Mechanism of Al Coordination Change in Alkaline-earth Aluminosilicate Glasses: An Application of Bond Valence Model

Abstract

Aluminum cations are generally present in four-fold ([4]Al3+) or five-fold coordination ([5]Al3+) in aluminosilicate slags, where the concentration of [5]Al3+ varies depending on the type of charge compensator, for example, Mg2+ and Ca2+. Although it has been reported that the amount of [5]Al3+ species increases with the replacement of CaO with MgO in the CaO–MgO–SiO2–Al2O3 system, the detailed mechanism underlying the change in the local structure near the aluminum cations remains unclear. Because the residual negative charge on the bridging oxygen between [4]Si4+ and [5]Al3+ ([4]Si4+–OBO–[5]Al3+) is larger than that of [4]Si4+–OBO–[4]Al3+, it is essential to understand the positive charge contributions of alkaline-earth cations to compensate for these negative charges on the bridging oxygens. In the present study, the valence of a single chemical bond near Mg2+ and Ca2+ cations in the chosen aluminosilicate glasses was determined using a simple empirical model, which enabled calculation of the bond valence from the observed interatomic distance of near alkaline-earth cations by synchrotron X-ray total scattering. Magnesium cations had a larger average bond valence (+0.39) than calcium cations (+0.31). The difference in the positive charge contribution from Mg2+ and Ca2+ should explain the variation in the coordination number of aluminum cations.