Condensation of [Si3O9]6- anions in the solid state to the dimeric cyclotrisilicate anion [Si6O17]10-

The structure of the silicate Rb10Si6O17] containing a novel dimeric cyclotrisilicate anion is reported. The compound is formed by the reaction of a mixture of SiO2 and Rb at temperatures above 700 degrees C. Systematic investigations by means of differential thermal analysis and temperature-dependent powder X-ray diffraction experiments revealed that the new compound evolved from Rb6Si3O9], which occurred as an intermediate product. Thus, the dimeric anion Si6O17]10- is formed by condensation of the monomeric cyclotrisilicate Si3O9]6-. For both silicates, Si6O17]10- and Si3O9]6-, the characteristic ring vibration modes were observed in the IR spectrum. The structure of Rb10Si6O17] was solved and refined from single-crystal X-ray diffraction data in the orthorhombic space group Pbca (No. 61). Synthesis and structure determination of Rb10Si6O17] bridge the gap to show that the recently reported structures of Rb14Si4]Si6O17] and Rb14Ge4]Si6O17] are indeed fascinating intergrowth structures of the stable oxide Rb10Si6O17] and the Zintl phases RbSi (Rb4Si4) and RbGe (Rb4Ge4), respectively.