COMPOSITIONAL AND TEMPERATURE EFFECTS ON 5-COORDINATED SILICON IN AMBIENT-PRESSURE SILICATE-GLASSES

Using Si-29 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, silicon coordinated by five oxygens has recently been found in a number of alkali silicate glasses quenched from liquids both at high and ambient pressure. In this report, both the presence of this species in other ambient-pressure samples and the increase in its concentration with fictive temperature are confirmed. However, it has not been possible to detect this species in a hydrated cesium silicate glass sample or in a silica gel, the latter at a detection limit of about 0.02%. Also as previously noted for high-pressure samples, it is found that five-coordinated Si is more abundant in K2Si4O9 than in Na2Si4O9 glass. Given the effects of alkali size on the coordination and bonding to oxygen known from crystalline silicates, this finding supports a mechanism for the formation of the high-coordinate species that involves the consumption of non-bridging oxygens.