High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal Structure, and Solid-State NMR Spectroscopy of Cs₂(UO₂)(Si₂O₆) and Variable-Temperature Powder X-ray Diffraction Study of the Hydrate Phase Cs₂(UO₂)(Si₂O₆)·0.5H₂O

A new uranium(VI) silicate, Cs₂(UO₂)(Si₂O₆), has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It crystallizes in the orthorhombic space group Ibca (No. 73) with a = 15.137(1) Å, b = 15.295(1) Å, c = 16.401(1) Å, and Z = 16. Its structure consists of corrugated achter single chains of silicate tetrahedra extending along the c axis linked together via corner-sharing by UO₆ tetragonal bipyramids to form a 3-D framework which delimits 8- and 6-ring channels. The Cs⁺ cations are located in the channels or at sites between channels. The ²⁹Si and ¹³³Cs MAS NMR spectra are consistent with the crystal structure as determined from X-ray diffraction, and the resonances in the spectra are assigned. Variable-temperature in situ powder X-ray diffraction study of the hydrate Cs₂(UO₂)(Si₂O₆)·0.5H₂O indicates that the framework structure is stable up to 800 °C and transforms to the structure of the title compound at 900 °C. A comparison of related uranyl silicate structures is made.