Effective misorientation

We can begin to understand the contrast mechanism by considering the ciystal to be made up of three distinct regions, namely the perfect crystal above the defect, the perfect crystal below the defect, and the deformed region around the defect (Figure 8.15(a)). We set the hmit of the deformed region as that where the effective misorientation ( ) arotmd a defect exceeds the perfect crystal... 

The width of the image can be deduced using this simple idea of contrast being formed when the misorientation around the defect exceeds the perfect crystal reflecting range. We consider the case of a screw dislocation nmning normal to the Bragg planes, where the line direction / coincides with the diffraction vector g. The effective misorientation at distance r from the core is =bH r (8.41)... 

Figure 9.8 Composite image of a series of double-crystal topographs recorded at different incidence angles of a SI LEC GaAs sample. The image is a set of contours of equal effective misorientation. (Courtesy S.J.Bamett)...

To demonstrate the effect of misorientation or even isotropization let us consider a structural entity52 which is a perfect lamellar stack. Figure 8.12 demonstrates... 

History. Wilke [129] considers the case that different orders of a reflection are observed and that the orientation distribution can be analytically described by a Gaussian on the orientation sphere. He shows how the apparent increase of the integral breadth with the order of the reflection can be used to separate misorientation effects from size effects. Ruland [30-34] generalizes this concept. He considers various analytical orientation distribution functions [9,84,124] and deduces that the method can be used if only a single reflection is sufficiently extended in radial direction, as is frequently the case with the streak-shaped reflections of the anisotropic... 

Figure 9.6. Fanning-out of an equatorial streak in a fiber pattern caused from misorienta-tion. Dashed arcs indicate azimuthal scans that are performed in practical measurements. The recorded scattering curves are used to separate the effects of misorientation and extension of the structural entities... 

Misorientation can be an issue during the time of collection of ED patterns as sometimes this can exceed 60 min in accumulation mode, and d5mamical diffraction contribution is observed (we may anticipate its presence due to the appearence of forbidden kinematically reflections in the pattern like +- 002). However, is important to note that misorientation effects become less critical and intensity of such forbidden reflections is lowered after applying precession mode to the ED pattern. Similar results have also been observed by M.Gemmi with Si samples. 

The determination of crystal structure in synthetic polymers is often made difficult by the lack of resolution in the diffraction data. The diffuseness of the reflections observed in most x-ray fiber patterns results from the small size and imperfect lattice nature of the polymer crystallites. Resolution of individual reflections is also made difficult from misorientation of the crystallites about the fiber axis. This lack of resolution leads to poor accuracy in measurement of peak positions. In particular, this lack of accuracy makes determination of layer line heights difficult with a corresponding loss of significant figures in evaluation of the repeat distance for the molecular conformation. In the case of helical conformations, the repeat distance may be of considerable length or, as we shall show, indeterminate and, in effect, nonperiodic. This evaluation requires high accuracy in measurements of layer line heights. 

Figure 4. Effect of cumulative strain on the distribution of boundaries by misorientation angles in Ti.

In the optimal conditions of epitaxy, the analysis of the current transients shows that the best fit is obtained using the Scarifker model, assuming a 3D instantaneous nucleation followed by a rapid diffusion control which is effective less than 0.1 s after the beginning of the pulse. TEM observations of very thin electrodeposits confirm that the coalescence of the first nuclei is achieved. Immeiatly after new epitaxial nuclei appear which coalesce when the the thickness of the deposit reaches 4 nm. The analysis of the moire patterns reveals that these nuclei present small misorientations. 

---