Diffraction contrast dislocations

There are established criteria for obtaining b by using diffraction contrast (23). Briefly, the dislocation intensity (contrast) is mapped in several Bragg reflections (denoted by vector, g) by tilting the crystal to different reflections and determining the dot product of the vectors g and b (called the g b product analysis). 

The reflections include a particular g in which the dislocation is invisible (i.e., g b = 0 when b is normal to the reflecting plane). With these criteria in diffraction contrast, one can determine the character of the defect, e.g., screw (where b is parallel to the screw dislocation line or axis), edge (with b normal to the line), or partial (incomplete) dislocations. The dislocations are termed screw or edge, because in the former the displacement vector forms a helix and in the latter the circuit around the dislocation exhibits its most characteristic feature, the half-plane edge. By definition, a partial dislocation has a stacking fault on one side of it, and the fault is terminated by the dislocation (23-25). The nature of dislocations is important in understanding how defects form and grow at a catalyst surface, as well as their critical role in catalysis (3,4). 

Figure 3.25. In situ catalysis (a) fresh VPO catalyst (b) dynamic real-time formation of atomic scale catalyst restructuring in butane after 2 min at 400 °C (c) enlarged image of (b) showing two sets of partial dislocations and (d) dynamic image of two sets of extended defects along symmetry-related (201) in (010) VPO after reduction in butane for several hours (diffraction contrast). The inset shows the defect nucleation near the surface. Careful defect analysis shows them to be formed by novel glide shear, (e) One set of the defects in high resolution (f) and (g) show diffraction contrast images of defects in 201 and 201. (After Gai et al, Science, 1995 and 1997 Acta Cryst. B 53 346.)...

Hirsch, P. B., Howie, A., Whelan, M. J. (1960). A kinematical theory of diffraction contrast of electron transmission microscope images of dislocations and other defects. Phil. Trans. Roy. Soc. (London), A252, 499-529. 

Tholen, A. R. (1970). On the ambiguity between moire fringes and the electron diffraction contrast from closely spaced dislocations, phys. slat, sol., (a)2, 537-50. 

In accord with the optical examination, both specimens show under the TEM piogressivc zones of shock damage, reflecting the changing shock conditions in the depth of the sample. The laser-shocked specimen displays the largest variation in microstructures. Just below the excavated zone of this specimen, i.e., within a depth of -50 pm below the original preshock surface, we observe a spotty diffraction contrast (Fig. 1.11a). The spots represent tiny dislocation loops that may result from decomposition of CaCOs into CaO and CO2 [41,42]. 

The reason we see a dislocation is that it bends a crystal plane near its core region. Indicate a case where we may not be able to see the dislocation in diffraction contrast under a two-beam condition (a two-beam condition refers to a crystal orientation in which the intensity of one diffraction spot is much higher than those of the other diffraction spots). Indicate the answer either graphically or using the relation of vectors g (the normal of planes which generate the diffraction beam) and b (Burgers vector of dislocation). 

Dislocations. Dislocation configurations have been studied mainly by diffraction contrast bright-field imaging since this method is very sensitive to lattice strain fields. Under these conditions, dislocations appear as dark lines when two-beam diffraction conditions close to s 0 are used. A network of dislocations in the basal plane (0001) of zinc is visible in Figure 66. 

Loss of crystallinity causes all diffraction contrast features in the TEM image to fade away. Moire fringes, lattice fringes, bend contours and the like will all lose contrast during irradiation [118]. Features that depend on orientation such as bend contours or dislocation strain field images will become smeared out, as directions in imperfect or very small crystals are less well defined (the reciprocal lattice spot increases in size). During irradiation, new contrast features -radiation artifacts - can appear temporarily and then fade with the rest. 

Fig. 5.4 Diffraction contrast images of (a) stacking faults, (b) dislocations, and (c) a dislocation loop...

Diffraction contrast imaging is also widely used in analysis other defects such as dislocation loops, partial dislocations, strain field introduced by small precipitates, etc. In Eig. 5.4c, the Burgers vector for the dislocation loop always points to the center, so the areas perpendicular to the diffraction g show no contrast since g-b = 0. 

FIGURE 12.8 Diffraction contrast from dislocations in AI2O3. 

Dolomite is much less prone to electron damage than calcite and so it is easier to determine Burgers vectors using diffraction contrast methods. The vectors for perfect dislocations are 2 110 and 12 01). However, dislocation dissociation is also possible in dolomite when it occurs in the basal planes, pairs of... 

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