Structural role of fluorine in amorphous silica

The composition and temperature dependence of the coordination environments of F atoms in fluorinated silica glasses have been studied using high-resolution 19F and 29Si NMR spectroscopies. 19F MAS and wideline NMR spectra have revealed the presence of two distinct types of fluorine environments in glasses containing 1–3.3 wt% fluorine. The majority of the fluorine environments are formed by replacing one of the bridging oxygens around a silicon atom with a non-bridging fluorine atom, forming SiO3/2F polyhedra. The less abundant species is found to be highly unusual in that it involves bonding of a non-bridging fluorine to a silicon atom that is already coordinated to four bridging oxygens, yielding a fivefold coordinated silicon of the type SiO4/2F. The relative concentration of the SiO4/2F species in these glasses is found to increase with both increasing fluorine content and fictive temperature. The possible role of these structural elements in controlling diverse physical properties of these glasses such as the optical absorption edge and viscosity is discussed.