Deviations from Perfect Diffraction

Another argument in favor of XRD is that catalytic activity should be related to the defects (or "nanostructure") of a solid catalyst. Active sites are nonequilibrium structures and are therefore not part of the bulk structures that are most commonly determined by diffraction analysis. XRD is, however, in many ways sensitive to the deviations from perfect ordering of the unit cells in a perfect crystal hence, XRD can be used to detect the nanostructure representing deviations from an ideal crystal. 

Actually, X-ray diffraction in general is a useful supplement to high-resolution electron microscopy because it measures the mean characteristics of the whole sample, whereas the HRTEM can only cover ejctremely small sections of it. The XRD results confirm the sp -character of carbon atoms in onions, low particle size as well as a certain deviation from perfect crystaUinily. This imperfection results from the nonordered packing of individual shells, the different sizes of individual onions, and the defects present in the fullerene shells. 

The effects of non-parallelism of the mirrors in a nominally plane-parallel cavity have been studied by Fox and Li (1963). The diffraction loss is found to increase rapidly for only slight deviations from perfect alignment or from perfect optical quality of the windows and mirrors. 

The carbonized material is often called amorphous or baked carbon . It is without long-range crystalline order and the deviation of the interatomic distances of the carbon atoms (from the perfect graphite crystal) is greater than 5% in both the basal plane ab directions) and between planes (c direction), as determined by x-ray diffraction. 

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