High-Temperature, High-Pressure Hydrothermal Synthesis and Characterization of a Salt-Inclusion Mixed-Valence Uranium(V,VI) Silicate: [Na9F2][(UVO2)(UVIO2)2(Si2O7)2]

A salt-inclusion mixed-valence uranium(V,VI) silicate, [Na9F2][((UO2)-O-V)center dot((UO2)-O-VI)(2)(Si2O7)(2)], was synthesized under hydrothermal conditions at 585 degrees C and 160 MPa and structurally characterized by powder and single-crystal X-ray diffraction (XRD). The valence states of uranium were established by U 4f X-ray photoelectron spectroscopy (XPS). The structure contains two-dimensional (2D) sheets of uranyl disilicate with the composition [UO2Si2O7], which are connected by U(1)O-V(6) tetragonal bipyramids to form thick layers. The Na+ cations are located at sites in the intralayer and interlayer regions. In addition to Na+ cations, the interlayer region also contains F- anions such that infinite chains with the formula FNa1/1Na4/2 are formed. The same type of chain was observed in K2SnO3. The title compound is not only the first example of salt-inclusion metal silicate synthesized under high-temperature, high-pressure hydrothermal conditions, as well as the first salt-inclusion mixed-valence uranium silicate, but it is also the first mixed-valence uranium(V,VI) silicate in the literature. Crystal data: [Na9F2][((UO2)-O-V)((UO2)-O-VI)(2)(Si2O7)(2)], triclinic, P (1) over bar (No. 2), a = 5.789(1) angstrom, b = 7.423(2) angstrom, c = 12.092(2) degrees, alpha = 90.75(3), beta = 96.09(3)degrees, gamma = 90.90(3)degrees, V = 516.5(2) angstrom(3), Z = 1, R-1 = 0.0241, and wR(2) = 0.0612.