Understanding the Formation of Salt-Inclusion Phases: An Enhanced Flux Growth Method for the Targeted Synthesis of Salt-Inclusion Cesium Halide Uranyl Silicates

Salt-inclusion compounds (SICs) are known for their structural diversity and their potential applications, including luminescence and radioactive waste storage forms. Currently, the majority of salt-inclusion phases are grown serendipitously and the targeted growth of SICs has met with only moderate success. We report an enhanced flux growth method for the targeted growth of SICs. Specifically, the use of (1) metal halide reagents and (2) reactions with small surface area to volume ratios are found to favor the growth of salt inclusion compounds over pure oxides and thus enable a more targeted synthetic route for their preparation. The Cs-X-U-Si-O (X = F, Cl) pentanary phase space is used as a model system to demonstrate the generality of this enhanced flux method approach. Single crystals of four new salt-inclusion uranyl silicates, [Cs3F][(UO2)(Si4O10)], [Cs2Cs5F][(UO2)(2)(Si6O17)], [Cs9Cs6Cl] [ (UO2)(7)(Si6O17)(2)(Si4O12)], and [Cs2Cs5F]-[(UO2)(3)(Si2O7)(2)], were grown using this enhanced flux growth method. A detailed discussion of the factors that favor salt inclusion phases during synthesis and why specifically uranyl silicates make excellent frameworks for salt-inclusion phases is given.