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Phase‐Transformation‐Induced Twinning in Orthorhombic LaGaO 3 : {121} and [010] Twins
Author(s) -
Wang WeiLin,
Lu HongYang
Publication year - 2006
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.1551-2916.2005.00673.x
Subject(s) - crystal twinning , orthorhombic crystal system , crystallography , reflection (computer programming) , electron diffraction , phase (matter) , condensed matter physics , macle , materials science , diffraction , crystal structure , chemistry , optics , physics , microstructure , organic chemistry , computer science , programming language
Pressureless‐sintered lanthanum gallate (LaGaO 3 ) ceramics have been analyzed using X‐ray diffractometry (XRD) and scanning electron and transmission electron microscopy (SEM and TEM). Twins are induced by solid‐state phase transformation upon cooling from rhombohedralto orthorhombic ( o , Pnma ) symmetry at ∼145°C. Two types of transformation twins have been identified, and they co‐exist in some of the grains. The {121} twins are generated by loss of the mirror plane symmetry upon phase transformation. The twin planes are on {121} about which the two crystal parts are related by a mirror operation. The fact that twins are of reflection type is also confirmed by a tilting experiment in the microscope. The other, often termed the‐type rotation, is a rotation twin induced by loss of fourfold rotation axes upon phase transition. The spot splitting of 2φ≈0.22° because of the orthorhombic obliquity (φ) is evidenced from the corresponding electron diffraction patterns (SADPs) where triplet reflection spots are clearly identified. The {121} twin domain boundaries exhibit δ‐fringes. The fault vector across twin boundaries R =ɛis determined by applying the invisibility criteria of 2π g · R =0, or 2 n π. Such a translation is not related in a simple way to the structure of LaGaO 3 , and the length of the fault vector is not a fractional of the lattice displacement vector.