compositions underwent a liquidliquid phase transition which resulted in glasses with a cloudy appearance due to spheroids of one glass in a matrix of a second glass. The two glasses were isocompositional within the limits of experimental error. Clear, brilliant, single-phase glasses were obtained from La 3 Al 5 O 12 , ErLaYAl 5 O 12 , and compositions containing >5 mol% La 2 O 3 substituted for the other rare-earth oxides. Formation of two glasses is attributed to nucleation and growth of the second liquid at a temperature below the equilibrium liquid-liquid transition temperature. Addition of lanthanum depresses the phase transition temperature below the glass transition temperature and the liquidliquid phase transition is not observed. The results are discussed in the context of first-order liquid-liquid phase transitions (polyamorphism) and formation of single-phase glass from liquids that contain a high proportion of 4-coordinate aluminum ions.

Abstract. Here we report the first synthesis of a forsterite (Mg2SiO4) composition glass as an essentially phase-pure bulk material. Under containerless conditions, with heterogeneous nucleation sites minimized, glass forms by cooling ca. 1 mm liquid Mg2SiO 4 droplets in oxygen at 700 K/s. 29Si NMR spectroscopic data indicate that the SiO n tetrahedra and MgO6 octahedra exist in a comer sharing arrangement in the glass, but upon crystallization the polyhedral units reorganize to form edge-sharing linkages. Transposed temperature drop calorimetry shows that the glass is 61.4 + 1.3 kJ/mol higher in enthalpy than the crystal.

The total structure factor S(Q) and the radial distribution function G(r) of liquid Y3Al5O12 (YAG) were measured at 1770-2230 K by x-ray scattering from samples under containerless conditions in Ar and O2. Nominal coordination numbers are 4 for Al3+ and 6 for Y3+ ions. The G(r) has peaks at r approximately 1.8 A for Al-O, r approximately 2. 25 A for Y-O, and r approximately 3.3-3.6 A assigned to metal ions in adjacent AlO5-4 and YO9-6 polyhedral ions. Relative to the pure oxides, G(r) for molten YAG has smaller half-widths for the Al-O and Y-O peaks, and an increased sensitivity to temperature and the ambient gas composition.

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