Crystal lattices forming images

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. 

Figure 6.7 Schematic drawing of the crystal structure of glycolate oxidase. Each disc is an octamer, and these octamers are twisted relative to each other to form an open three-dimensional lattice. The image is adapted from [1].

The crystal lattice, however, plays a second role. It not only amplifies the diffraction signal from individual molecules, it also serves as half a lens. The X rays scattered by the atoms in a crystal combine together, by virtue of the periodic distribution of their atomic sources, so that their final form is precisely the Fourier transform, that is, the diffraction pattern that we would ordinarily observe at / if we did in fact have an X-ray lens. Thus the situation is not intractable, only difficult. We find in X-ray crystallography that while we cannot record the image plane, we can record what appears at the diffraction plane. It is then up to us to figure out what is on the image plane from what we see on the diffraction plane. 

Twinned crystals of many materials are produced during both growth and deformation. Many mineral crystals are found in a twinned form in nature and deformation twins are often obtained when metals are deformed, especially at low temperatures. The most commonly obtained types of twinned crystals are those in which one part of the crystal is a mirror image of another part. The boundary between the two regions is called the twinning plane. The particular types of twins that form in a crystal depend upon the structure of the crystal lattice and deformation twins can be explained in terms of a simple shear of the crystal lattice. 

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

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