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 HfO 2 –La 2 O 3 (–Gd 2 O 3 ) systems. All melts crystallized upon quenching in oxygen. Hf 2 La 2 O 7 pyrochlore and Gd 0.5 Hf 0.5 O 1.75 fluorite phases were identified. Gd 0.5 Hf 0.5 O 1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO 2 ‐ La 2 O 3 melts contained 31.6–34.2 mol% La 2 O 3 . The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na 2 O·4MoO 3 melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf 2 La 2 O 7 and −48.8±4.7 kJ/mol for Hf 2 Gd 2 O 7 . The enthalpy of the pyrochlore–fluorite phase transition in Hf 2 Gd 2 O 7 is 23.6±3.1 kJ/mol.