Calorimetric study of glasses and liquids in the polyamorphic system Y₂O₃-Al₂O₃

Liquids in the Y₂O₃-Al₂O₃ system exhibit liquid state 'polyamorphism' with a transition occurring between two liquid phases (in the supercooled state) with the same chemical composition but different density and structure. Differential scanning and high temperature solution calorimetry experiments were performed on Y₂O₃-Al₂O₃ glasses quenched from polyamorphic liquids to establish the thermodynamic properties of the high and low density amorphous phases (HDA, LDA) and the energetic factors that drive the liquid-liquid phase transition. Glasses produced from the high density, high temperature liquid by rapid quenching show the onset of a glass transition for the HDA form at 1120-1150 K. The HDA liquid is highly 'fragile' with a nonArrhenian temperature dependence consistent with its large configurational entropy. The HDA-LDA transition occurs at 1450-1575 K, depending upon the composition, during quenching experiments. The HDA form can be recovered metastably to ambient conditions providing samples for calorimetric study. Transition to the more stable LDA 'polyamorph' then occurs when the HDA form is heated above its glass transition. This transition is observed as an exothermic signature in heat capacity traces at 1211-1277 K. The glass transition for the LDA polyamorph occurs at ∼1320-1480 K. It is broader than that observed for the HDA form, indicating a 'stronger' (less fragile) liquid state, associated with smaller configurational entropy. Heats of solution measured in molten 2PbO. B₂O₃ at 1073 K indicate a large enthalpy difference between the HDA and LDA phases (ΔH°=182±5 kJ/mol at 298 K). The transition enthalpy is strongly temperature dependent and ΔH (LDA=HDA) is 30-50 kJ/mol at the liquid-liquid transition temperature (1450-1575 K depending on the composition).