High-temperature, high-pressure hydrothermal synthesis and characterization of an open-framework uranyl silicate with nine-ring channels: Cs ₂UO₂Si₁₀O₂₂

A new uranium(VI) silicate, Cs₂UO₂Si ₁₀O₂₂, has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, luminescence, and solid state NMR spectroscopy. It crystallizes in the monoclinic space group P2₁/c (No. 14) with a = 12.2506(4) Å, b = 8.0518(3) Å, c = 23.3796(8) Å, β = 90.011(2)°, and Z = 4. Its structure consists of silicate double layers in the ab plane which are connected by UO₆ tetragonal bipyramids via four equatorial oxygen atoms to form a 3D framework with nine-ring channels parallel to the b axis where the Cs⁺ cations are located. The photoluminescence emission spectrum at room temperature consists of one broad structured band which is typical of uranyl. The ²⁹Si MAS NMR spectrum is consistent with the crystal structure as determined from X-ray diffraction, and the resonances in the spectrum are assigned. A comparison of related uranyl silicate structures is made.