High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal Structure, and Solid State NMR Spectroscopy of a New Vanadium(IV) Silicate: Rb₂(VO)(Si₄O₁₀)·<i>x</i>H₂O

A new vanadium(IV) silicate, Rb2(VO)(Si4O10)·xH2O (x ≈ 0.1), has been synthesized by a high-temperature, high-pressure hydrothermal method. It crystallizes in the tetragonal space group I41md (No. 109) with a = 12.2225(7) Å, c = 7.7948(6) Å, and Z = 4. The structure consists of spiral chains of corner-sharing SiO4 tetrahedra linked to neighboring chains via corner sharing to form a 3-D silicate framework which delimits channels to accommodate the VO2+ groups. The Rb+ ions are located in the cavities within the silicate framework. Magnetic susceptibility confirms the valence of vanadium. A partially occupied lattice water site is confirmed by IR and solid state 1H NMR spectroscopy. The structure of the title compound is considerably different from those of the synthetic silicate K2(VO)(Si4O10)·H2O and the two polymorphs of the natural mineral Ca(VO)(Si4O10)·4H2O, although they have identical framework stoichiometry.