Coordination evolution and stability of Al3+ in Al2O3-rich melts: Modulated by alkaline oxide type (M=Ca, Mg, Fe) and MO/Al2O3 ratio

As an amphoteric oxide, the structural behavior of Al₂O₃ in aluminosilicate melts is complex. Current understanding of the coordination structure of Al³⁺ in different alkaline oxide environments remains limited. This study employed molecular dynamics simulations to investigate the coordination structure and stability of Al³⁺ in MO-SiO₂-Al₂O₃ melts with different alkaline oxide types (CaO, MgO, FeO) and concentrations (MO/Al₂O₃=0.25–3). The results indicate that different alkaline oxides exhibit varying capabilities in achieving charge compensation balance for Al³⁺: CaO/Al₂O₃=0.5 for Ca²⁺ and FeO/Al₂O₃=1.0 for Fe²⁺, whereas Mg²⁺ cannot achieve charge balance even at MgO/Al₂O₃ ratio exceeding 3.0. Kinetic lifetime analysis reveals that only tetrahedrally coordinated [SiO₄] and [AlO₄] exhibits significant structural persistence. Oxygen linkage analysis shows that Si-O-Si structures possess the highest persistence, substantially exceeding Si-O-Al and Al-O-Al structures. These results fundamentally elucidate the structural behavior of Al³⁺ in different alkaline environments and their implications for regulating the properties of high-Al₂O₃ aluminosilicate melts.