Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates

Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO2–La2O3 (–Gd2O3) systems. All melts crystallized upon quenching in oxygen. Hf2La2O7 pyrochlore and Gd0.5Hf0.5O1.75 fluorite phases were identified. Gd0.5Hf0.5O1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO2- La2O3 melts contained 31.6–34.2 mol% La2O3. The unit cell parameter increased linearly with La content from 10.736 to 10.789 A. Drop solution calorimetric experiments were performed in 3Na2O·4MoO3 melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf2La2O7 and −48.8±4.7 kJ/mol for Hf2Gd2O7. The enthalpy of the pyrochlore–fluorite phase transition in Hf2Gd2O7 is 23.6±3.1 kJ/mol.