Structure of liquid tricalcium aluminate

The atomic-scale structure of aerodynamically levitated and laser-heated liquid tricalcium aluminate (${\mathrm{Ca}}₃{\mathrm{Al}}₂{\mathrm{O}}₆$) 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, ${S}_{\mathrm{CaCa}}(Q)$, and partial pair distribution function, ${g}_{\mathrm{CaCa}}(r)$. 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 ${\mathrm{Ca}}₃{\mathrm{Al}}₂{\mathrm{O}}₆$ lies at the CaO-rich limit of the CaO:${\mathrm{Al}}₂{\mathrm{O}}₃$ glass-forming system. Our results show that, although significantly depolymerized, liquid ${\mathrm{Ca}}₃{\mathrm{Al}}₂{\mathrm{O}}₆$ is largely composed of ${\mathrm{AlO}}₄$ 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 $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}%$ unconnected ${\mathrm{AlO}}₄$ monomers and ${\mathrm{Al}}₂{\mathrm{O}}₇$ 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:${\mathrm{Al}}₂{\mathrm{O}}₃$ liquids could be described in terms of a percolation threshold at which the glass can no longer support the formation of an infinitely connected ${\mathrm{AlO}}₄$ network.