2 SIMPLE RULES FOR COVALENT BINARY GLASSES

We describe briefly a model of the effects of network additives on gla ss formation that was successfully developed and applied to several ex perimentally studied covalent glasses [Kerner and dos Santos, Phys. Re v. B 37 (1988) 3881; Kerner, J. Non-Cryst. Solids 135 (1991) 155; Kern er and Micoulaut, C.R. Acad. Sci. (Paris) 6. 315, II(1992) 1307; J. No n-Crystal. Solids 176 (1994) 271]. We derive two simple relations invo lving average coordination numbers and binding energies, which are fun damental for the glass forming tendency and the behavior of the glass transition temperature as a function of the modifier rate n. These rel ations are then compared with the experimental data for B2O3 and SiO2 based glasses, and a good agreement of our formulae with the experimen tal data is shown.