Crystal twinning defects

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 total excess free energy can further decrease by introducing crystal defects. Twinning defects, those in (111) plane for fee metals in particular, are the most common and widely observed type in the shape control of metal nanostructures [38, 39]. The introduction of twin planes reduces the symmetry and alters the... 

As with the 1-D nanostructures discussed in Section 10.5, the formation of 2-D Pt nanostructures relies upon either defects in seed crystals or using templates. The most commonly encountered defects with platinum metal are those due to stacking faults, such as twin defects in (111) planes. Lipids and micelles at the interface are the types of soft template most useful for the generation of 2-D nanostructures. Some TEM images of representative, recently created 2-D Pt nanostructures are shown in Figure 10.10 these structures include planar multipods (bipods and tripods), triangular plates and dendritic sheets. 

Figure 11.5 Schematic of conventional shapes of face-centered cubic (fee) metal nanostructures. The shapes in the top row are single crystals, in the second row are particles with twin defects or stacking faults, and in the...

Although specific calculations for i and g are not made until Sect. 3.5 onwards, the mere postulate of nucleation controlled growth predicts certain qualitative features of behaviour, which we now investigate further. First the effect of the concentration of the polymer in solution is addressed - apparently the theory above fails to predict the observed concentration dependence. Several modifications of the model allow agreement to be reached. There should also be some effect of the crystal size on the observed growth rates because of the factor L in Eq. (3.17). This size dependence is not seen and we discuss the validity of the explanations to account for this defect. Next we look at twin crystals and any implications that their behaviour contain for the applicability of nucleation theories. Finally we briefly discuss the role of fluctuations in the spreading process which, as mentioned above, are neglected by the present treatment. 

The achievement of the corresponding monocrystals of sufficient optical and crystalline quality is made possible only after very thorough purification. Chemical impurities are known to disturb the crystal lattice through the occurence of twins, veils dislocations, rounding-off of faces ultimately quenching further growth. Any crystalline defect dramatically increases the residual absorption coefficient and lowers the optical damage threshold. 

Single crystal and bulk BaTiOs exhibits a sharp paraelectric-to-ferroelectric transition at 393K. In the presence of submicron grains, the transition becomes diffuse and can be absent for polycrystalline BaTiOs. Twin boundaries along the four crystallographically equivalent 11 planes constitute the main lattice defects. Junctions between such twin boundaries can be frequently observed within a grain. The local atomic arrangement of the core of twin intersections was studied by focal-series reconstruction (Jia etal. 1999). 

The introduction to this chapter mentions that crystals often contain extended defects as well as point defects. The simplest linear defect is a dislocation where there is a fault in the arrangement of the atoms in a line through the crystal lattice. There are many different types of planar defects, most of which we are not able to discuss here either for reasons of space or of complexity, such as grain boundaries, which are of more relevance to materials scientists, and chemical twinning, which can contain unit cells mirrored about the twin plane through the crystal. However,... 

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