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

4-Fold oxygen coordinated tetrahedral silicon ( IV Si) 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 coordination 1 . The most notable is stishovite, a high-pressure SiO 2 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 ( VI Si) which cannot be synthesized from SiO 2 with IV Si at ambient pressures 4,5 . That such an apparent nucleation barrier is not limited to silicon oxygen coordination is shown by aluminium. Thus, Jadeite (NaAlSi 2 O 6 ) with VI Al will not precipitate from its isochemical glass, in which only IV Al 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 SiO 2 –P 2 O 5 system. X-ray diffraction studies on one of the phases in this system, SiO 2 ·P 2 O 5 , show the presence of VI Si only 8 . The aforementioned examples indicate that the silicon-phosphate glass from which this phase precipitates will also contain VI Si, as no external pressure is used. Zachariasen 9 , 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 29 Si MAS (magic angle spinning) NMR results show that the SiO 2 –P 2 O 5 glass contains only IV Si which on devitrification at ambient pressures transforms to crystalline silicon-phosphate with VI Si.