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 polymorph^2,3. 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 pressures^4,5. 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 kbar^6,7. 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.