Silicon coordination changes from 4-fold to 6-fold on devitrification of silicon phosphate glass

Abstract

4-Fold oxygen coordinated tetrahedral silicon (IVSi) is the common building block in silicas and silicates. However, 17 crystallographically well-characterized inorganic silicates are known in which silicon exists in 6-fold oxygen octahedral coordination1. The most notable is stishovite, a high-pressure SiO2 polymorph2,3. An interesting question is whether differences in silicon oxygen coordination between liquid and solid silicates impose a barrier to nucleation. One such case is stishovite (VISi) which cannot be synthesized from SiO2 with IVSi at ambient pressures4,5. That such an apparent nucleation barrier is not limited to silicon oxygen coordination is shown by aluminium. Thus, Jadeite (NaAlSi2O6) with VIAl will not precipitate from its isochemical glass, in which only IVAl has been proven to exist, except when subjected to pressures >60 kbar6,7. To test if transitions of silicon coordination from 4- to 6-fold impose, in general, a barrier to nucleation, we studied the SiO2–P2O5 system. X-ray diffraction studies on one of the phases in this system, SiO2·P2O5, show the presence of VISi only8. The aforementioned examples indicate that the silicon-phosphate glass from which this phase precipitates will also contain VISi, as no external pressure is used. Zachariasen9, on the other hand, has argued against 6-fold coordination in glasses, predicting that octahedral coordination would force periodicity on the lattice, thereby disrupting the vitreous state. Our 29Si MAS (magic angle spinning) NMR results show that the SiO2–P2O5 glass contains only IVSi which on devitrification at ambient pressures transforms to crystalline silicon-phosphate with VISi.