34.5: Using Polycrystalline Materials in the TEM
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Just as with X-rays, a completely isotropic fine-grained polycrystalline sample will give a diffraction pattern of concentric rings in the zero order Laue zone (ZOLZ), as the many small crystals at random orientations produce a continuous angular distribution of hkl spots at distance 1/dhkl from the 000 spot - a ring of radius 1/dhkl around the 000 spot for each allowed reflection. The rings are then indexed according to the order of allowed reflections within the ZOLZ.
As the grain size increases, the rings within the diffraction pattern break up into discontinuous rings containing discrete reflections. If there is any texture (preferred orientation) within the specimen, arcs may be seen instead of complete rings.