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

Aluminum cations are generally present in four-fold (([4])AI(3+)) or five -fold coordination (Al-[5](3+)) in aluminosilicate slags, where the concentration of Al-[5](3+) varies depending on the type of charge compensator, for example, Mg2+ and Ca2+. Although it has been reported that the amount of Al-[5](3+) 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 Si-[4](4+) and Al-[5](3+) (Si-[4](4+)-O-BO-Al-[5](3+)) is larger than that of Si-[4](4+)-O-BO-Al-[4](3+), 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 alarger 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.

Automated summary

Aluminum cations in aluminosilicate slags are present in four-fold or five-fold coordination, depending on the charge compensator. This study determines the valence of chemical bonds near alkaline-earth cations in aluminosilicate glasses, and explains the variation in coordination number of aluminum cations.