BaTiO, single crystals were grown by the top-seeded solution growth (TSSG) technique and the spatial domain configurations in the tetragonal crystals were observed by using a polarizing microscope, SEM, TEM, and X-ray topography. The 90" domain boundaries were stopped, forming wedge shapes, by another 90" boundary. All of the straight lamellar boundaries in the observed domain patterns, even in the herringbone structure, can be interpreted as head-totail 90" walls. The irregular overlapped boundary can be observed by using a polarizing microscope and X-ray topography. It looks as if the lamellar domains are penetrating one another because the wedge-shaped lamellar domains are spatially overlapped at those boundaries. After thinning and polishing {OOl} crystal plates, some 90" a-c boundaries disappear and a-domains become predominant.
The relationships between cracking and mechanical twinning, and between ferroelectric polarization and mechanical twinning, in stoichiometric LiNbO 3 were investigated in the present study. Three sets of mechanical twins crossed one another, and the crossed points of those twins provided preferred sites for the nucleation of cracks. Mechanical twins of LiNbO 3 revealed a head-to-tail arrangement of ferroelectric polarization, and complicated polarization states were observed at the crossed points. Some possible mechanisms of crack nucleation and propagation are proposed in this paper.
In an {loo) BaTiOs single crystal plate of 0.2mm thickness, it was observed that the surface of thin plate was deformed during the polishing process and, as a result, the 90" a-a boundaries were partially stabilized. These stabilized 90" a-a walls were reappeared at the exact their positions after repeated paraelectric-to-ferroelectric phase transition, i.e., they show a memory effect. These effects can be explained by the deformed surface layers and the characteristic anisotropy of 90" walls.
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