Line defects in barium titanate observed by polarized light microscopy

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Abstract
Line defects lying parallel to the tetragonal c axis within ferroelectric a domains of barium titanate have been observed using polarized light. When the domain wall is inclined to the light beam, the-wall may exhibit thickness fringes, and the defects, which lie normal to the light beam, have oscillatory contrast of the same periodicity. A theoretical analysis of stress birefringence contrast due to dislocations in an anisotropic material shows that the contrast is consistent with the defects being screw dislocations. Analysis of the contrast of the defects in X-ray topographs confirms this conclusion. The stress birefringence analysis shows that, as a result of the optical anisotropy of the unstrained crystal, the contrast of a dislocation viewed from the side is much higher than in an isotropic material. The behaviour of the normal modes of propagation in the vicinity of a dislocation is compared with the behaviour of Bloch waves in electron or X-ray diffraction.