Structural properties of molten silicates from ab initio molecular-dynamics simulations: Comparison between ${\mathrm{C}\mathrm{a}\mathrm{O}-\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3{-\mathrm{S}\mathrm{i}\mathrm{O}}_2$ and ${\mathrm{SiO}}_2$

We present the results of first-principles molecular-dynamics simulations of molten silicates, based on the density functional formalism. In particular, the structural properties of a calcium aluminosilicate $[\mathrm{CaO}-{\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{SiO}}_2]$ melt are compared to those of a silica melt. The local structures of the two melts are in good agreement with the experimental understanding of these systems. In the calcium aluminosilicate melt, the number of nonbridging oxygens found is in excess of the number obtained from a simple stoichiometric prediction. In addition, the aluminum avoidance principle, which states that links between ${\mathrm{AlO}}_4$ tetrahedra are absent or rare, is found to be violated. Defects such as two-fold rings and five-fold coordinated silicon atoms are found in comparable proportions in both liquids. However, in the calcium aluminosilicate melt, a larger proportion of oxygen atoms are three-fold coordinated. In addition, fivefold coordinated aluminum atoms are observed. Finally evidence of creation and anihilation of nonbridging oxygens is observed, with these oxygens being mostly connected to Si tetrahedra.