2 SIMPLE RULES FOR COVALENT BINARY GLASSES

We describe briefly a model of the effects of network additives on glass formation that was successfully developed and applied to several experimentally studied covalent glasses [Kerner and dos Santos, Phys. Rev. B 37 (1988) 3881; Kerner, J. Non-Cryst. Solids 135 (1991) 155; Kerner and Micoulaut, C.R. Acad. Sci. (Paris) 6. 315, II(1992) 1307; J. Non-Crystal. Solids 176 (1994) 271]. We derive two simple relations involving average coordination numbers and binding energies, which are fundamental for the glass forming tendency and the behavior of the glass transition temperature as a function of the modifier rate n. These relations are then compared with the experimental data for B2O3 and SiO2 based glasses, and a good agreement of our formulae with the experimental data is shown.