Étude du polymorphisme des sesquioxydes de terres rares à haute température

Abstract

Les courbes obtenues lors de l'analyse thermique des sesquioxydes de terres rares à haute température présentent, entre 1 800° et 2 400° C, un certain nombre d'anomalies susceptibles de correspondre à des changements de forme cristalline. L'étude aux rayons X à haute température de ces phénomènes a permis de confirmer cette hypothèse et de mettre en évidence deux formes cristallines nouvelles. Tous les sesquioxydes de terres rares, à l'exception probablement du dernier terme de la série, Lu₂O₃, cristallisent sous une forme hexagonale II stable à haute température. On étudie le passage de cette forme aux formes classiques de ces oxydes : A hexagonale, B monoclinique et C cubique. En outre, les premiers sesquioxydes de la série (La₂O₃ à Gd₂O₃) présentent, au voisinage de leur fusion, une autre forme cristalline de structure non encore identifiée.

Synthesis, characterization and lattice thermal expansion of some compounds in the system Gd2CexZr2−xO7

“…In general on going from La to Lu, the structure of RE 2 O 3 changes from A-type to B-type to C-type. Foex and Traverse [24] have reported that the stability of C-type modification increases on decreasing the RE 3+ ionic size. In fact, Lu 2 O 3 , which is C-type, does not undergo any phase transition as it directly melts.…”

Systematics of Phase Transition and Mixing Energetics in Rare Earth, Yttrium, and Scandium Stabilized Zirconia and Hafnia

“…Owing to the high transition temperature (>2000°C), structural studies on these materials remain challenging and phase stabilities are difficult to verify. For Sm 2 O 3 and Gd 2 O 3 , a hexagonal ( H phase) to cubic ( X phase, Im ‐3 m ) phase transition has been reported at 2250° and 2360°C 37 . Therefore the presence of a cubic fluorite phase at high temperature is possible.…”

“…For Sm 2 O 3 and Gd 2 O 3 , a hexagonal (H phase) to cubic (X phase, Im-3m) phase transition has been reported at 22501 and 23601C. 37 Therefore the presence of a cubic fluorite phase at high temperature is possible. (4) The interaction parameters of both ZrO 2 -REO 1.5 and HfO 2 -REO 1.5 are strongly negative, lying between À107.5 and À89.2 kJ/mol for ZrO 2 -REO 1.5 and between À138.1 and À93.7 kJ/mol for HfO 2 -REO 1.5 solid solutions.…”

High-pressure Syntheses and Crystal Structures of Monoclinic B-Ho2O3 and Orthorhombic HoGaO3

“…Additional reaction components can help to transform the sesquioxides during the temperature treatment. For example, Foex et al achieved a transformation from the C-type to the B-type structure by adding some lime [33], or by offering support with oxides like CaO and SrO [37].…”