Energetics of the Charge-Coupled Substitution Si4+→ Na++ T3+ in the-Glasses NaTO2–SiO2 (T = Al, Fe, Ga, B)

Heats of solution in molten 2Pb0.B203 at 973 K are reported for glasses xNaT3+02-(l x)Si02 for T = Fe, Ga. These measurements, combined with previous data for T = Al, B, give a relative measure of the enthalpy of the charge-coupled substitution Si4+ + Na+ + T3+. The heats of solution become more endothermic with increasing x for x S 0.5 and exhibit a maximum near x = 0.5. This indicates an exothermic enthalpy for the substitution and an overall stabilization of the glasses. The degree to which the glasses are stabilized decreases in the order A1 > Ga > Fe > B. On the basis of molecular orbital calculations, X-ray scattering, and Raman spectroscopy, it is argued that this trend is primarily due to a decrease in the range of energetically favorable T-0-T bond angles as Al, Ga, Fe, and B are substituted for Si. [