Immiscibility in binary silicate liquids: Insight from ab initio molecular dynamics simulations

Liquid-liquid phase separation (LLPS) is a prevalent phenomenon in silicate liquids. The ionic potential of the cations is widely recognized as a pivotal factor controlling the immiscibility extent of silicates; nonetheless, the intricate relationship between the two has yet to be fully understood. Here, using ab initio molecular dynamics simulations, we study the static and dynamic structural evolutions in a prototypical LLPS system ($\mathrm{Ti}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$), aiming to decode the structural origin of the nonmonotonic dependence of ionic potential on immiscibility extent. The simulations reproduce the initial stage of phase separation as represented by formation of microscale aggregative Ti-Ti clusters upon cooling. Such microphase separation primarily arises from the Coulombic repulsion between $\mathrm{T}{\mathrm{i}}^{4+}$ cations and adjacent $\mathrm{S}{\mathrm{i}}^{4+}$ nodes, rather than the previously believed repulsion between poorly shielded $\mathrm{T}{\mathrm{i}}^{4+}$ cations. Analysis of dynamics reveals that the transport of $\mathrm{T}{\mathrm{i}}^{4+}$ cations across the Si-O-Si network is more sluggish than that of alkali (alkaline)-earth cations. Slow dynamics of $\mathrm{T}{\mathrm{i}}^{4+}$ cations are decoupled from their local coordination structure, but instead, it highly depends on the topological rigidity of these nearest-neighbor Ti--O bonds. As such, the high ionic potential of $\mathrm{T}{\mathrm{i}}^{4+}$ cations drives them away from nearby-network $\mathrm{S}{\mathrm{i}}^{4+}$ nodes, promoting immiscibility. On the other hand, this same potential causes strong topological rigidity, and hence, suppresses immiscibility by hindering the $\mathrm{T}{\mathrm{i}}^{4+}$ migration. This dual effect of the ionic potential questions the classical structural model in LLPS and provides insights into the association between immiscibility extent and ionic potential.