Structural Changes in Borate Glasses

Abstract

A series of potassium borate glasses (100-x) B2O3.K2O) was prepared from analytical reagent chemicals and the glass samples were analysed for the determination of potassium and boron concentrations. Many physical and chemical properties of these glasses were studied and the property-composition curves are presented. Four structural changes are observed in the property-composition curves at about 10, 20 28 and 32 mol% K2O. Detailed computer calculations for the number of BO4 groups, bridging oxygens, and non-bridging oxygens as a function of the alkali oxide concentration are also presented. These calculations are based on recently published hypotheses for the structure of alkali borate glasses. Comparison between the experimental results and the calculations indicates the following: a) The change at about 10 mol% K2O is due to the appearance of very small concentrations of non-bridging oxygens, while the change at about 20 mol% K2O is due mainly to the large increase in the non-bridging oxygens concentration at that composition. b) The change at about 28 mol % K2O is due to a maximum in the number of bridging oxygens. c) The change at about 32 mol% K20 is due to a maximum in the fraction of the four-fold coordinated borons. Some properties are determined either by the number of BO4 groups or by the number of non-bridging oxygens, others by the combined effect of these configurations. The possibility of extending the calculations to other borate system is indicated.