Pentacoordinate Silicon Complexes as Precursors to Silicate Glasses and Ceramics

The recent discovery that pentacoordinate alkali glycolato silicates such as monomeric MSi(OCH,CH,O),OCH,CH,OH or dimeric M,Si,(OCH,CH,O), (where M = Li, Na, K, or Cs) can be synthesized directly from SO,, ethylene glycol, and MOH suggests that these compounds may serve as useful, inexpensive precursors to alkali silicate glasses or ceramics, either by sol-gel processing or by simple pyrolysis. We report here studies on the chemical changes and phase transformations that occur during the pyrolytic transformation of these compounds to ceramic and glassy materials. The evolutionary processes encountered as the materials are heated to selected temperatures were followed by X-ray powder diffraction, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermal analysis techniques. In general, the crystalline compounds oxidatively decompose at temperatures above 300°C to form amorphous materials. At higher temperatures, the line compounds M,Si,O, begin to crystallize. During the transformation process, ligand oxidation forms CO, and H,O, which react with the alkali metals to form small amounts of carbonates as seen by DRIFTS. At higher temperatures, the carbonates decompose with coincident formation of the primary crystalline phase, except in the case of the potassium compound, which exhibits some phase segregation.