Novel Silicate Anion: Si₈O₂₂^12-. Hydrothermal Synthesis and X-ray Powder Structure of Three New Niobium Silicates

A sodium niobium(V) tetrasilicate, Na2H(NbO)Si4O11·1.25H2O (1), was synthesized hydrothermally at 190 °C from a mixture of silicic acid, NaOH, NbCl5, and H2O2 and added hydrochloric acid. The successive treatment of 1 with nitric acid yielded HNb(H2O)Si4O11·H2O (2). Contact of 2 with cesium hydroxide solutions converted it to the partially exchanged Cs+ phase Cs0.66H0.33Nb(H2O)Si4O11 (3). The three structures were solved from X-ray powder diffraction data. All compounds crystallize in the monoclinic space group P21/m with Z = 2, containing the Si8O2212- anion. This new silicate anion type is related to the Si4O116- unit, which is present in the amphibole series. The difference between both anion types lies in the chain periodicity: two for amphiboles or four for the new niobium silicates. These niobium silicates have framework structures enclosing tunnels in which the alkali cations reside.