Energetics of the Charge-Coupled Substitution Si4+ Na++ T3+ in the-Glasses NaTO2SiO2 (T = Al, Fe, Ga, B)
Heats of solution in molten 2PbO\xb7B2O3 at 973 K are reported for glasses xNaT3+O2\u2013(1 \u2013x)SiO2 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+\u2192 Na++ T3+. The heats of solution become more endothermic with increasing x for x\u2192 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 Al > 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\u2013O\u2013T bond angles as Al, Ga, Fe, and B are substituted for Si.