Abstract
In-situ high-temperature Raman spectroscopy combined with classical molecular dynamics (CMD) simulations was employed to elucidate the structural characteristics and the transition mechanism of Ca12Al14O33 (C12A7) in both melt and glass states across a compositional range of 12CaO and 7Al2O3, specifically at temperatures from room temperature to 1723 K. CMD results reveal that Al-O bonds mainly form [AlO4] tetrahedra in both phases, while the melt exhibits a higher proportion of [AlO5] species and highly fluctuating Ca-O coordination. Oxygen speciation indicates pronounced network depolymerization in the molten state, characterized by a reduction in bridging oxygens and Q4 units, accompanied by an enrichment in non-bridging oxygens as well as Q1 and Q2 units. Upon cooling, the network undergoes repolymerization, resulting in a glass framework dominated by [AlO4] tetrahedra and bridging oxygens, that retains the short-range order but exhibits reduced connectivity. Raman spectroscopy confirms these results, showing the progressive red shift and band broadening with heating, and the emergence of [AlO5] species in the melt. Gaussian deconvolution of Raman spectra quantifies the dominance of [AlO4] tetrahedra and confirms the increased fraction of [AlO5] species in the melt. As the temperature increases, the fraction of [AlO5] species in the melt significantly increases. Specifically, the Al atoms are coordinated in [AlO4] tetrahedra in the glass state, whereas in the melt, this proportion increases to approximately 10% for [AlO5] species. Together, these results show the depolymerization-repolymerization mechanism that governs the melt-to-glass transition in C12A7.