Material defects cautions

MCM-41 is considered as crystalline on a macroscopic level because of the regular arrangement of the mesopores in a honeycomb fashion therefore, it is possible to estimate crystaUite size perpendicular to the basal plane with the help of the X-ray diffraction pattern using the Scherrer equation, which has been widely used e to estimate the crystallite diameter for various materials. However, this technique must be used with caution, since faults or other defects can also result in fine broadening. The calculated crystallite sizes are shown in Table 1 and... 

A word of caution is needed at this point on the above observations and following comments. It is clear from Figure 9.1 that the defect structure has a strong concentration dependence, and thus for meaningful comparisons between different materials it is necessary to compare like with like. However, the concentration dependence of the activation energy curve is different for each substitutional cation indeed, Andersson et al. [22] showed that the interplay between the electronic and elastic component depends heavily on the dopant type. It is probably the case that the type of defect clusters observed is very dependent upon concentration, and that typically around the concentration for the minimum activation energy a change in the defect structure takes place. 

---