Polyamorphic transitions in yttria-alumina liquids

There has been much recent discussion given to the phenomenon of polyamorphism where distinct, different states of amorphous liquids and solids are observed as a function of density. Underlying this phenomenon is the possibility of a first order liquid-liquid phase transition driven by the density and entropy differences between the two amorphous phases. Differential scanning calorimetry (DSC) of aluminate glasses containing 32-20 mol% Y2O3 has enabled characterisation of the thermodynamic differences between the high and low density amorphous phases. The DSC measurements show a glass transition temperature with onset at 1135 K. The width and the magnitude of jump in heat capacity at this glass transition indicates an extremely fragile liquid, that is there is a pronounced non-Arrhennian viscosity-temperature relation. There are two additional features of interest. A second glass transition at 1300 K is attributed to the low-density liquid (LDA) more stable at low temperature. This low-density liquid has a more Arrhenian viscosity temperature curve. A strong exothermic signal is seen between the two glass transitions and represents the transition between the supercooled, high-density liquid (HDA) and a low-density glass. Neutron scattering data suggest that the polyamorphic transition is accompanied by minor changes in the aluminate framework and more prominent changes in the local environment surrounding the yttrium ion, seen as an increase in the first neighbor yttrium-oxygen distance. (C) 2001 Elsevier Science B.V. All rights reserved.