Six-coordinated silicon in glasses

Abstract

It has been accepted since 1932 that the building block of silicate glasses consists of silicon tetrahedrally coordinated to 4 oxygen atoms¹. Although the existence of 6-coordinated silicon in a few crystalline materials, such as stishovite, and SiP₂O₇ is known^2,3, the presence of (SiO₆) units has not yet been observed experimentally in glasses. Recent X-ray diffraction and magic-angle-spinning (MAS) nuclear magnetic resonance investigations^4,5 of SiO₂-P₂O₅ glasses found only 4-coordinated silicon although SiP₂O₇ was produced by devitrification. Raman studies⁶ of Na₂O-SiO₂-P₂O₅ glasses were interpreted as indicating the presence of a structural unit containing Si-O-P bonding but otherwise unidentifiable. We have applied MAS nuclear magnetic resonance spectroscopy to similar glasses and here we report the first observation of [SiO₆] units in glasses.