Cs[Si₃O₆(OH)] and Rb[Si₂O₄(OH)]: two novel phyl­losilicates

The crystal structures of two novel phyl­losilicates with compositions Cs[Si₃O₆(OH)] (caesium hydroxo­hexa­oxotetra­otri­silicate) and Rb[Si₂O₄(OH)] (rubidium hydroxo­hexa­oxotetrao­di­silicate) have been characterized by X-ray diffraction. The topology of the caesium phyl­losilicate silica sheet consists of interconnected four- and six-membered rings and thus differs from all of the previously reported phyl­losilicates. The topology of the rubidium phyl­losilicate silica sheet consists of six-membered rings only, in boat conformations, resulting in a corrugated sheet similar to that observed in δ-Na₂Si₂O₅. Both of the title compounds exhibit the characteristic sandwich structure of sheet silicates, with the Cs atom ninefold coordinated and the Rb atom eightfold coordinated to the framework O atoms.