Crystallite Size from XRD is smaller than SEM particle size !!!

"Why the crystallite size calculated from X ray diffraction (XRD) and observed in Scanning electron microscope (SEM) are not matching?" 

Infact XRD gives small crystallite size in many instances. 
This is a recurring question from many research students working in material science.

Here is the hint....

 Please look at the figure ..... this explanation i have read long before in a tutorial handout, i couldn't dig that out. Once i get that i will give the links.

In X ray diffraction, Bragg Peaks are from the 'coherently scattering domains' (as found in text books) in the sample. These are the entities which,  X ray will see as depicted in the  left side of the figure below. In a powder sample,if prepared carefully, the orientation of crystallites are supposed to be random. 

Now we will discuss what SEM can give us? In SEM we always see/observe morphology. The crests and troughs on the surface of the sample. It can't see anything inside of the sample. Here we call those crests as particles and we measure particle size. A particle can have one or more crystallites with same size or with a range of sizes.

That's how we always have discrepancy in the Crystallite size and Particle size. If you want to compare numbers from XRD with some imaging technique then go for Transmission electron microscopy (TEM). These two will match with in the tolerance limits.

As such the image is self explanatory, still some one wants more details please post as comment.