Effect of rare earth size on network structure and glass forming ability in binary aluminum garnets

Rare earth aluminate glasses are potentially useful for optical, luminescence, and laser applications. As reluctant glass formers, these materials exhibit unconventional atomic structures. To better understand how their structures correlate with glass formation, we investigate two rare earth aluminum garnet melts, La₃Al₅O₁₂ (LAG) and Yb₃Al₅O₁₂ (YbAG), which represent the relative extremes of good and poor glass forming ability in rare earth aluminates. Structural models have been refined to high-energy X-ray diffraction data over 1340-2740 K. Both melts contain mixtures of AlO₄, AlO₅, and AlO₆ polyhedra, with larger fractions of ^[5]Al and ^[6]Al in YbAG. Extrapolation of the Al-O coordination distributions to the glass transition match closely with ²⁷Al nuclear magnetic resonance measurements of (La_1-zY_z)₃Al₅O₁₂ glasses, z = 0 to 1. During cooling, the mean coordination numbers increase for La-O in LAG from 6.45(8) to 6.98(8) and for Yb-O in YbAG from 6.02(8) to 6.21(8). Linkedness among Al-O polyhedra at ∼2450 K is mostly corner-sharing, with 9% edge-sharing in LAG and 19% in YbAG. Among ^[4]Al units, both melts have 6% edge-sharing that convert to all corner-sharing upon cooling. Network connectivity is compared using a newly defined metric, Kⁿ, that is similar to the Qⁿ distribution but that accounts for the edge-sharing and triply bonded oxygen present in these melts. The lower glass forming ability in YbAG as compared to LAG correlates with more edge-sharing, associated with the larger fractions of ^[5]Al and ^[6]Al, and lower connectivity among ^[4]Al units.