Structure of liquid tricalcium aluminate

The atomic-scale structure of aerodynamically levitated and laser-heated liquid tricalcium aluminate (Ca₃Al₂O₆ ) was measured at 2073(30) K by using the method of neutron diffraction with Ca isotope substitution (NDIS). The results enable the detailed resolution of the local coordination environment around calcium and aluminum atoms, including the direct determination of the liquid partial structure factor, S_CaCa(Q ) , and partial pair distribution function, g_CaCa(r ) . Molecular dynamics (MD) simulation and reverse Monte Carlo (RMC) refinement methods were employed to obtain a detailed atomistic model of the liquid structure. The composition Ca₃Al₂O₆ lies at the CaO-rich limit of the CaO:Al₂O₃ glass-forming system. Our results show that, although significantly depolymerized, liquid Ca₃Al₂O₆ is largely composed of AlO₄ tetrahedra forming an infinite network with a slightly higher fraction of bridging oxygen atoms than expected for the composition. Calcium-centered polyhedra exhibit a wide distribution of four- to sevenfold coordinated sites, with higher coordinated calcium preferentially bonding to bridging oxygens. Analysis of the MD configuration reveals the presence of ∼10 % unconnected AlO₄ monomers and Al₂O₇ dimers in the liquid. As the CaO concentration increases, the number of these isolated units increases, such that the upper value for the glass-forming composition of CaO:Al₂O₃ liquids could be described in terms of a percolation threshold at which the glass can no longer support the formation of an infinitely connected AlO₄ network.