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〈111〉 Rotation Twins in an Orthorhombic LaGaO3 Perovskite

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Phase-transformation-induced twins in pressureless-sintered lanthanum gallate (LaGaO3) ceramics have been analyzed using transmission electron microscopy. Twins are induced by solid-state phase transformation upon cooling from rhombohedral (r, R 3c) to orthorhombic (o, Pnma) symmetry at 145°C. Domains with a 150°–60°–150° configuration were frequently detected when viewed along [210]. This observation representing the co-existence of the {121} and {123} twins is suggested by analyzing corresponding selected area diffraction patterns across the domain boundaries. The former, with the twin plane lying on {121}, is the reflection type whose twin variants are related by mirror plane symmetry. The latter, although its nature was confirmed by tilting experiments along an unsplit row of reflections, exhibits characteristic crystallographic orientation relationships that are distinctive from those of the {121} twins. The twin laws represented in the matrix form are also derived accordingly from corresponding orientation relationships. Crystallographic analysis indicates that these domains commonly possess an orientation relationship that can be described by the twofold rotation axis about 〈111〉 lost upon the rhombohedral→orthorhombic phase transition. They are therefore the 180° parallel-rotation twin, with the twin axis 〈111〉 lying in {123}. Twins generated by the ro phase transition between crystals of non-group–subgroup relations are discussed in terms of an intermediate metastable cubic phase of the lowest common supergroup symmetry.
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Document Type: Research Article

Affiliations: Centre for Nanoscience, Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan

Publication date: 2007-01-01

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