Antiphase boundary crystals

If both these partial dislocations exist in the crystal, they will be linked by an antiphase boundary (Fig. 3.12d). 

For energetic reasons, internal boundaries are almost always planar in crystals. This is not a mle, though, and in some circumstances curved boundaries can occur. These are frequently found when the boundary is simply a variation in metal atom ordering of the type characterized by antiphase boundaries (see below). 

Antiphase boundaries (APBs) are displacement boundaries within a crystal. The crystallographic operator that generates an antiphase boundary in a crystal is a vector R parallel to the boundary, specifying the displacement of one part with respect to the other (Fig. 3.27), whereas the crystallographic operator that generates a twin is reflection (in the examples considered above). 

Figure 3.27 Antiphase boundaries (a, b) antiphase boundaries are formed when one part of a crystal is displaced with respect to the other part by a vector parallel to the boundary.

Figure 3.28 Antiphase boundary affecting only one atom type in a crystal.

The mechanics of the TiNi transition is, as shown by the X-ray, quite complex particularly when it comes to formation of twin or antiphase boundary. Although, personally I consider them as secondary importance toward the understanding of Nitinol transition itself, perhaps they should be mentioned to complete the picture. It has been known that the martensite (low-temperature phase) always has a crystal structure with lower symmetry than the austenite (high-temperature phase). In order to lower the free-... 

A second type of boundary, in which there is no misorientation between grains, is the antiphase boundary. This occurs when wrong atoms are next to each other on the boundary plane. For example, with hexagonal close-packed (HCP) crystals, the sequence. .. ABABAB... can be reversed at the boundary to ABABA ABABA, where represents the boundary plane. Antiphase boundaries and stacking faults are typically of very low energy, comparable to that of a coherent twin boundary. 

In a crystal containing twin defects, the crystal lattices continue across the twin boundaries without a break. Another similar defect, the antiphase defect, is formed by a shift of the crystal by half a unit cell along the antiphase boundary. This defect can also contribute to strong image contrast as shown in Figure 10.3b. 

The limited number of high quality substrates suitable for oxide epitaxy, together with the wide range of structural properties exhibited by oxides, may require the use of a substrate with a different crystal symmetry than the film material. If the crystal symmetries are sufficiently different, antiphase boundaries (ABPs) may result during nucleation of the initial monolayers. Such APBs tend to be very stable and thus typically become permanently ingrained in the film structure. The question we address here is the effect that APBs in the bulk of the film have on the surface structure. 

Crystallographic shear planes (CS planes) are planar faults in a crystal that separate two parts of the crystal which are displaced with respect to each other. The vector describing the displacement is called the crystallographic shear vector (CS vector). Each CS plane causes the composition of the crystal to change by a small increment because the sequence of crystal planes that produces the crystal matrix is changed at the CS plane. (From this it follows that the CS vector must be at an angle to the CS plane. If it were parallel to the plane, the succession of crystal planes would not be altered and no composition change would result. A planar boundary where the displacement vector is parallel to the plane is more properly called an antiphase boundary.)... 

Each of SiC s crystalline polytypes has a distinct oxidation rate under the same oxidation conditions [1,3,4]. For the various SiC polytypes, the oxidation rate on the (0001) Si faces increases with the decrease in the percentage of hexagonality of the SiC polytype, while the growth rate on the (0001) C faces does not depend dramatically on polytype [3]. The dramatic difference in oxidation rates between opposite faces of the polar SiC crystal has long been known. Intermediate faces have intermediate oxidation rates [3]. As with other semiconductors, conduction type, dopant density, surface roughness and crystalline quality should also be expected to have an effect on the oxidation rate [5-7]. Selective oxidation at antiphase boundaries has been reported for wet oxidation of 3C-SiC heteroepitaxial layers, but not for dry oxidation [8-10]. 

Despite continuing progress in the crystal growth, 3C-SiC films still contain many lattice defects. In particular, twins, stacking faults and antiphase boundaries (APBs) have been reported [64,65]. APBs occur as a common defect when a polar film, SiC in this case, is heteroepitaxially grown on a non-planar substrate. To eliminate this particular defect in 3C-SiC films, Si substrates misoriented from the (100) plane have been used, as stated above [39,53],... 

Figure 3.24 Surfaces and boundaries in a crystal (a) the external surface (b) grain boundaries (c) a twin plane (d) an antiphase boundary and (e) a crystallographic shear plane...

Such interconnections are sometimes observed only in one slice of the crystals. An example is shown in the HREM micrograph of Fig. 33a. The contrast at the edge of crystd, characterized by double rows of white dots corresponds to a triple slab "PbO-Cu-PbO" at the level of the defect such a double row disappears and is replaced by a triple row of small spots. This defect is explained by the fact that a "45°" antiphase boundary is interrupted at the level of the "SrO" planes as a consequence, this limited defects can be interpreted as the replacement of triple "PbO-Cu-PbO" layer by a "Cu02-Y-Cu02" layer (Fig. 33b). 

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). 

An antiphase boundary (APB) is a type of stacking fault. Stacking faults also occur in materials that do not order, the simplest example being a fault in the normal close-packed layering in face-centered cubic metals, like copper, represented by CABCAB ABCABC. Stacking faults, formed during crystal growth or as a result of deformation (shp), are bounded by partial dislocations. 

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