![]() The first transmission-electron-microscopy (TEM) images of an inorganic crystal, the metal oxide Ti 2Nb 10O 29 at 3 Å resolution (Iijima, 1971 ), were almost simultaneous with the first protein crystals studied by TEM: bacteriorhodopsin (7 Å) and catalase (9 Å) (Unwin & Henderson, 1975 ). Protein crystals, on the other hand, have much larger unit cells but are often too thick (>100 nm) to be studied by EM and they are very radiation sensitive. On one hand, inorganic crystals mostly have unit cells smaller than 10 Å and so could not be studied until the resolution of the EM surpassed the 4 Å point-to-point resolution in around 1970. For crystals, the situation was different. Indeed, electron microscopy (EM) could reveal details within living cells, with a resolution 100 times higher than the light microscope (Porter et al., 1945 ) and played a crucial role in the birth of cell biology in the 1950s. The ability to form images in the electron microscope makes it very powerful for looking at non-crystalline as well as crystalline objects. Soon after, the fact that electrons, unlike X-rays, can be focused into an image was exploited in the invention of the electron microscope (Knoll & Ruska, 1932 ). There is in principle no limitation to the complexity of the structures that can be solved by electron crystallography.Įlectron diffraction (ED) of crystals was discovered in 1927, only 15 years after the discovery of X-ray diffraction. Examples of electron-crystallography applications are given. Different techniques developed for electron crystallography, including three-dimensional reconstruction, the electron precession technique and ultrafast electron crystallography, are reviewed. Crystal structures can be solved to atomic resolution in two dimensions as well as in three dimensions from both TEM images and electron diffraction. In this paper, some recent developments of electron crystallography and its applications, mainly on inorganic crystals, are shown. There are two main advantages of structure determinations by electron crystallography compared to X-ray diffraction: (i) crystals millions of times smaller than those needed for X-ray diffraction can be studied and (ii) the phases of the crystallographic structure factors, which are lost in X-ray diffraction, are present in transmission-electron-microscopy (TEM) images. The method is demonstrated by experiments on several materials, but particularly on germanium and gallium-arsenide specimens since the similarity of these materials exemplifies the sensitivity of the technique.The study of crystals at atomic level by electrons – electron crystallography – is an important complement to X-ray crystallography. By using the tables, crystal point groups can be obtained from convergent beam or bend contour patterns. These tables assume the symmetric Laue condition and ignore the presence of irreducible lattice translations normal to the slab. A graphical representation of each diffraction group is given, together with tables showing how the diffraction groups are related to the specimen point groups and under certain assumptions to the crystal point groups. We find that the pattern symmetries may be described by thirty-one diffraction groups and that each of these diffraction groups is isomorphic to one of the point groups of diperiodic plane figures and to one of the thirty-one Shubnikov groups of coloured plane figures. The diffraction of fast electrons by a thin parallelsided slab has been studied by group theory and by a graphical construction. The convergent beam and bend extinction contour techniques of electron microscopy are capable of providing much more information than can be obtained from conventional diffraction patterns and it is the objective of this work to examine the symmetry properties of each of these patterns. ![]()
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