NMR EVIDENCE FOR 5-COORDINATED SILICON IN A SILICATE GLASS AT ATMOSPHERIC-PRESSURE

KNOWLEDGE of the structure of liquid silicates is essential to understanding the properties of materials ranging from magmas in lava flows to melts in glass processing. At 1 atmosphere pressure, a wide range of evidence indicates that most silicon cations in these systems are coordinated by four oxygens in a tetrahedral configuration (Si(IV)). Molecular dynamics computer simulations of these liquids have, however, predicted that defect complexes (of relatively low abundance) consisting of silicon with five oxygen neighbours (Si(V)) are of key importance in the mechanism by which viscous flow takes place 1-5. I present here direct experimental evidence from Si-29 NMR studies of K2Si4O9 glass that Si(V) does exist in silicate liquids at low pressures, and that the abundance of this species increases with temperature, supporting the idea that Si(V) defects contribute to 'weakening' of the structure of molten silicates.