Insight into the structure and crystallization of SiO2-Al2O3-P2O5-Na2O-MgO-CaO glass system with Mg-Ca substitution: A molecular dynamics study

Transparent glass ceramics (TGCs) have shown great prospects in many application fields. Understanding the effect of oxides on glass network structure and crystallization is essential for designing TGCs. A series of 18Na2O-(16-x)CaO-xMgO-3P2O5-2.5Al2O3-60.5SiO2 (mol%, x = 0, 4, 8, 12, 16) glass-ceramics were studied with Molecular dynamics (MD) simulations, and the effect of CaO substituted by MgO on the glass structure and crystallization was analyzed in details. The results show that MgO statistically behaves as a network former, and the overall network connectivity increases with the substitution of MgO for CaO. Further structure analysis shows that the complex glass system consists of Si-rich and P-rich phases, which respectively contain Si-Mg-rich regions with 2Mg1Si-related tricluster oxygens (TBOs) and P-Mg-rich regions. The energetically unfavorable TBOs and Mg4-O-Mg4 linkages provide favorable nucleation sites for homogeneous nucleation, and the so-called ‘Coordination matching’ principles are proposed to explain the type of crystals. These results help us understand the crystallization of Mg-containing glass ceramics and benefit the future design of novel spontaneous TGCs.