Antiphase boundaries ideal

Using the constructed potentials the y-surface for the (111) plane was calculated. (For more details see Girshick and Vitek 1995). T e lowest energy minimum on this surface corresponds to the ideal Llo structure. However, there are three different metastable stacking fault type defects on (111) the antiphase boundary (APB), the complex stacking fault (CSF) and the superlattice intrinsic stacking fault (SISF). The displacements... 

Figure 31 Pbi.4Bio.6Sr2Cao.5Yo.5Cu308 (a) [110] HREM image of a 45° antiphase boundary (AB). The nature of the cationic strontium and lead layers is indicated as S and P respectively the boundary is marked by a row of small black arrows, (b) Idealized drawing of the connection of the different layers through the 45° antiphase boundaiy. The SrO layers, which remain unchanged across the boundary, are shown with large black arrows. Tlie 45° boundary is parallel to an oxygen plane (plane of small anows).

Figure 32 (a) [110] image of an antiphase boundary parallel to the c axis in the thick part of the crystal. The shift of the layers accross the boundary is close to c/4 (indicated by rows of small dots parallel to the layers), (b) Idealized drawing of the composition of the layers through the antiphase boundary parallel to (110). The unchanged planes [PbOjoo and [Cu02]oo, are indicated by large and medium arrows. The boundary appears in an [AO] plane (row of small arrows). 

The BOg octahedra can not only deform but may also tilt and rotate along their fourfold or twofold axes, giving rise to different superstmctures or modulated structures. Besides, there is a strong dependence of structural symmetry on temperature at lower temperatures, numerous modifications or structural distortions from the ideal perovskite stmcture exist [43—45], and all of these causes lower the symmetry of the structure from cubic to tetragonal, orthorhombic, rhombohedral, or monoclinic. A lowering in symmetry will introduce different orientation variants (twins) and translation variants (antiphase boundaries). Stmctures with a lower symmetry, derived from the cubic structure by tilting and/or deformation of the BOg octahedra, become stable such that one (or several) phase transformation(s) may take place. 

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