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Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates
Author(s) -
Ushakov Sergey V.,
Navrotsky Alexandra,
Tangeman Jean A.,
Helean Katheryn B.
Publication year - 2007
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2007.01592.x
Subject(s) - pyrochlore , fluorite , analytical chemistry (journal) , materials science , enthalpy , crystallography , chemistry , phase (matter) , thermodynamics , metallurgy , physics , organic chemistry , chromatography
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.