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 fo und in a number of alkali silicate glasses quenched from liquids both at high and ambient pressure. In this report, both the presence of thi s species in other ambient-pressure samples and the increase in its co ncentration with fictive temperature are confirmed. However, it has no t been possible to detect this species in a hydrated cesium silicate g lass sample or in a silica gel, the latter at a detection limit of abo ut 0.02%. Also as previously noted for high-pressure samples, it is fo und that five-coordinated Si is more abundant in K2Si4O9 than in Na2Si 4O9 glass. Given the effects of alkali size on the coordination and bo nding to oxygen known from crystalline silicates, this finding support s a mechanism for the formation of the high-coordinate species that in volves the consumption of non-bridging oxygens.