Low Cation Coordination in Oxide Melts

The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on ${\mathrm{Y}}_2{\mathrm{O}}_3$, and ${\mathrm{Ho}}_2{\mathrm{O}}_3$ melts at 2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline ${\mathrm{Y}}_2{\mathrm{O}}_3$ (or ${\mathrm{Ho}}_2{\mathrm{O}}_3$). Investigation of ${\mathrm{La}}_2{\mathrm{O}}_3$, $\mathrm{Zr}{\mathrm{O}}_2$, and ${\mathrm{Al}}_2{\mathrm{O}}_3$ melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as $\mathrm{Si}{\mathrm{O}}_2$. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.