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Phase-Transformation-Induced Twinning in Orthorhombic BaCeO3

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Phase-transformation-induced twinning in orthorhombic barium cerate (BaCeO3) has been analyzed using transmission electron microscopy. Reflection twins with boundaries lying in {110) and {112) are generated by loss of point group symmetry elements during solid-state phase transition along the sequence of Pm 3mR 3co1-Ibmmo2-Pbnm when pressureless-sintered samples were cooled from 1400°C to room temperature. Fault vectors R=〈1 10] and 〈021], in which displacements are not related in a simple way to the lattice translation, are determined for the twin boundaries exhibiting -fringe patterns. The high- and low-symmetry phases, rhombohedral and orthorhombic, are not related by group–subgroup symmetry, and the transformation is discontinuous and first order in nature, where twin relationships in the low-symmetry phase are forbidden by the Landau theory. However, the transformation twins observed experimentally are consistent with those predicted directly from assuming a non-disruption condition between transition phases, where the new structure is described in the frame of the old one geometrically. Such a phase transition can be continuous and diffusionless if it occurs via second order to and from a metastable intermediate phase, which is a shared common space group of the high- and low-symmetry phases. Accordingly, two possible intermediate phases of the minimal common supergroup cubic Pm 3m (No. 221) and of the maximal common subgroup monoclinic C2/c (No. 15) for the two end phases are identified.
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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: 2008-07-01

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