Evolution of local structures in alkali-borate glasses

Abstract

We analyze the dependence of the relative proportion of various characteristic clusters in binary alkali borate glasses on modifier concentration x. Pure B2O3 glass contains, according to various estimates, from 75% up to 85% of boron atoms inside boroxol rings, wih the remaining boron atoms in between, linking the boroxol rings via oxygen bonds. The addition of Na2O modifier creates four-coordinated borons, but the resulting network glass remains totally connected. We study local transformations that lead to the creation of new configurations such as tetraborates, pentaborates, diborates, etc., and set forth a nonlinear differential system similar to the Lotka–Volterra equations. The resulting density curves of various local configurations as functions of x are obtained. Then the average rigidity is evaluated, enabling us to compute the glass transition temperature Tg(x) for a given value of x.