Abstract
Pressureless‐sintered lanthanum gallate (LaGaO3) 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 rhombohedral to 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=ɛ is determined by applying the invisibility criteria of 2πg·R=0, or 2nπ. Such a translation is not related in a simple way to the structure of LaGaO3, and the length of the fault vector is not a fractional of the lattice displacement vector.
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