Condensation of [Si₃O₉]^6- anions in the solid state to the dimeric cyclotrisilicate anion [Si₆O ₁₇]^10-

The structure of the silicate Rb₁₀[Si₆O₁₇] 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°C. Systematic investigations by means of differential thermal analysis and temperature-dependent powder X-ray diffraction experiments revealed that the new compound evolved from Rb₆[Si₃O₉], which occurred as an intermediate product. Thus, the dimeric anion [Si ₆O₁₇]^10- is formed by condensation of the monomeric cyclotrisilicate [Si₃O₉]^6-. For both silicates, [Si₆O₁₇]^10- and [Si ₃O₉]^6-, the characteristic ring vibration modes were observed in the IR spectrum. The structure of Rb₁₀[Si ₆O₁₇] was solved and refined from single-crystal X-ray diffraction data in the orthorhombic space group Pbca (No. 61). Synthesis and structure determination of Rb₁₀[Si₆O₁₇] bridge the gap to show that the recently reported structures of Rb₁₄[Si ₄][Si₆O₁₇] and Rb₁₄[Ge ₄][Si₆O₁₇] are indeed fascinating intergrowth structures of the stable oxide Rb₁₀[Si₆O₁₇] and the Zintl phases RbSi (Rb₄Si₄) and RbGe (Rb ₄Ge₄), respectively.