THE STRUCTURE OF (5X)P2O5.(1-X)K2O.4(1-X)SIO2 GLASSES

Glasses based on the potassium tetrasilicate composition with the addition of increasing amounts of P2O5, have been prepared by the melt quench route and their structure determined by magic angle spinning NMR of Si-29, P-31 and Al-27 (impurity). The base glass is composed of Q(3) and Q(4) units in the amounts predicted by stoichiometry. Introduction of small amounts of P2O5 removes potassium from the silicate matrix to produce ortho- and pyrophosphate units along with repolymerisation of Q(3) to Q(4) Increasing the concentration of P2O5 produces metaphosphate units of increasing length and Si-O-P bonds begin to form. When the concentration of P2O5 has reached between 25 and 30 mol%, 6-coordinated silicon begins to occur and increases in proportion relative to 4-coordinated silicon on further addition of P2O5. The impurity aluminium is present initially as 4-coordinated species but 5- and 6-cordinations appear at P2O5 contents of 10 mol%, i.e. earlier than 6-coordinated silicon. The appearance of 6-coordinated silicon is accompanied by a sharp drop in the glass transition temperature and by an increase in glass density