Abnormal orientation variants and interfaces in ferroelastics and precautions to be taken for the use of group theory (example : rare earth sesquioxides)

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Abstract
In group-subgroup phase transitions the orientation variants of the low symmetry structure are related by symmetry operations of the high symmetry structure which have been lost in the phase transition. In the case of the ferroelastic A (hexagonal) → B (monoclinic) phase transition of rare earth sesquioxides, orientation variants can occur which are not related by such symmetry operations but by the symmetry operations which would correspond to a twin (without change of the lattice) of the hexagonal structure. This phenomenon can occur when it reduces the high strain or high interface energies existing during the ferroelastic phase transition. Precautions necessary for the use of group theory in the study of ferroelastic orientation variants will be given. Special interest has also been taken in the study, by high resolution electron microscopy, of interfaces between a normal orientation variant and an unexpected (or abnormal) orientation variant. The interface has a certain thickness with possible atomic rearrangement

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