Flux synthesis of salt-inclusion uranyl silicates: [K3Cs4F][(UO2)3(Si2O7)2] and [NaRb6F][(UO2)3(Si2O7)2].

Two salt-inclusion uranyl silicates, [K(3)Cs(4)F][(UO(2))(3)(Si(2)O(7))(2)] (compound 1) and [NaRb(6)F][(UO(2))(3)(Si(2)O(7))(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 UO(6) 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 K(2)Cs(4)F and Rb(6)F 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 degrees C, as indicated from powder X-ray diffraction. The (19)F and (29)Si MAS NMR spectra and the second-harmonic-generation response of 1 are consistent with the crystal structure analysis results. Crystal data: [K(3)Cs(4)F][(UO(2))(3)(Si(2)O(7))(2)], Cmc2(1), a = 7.8095(3) A, b = 22.2819(7) A, c = 14.0861(4) A, and Z = 4; [NaRb(6)F][(UO(2))(3)(Si(2)O(7))(2)], Pnnm, a = 11.1429(2) A, b = 13.5151(3) A, c = 7.8868(1) A, and Z = 2.