Flux Synthesis of Salt-Inclusion Uranyl Silicates: [K<sub>3</sub>Cs<sub>4</sub>F][(UO<sub>2</sub>)<sub>3</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>] and [NaRb<sub>6</sub>F][(UO<sub>2</sub>)<sub>3</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>]
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Two salt-inclusion uranyl silicates, [K3Cs4F][(UO2)3(Si2O7)2] (compound 1) and [NaRb6F][(UO2)3(Si2O7)2] (compound 2), have been synthesized at high temperature using mixtures of alkali metal fluorides as fluxes and structurally characterized by single-crystal X-ray diffraction. Both compounds contain UO6 tetragonal bipyramids linked by disilicate groups to form three-dimensional framework structures with identical framework composition but different framework structures and salt inclusions. Both structures contain elliptical 12-ring channels where the K2Cs4F and Rb6F units are located. They are the first examples of salt-inclusion uranium silicates. The frameworks of 1 and 2 are thermally stable up to 850 and 750 °C, as indicated from powder X-ray diffraction. The 19F and 29Si MAS NMR spectra and the second-harmonic-generation response of 1 are consistent with the crystal structure analysis results. Crystal data: [K3Cs4F][(UO2)3(Si2O7)2], Cmc21, a = 7.8095(3) Å, b = 22.2819(7) Å, c = 14.0861(4) Å, and Z = 4; [NaRb6F][(UO2)3(Si2O7)2], Pnnm, a = 11.1429(2) Å, b = 13.5151(3) Å, c = 7.8868(1) Å, and Z = 2.
