Atomic scale structural analysis of liquid immiscibility in binary silicate melt: A case of SiO2‒TiO2 system

Abstract

Thermodynamic/dynamic modeling of liquid immiscibility in silicates is seriously hindered due to lack of in situ investigation on the structural evolution of the melt. In this work, atomic-scale structural evolution of a classic binary silicate immiscible system, SiO2–TiO2, is tracked by in situ high energy X-ray diffraction (HE-XRD). It is found that both the configuration of [SiO] and the polymerization between them are closely coupled with embedment and extraction of the metallic cations (Ti4+). [SiO] monomer goes through deficit-oxygen and excess-polymerization before liquid‒liquid separation and enables self-healing after liquid‒liquid separation, which challenges the traditional cognition that [SiO4] monomer is immutable. Ti4+ cations with tetrahedral oxygen-coordination first participate in the network construction before liquid separation. The four-fold Ti–O bond is broken during liquid separation, which may facilitate the movement of Ti4+ across the Si–O network to form TiO2-rich nodules. The structural features of nodules were also investigated and they were found highly analogous to that of molten TiO2, which implies a parallel crystallization behavior in the two circumstances. Our results shed light on the structural evolution scenario in liquid immiscibility at atomic scale, which will contribute to constructing a complete thermodynamic/dynamic framework describing the silicate liquid immiscibility systems beyond current models.