sr3(UO2)(V2O7)2

“…The present work continues our crystal chemical study of alkaline and alkaline‐earth uranyl vanadates and further develops understanding of uranyl‐based frameworks . Here we report the flux synthesis and crystal structures of the new uranyl vanadates M (UO 2 )V 2 O 7 ( M = Ca, Sr) and Sr 3 (UO 2 )(V 2 O 7 ) 2 with a framework and a layer‐type structure, respectively. Raman spectra, sheet‐anion topologies, and structural complexities are discussed.…”

Observation of the Same New Sheet Topology in Both the Layered Uranyl Oxide-Phosphate Cs11[(UO2)12(PO4)3O13] and the Layered Uranyl Oxyfluoride-Phosphate Rb11[(UO2)12(PO4)3O12F2] Prepared by Flux Crystal Growth

“…Compounds 1 - 4 were synthesized via molten flux methods using alkali chloride fluxes (Bugaris and zur Loye, 2012; Juillerat et al, 2019a). For all reactions UF 4 (International Bio-Analytical Industries, powder, ACS grade) was used as the uranium starting material, AlPO 4 (Alfa Aesar, powder, 99.99%) was used as the phosphate source, and an alkali halide, CsCl (Alfa Aesar, powder, 99.99%), KCl (Mallinckrodt Chemicals, powder, 99.6%), or RbCl (Alfa Aesar, powder, 99.8%), or a mix thereof was used as a flux.…”

“Soft” Alkali Bromide and Iodide Fluxes for Crystal Growth

“…In molten solutions, it is important to select a flux that is capable of dissolving the reactants and has a substantial change in solubility over the temperature range of interest, otherwise, the nucleated crystals will be re-dissolved and no single crystals will form. The optimal rate of nucleation occurs over a given temperature range which is specific to each system, thus exploratory crystal growth largely focuses on this determination (Figure 1; Bugaris and zur Loye, 2012;Juillerat et al, 2019). Indeed, selecting an appropriate reaction temperature is among the most critical considerations to make when conducting crystal growth experiments.…”

“…A convenient alternative to conventional solvents is various inorganic compounds that melt at readily achievable temperatures without reaching their boiling points during the reaction, circumventing the necessity of having expensive closed reaction vessels that can withstand high pressures. One well-known example of a high temperature flux is MoO 3 , which has been widely used for crystallizing various phases, for example, uranium oxides (Juillerat et al, 2019 ). This compound melts at 795°C, creating a reaction and crystallization medium for the starting materials, and can be slowly evaporated to promote the formation of large single crystals by slow oversaturation of the solution.…”