Configurational constraints on glass formation in the liquid calcium aluminate system

We report new time-resolved synchrotron x-ray diffraction (SXRD) measurements to track structural transformations in calcium-aluminate (CaO)_x(Al₂O₃)_1−x liquids during glass formation, and review recent progress in neutron diffraction with isotope substitution (NDIS) experiments, combined with aspherical ion model molecular dynamics (AIM-MD) simulations, to identify the atomic-scale configurational constraints on glass-forming ability. The time-resolved measurements reveal substantial changes in ordering on short- and intermediate-range occurring during supercooling. In the equimolar composition x  =  0.5 (CA), the liquid undergoes a remarkable structural re-organisation on vitrification as over coordinated AlO₅ polyhedra and oxygen triclusters breakdown to form a network of predominantly corner-shared AlO₄ tetrahedra. This is accompanied by the formation of branched chains of edge-and face-sharing Ca-centred CaO_y polyhedra contributing to cationic ordering on intermediate length-scales. The Ca-rich end-member of the glass-forming system x  =  0.75 (C3A) is largely composed of AlO₄ tetrahedra, but % unconnected AlO₄ monomers and Al₂O₇ dimers are present, representing a threshold after which the glass can no longer support the formation of an infinitely connected network. Overall, the AIM-MD simulations are in excellent agreement with the SXRD and NDIS experiments suggesting an accurate potential model. However, small discrepancies between the simulated glass structures and experimental measurements are apparent, indicating a small degree of liquid-like ordering persists in the simulated glass trajectories. This may be due to the short simulation time-scales which are unrepresentative of the viscous kinetic processes involved in supercooling and glass formation. One approach to improve future models could be the integration of rare event sampling techniques into MD simulation codes to massively extend equilibration time-scales and more accurately model vitrification and structural configurations in real glass systems. (ucgp7)