High-Temperature,
High-Pressure Hydrothermal Synthesis
and Characterization of a Salt-Inclusion Mixed-Valence Uranium(V,VI)
Silicate: [Na<sub>9</sub>F<sub>2</sub>][(U<sup>V</sup>O<sub>2</sub>)(U<sup>VI</sup>O<sub>2</sub>)<sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>]
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Abstract
A salt-inclusion mixed-valence uranium(V,VI)
silicate, [Na<sub>9</sub>F<sub>2</sub>][(U<sup>V</sup>O<sub>2</sub>)(U<sup>VI</sup>O<sub>2</sub>)<sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>], was synthesized under hydrothermal conditions at
585 °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
[UO<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>], which are connected by
U(1)<sup>V</sup>O<sub>6</sub> tetragonal bipyramids to form thick
layers. The Na<sup>+</sup> cations are located at sites in the intralayer
and interlayer regions. In addition to Na<sup>+</sup> cations, the
interlayer region also contains F<sup>–</sup> anions such that
infinite chains with the formula FNa<sub>1/1</sub>Na<sub>4/2</sub> are formed. The same type of chain was observed in K<sub>2</sub>SnO<sub>3</sub>. 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: [Na<sub>9</sub>F<sub>2</sub>][(U<sup>V</sup>O<sub>2</sub>)(U<sup>VI</sup>O<sub>2</sub>)<sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)<sub>2</sub>], triclinic, <i>P</i>1̅ (No. 2), <i>a</i> = 5.789(1) Å, <i>b</i> = 7.423(2) Å, <i>c</i> = 12.092(2) Å,
α = 90.75(3)°, β = 96.09(3)°, γ = 90.90(3)°, <i>V</i> = 516.5(2) Å<sup>3</sup>, <i>Z</i> = 1, <i>R</i><sub>1</sub> = 0.0241, and <i>wR</i><sub>2</sub> = 0.0612
