Conceptually, If we consider one particle only, regardless of the ability to achieve the XRD and SEM tests, then its dimensions measured by SEM should be comparable to that calculated from Scherrer's formula, supposing that we fulfill all Scherrer's conditions for such XRD analysis, if and only if that particle is a single crystal.

However, in reality, the diffraction data obtained from the XRD of powder represent the average of a huge number of particles; thus the calculated dimensions are the average dimensions also. Unfortunately, the SEM image shows the dimensions of certain particles but NOT the average. That is why such a comparison is questionable. Please note that the comparison will be more questionable when the powder consists of particles with different shapes.

@ Imad Disher

Thanks for kind Reply