Structure of molten ytterbium aluminum garnet

Rare earth aluminum garnets are important materials in optical, dielectric, and thermal barrier applications. To advance the understanding of their melt processing and glass forming ability, we report the atomic structure of molten Yb₃Al₅O₁₂ over 1770-2630 K, which spans the equilibrium and supercooled liquid regimes. The melt density at T_m = 2283 K is 5.50 g cm⁻³, measured via silhouette imaging of electrostatically levitated drops over 1010-2420 K. Four separate structure measurements were made with aerodynamically levitated melts using x-ray and neutron diffraction with isotope substitution of Yb (¹⁷²Yb, ¹⁷⁴Yb, or ^natYb). Empirical potential structure refinement models were developed, which are in excellent agreement with the experiments. Coordination environments for Al-O are predominantly 4- and 5-coordinate, with a mean coordination of n_AlO = 4.43(8), while Yb-O environments mostly range from 5- to 8-coordinate, with n_YbO = 6.26(8). The cation-oxygen polyhedra are connected primarily by corner-sharing, with edge-sharing constituting up to ∼1/3 of the connectivity among polyhedra with Yb or higher-coordinated Al-O. Structurally, the -Al-O- network in molten Yb₃Al₅O₁₂ appears conducive to glass formation: n_OAl = 1.85(3), there are 1.86 AlO_x-AlO_x connections per Al atom (e.g., a mixture of Q³ and Q⁴ units), and the modal ring size is six cations. These characterize a network that is somewhat less constrained compared to SiO₂ glass, yet Yb₃Al₅O₁₂ cannot be quenched into crystal-free glass. Aluminum garnet compositions with larger rare earth cations do form glass, so these characterizations help reveal the structural characteristics corresponding to the limit of glass forming ability in rare earth aluminates.