crystallite size from XRD is bigger but from TEM it is smaller, because you calculate the crystallite size from scherrer formula which is just approximation about the crystallite size however TEM demonstrate the actual crystallite size. 

crystallite size from XRD is bigger but from TEM it is smaller, because you calculate the crystallite size from scherrer formula which is just approximation about the crystallite size however TEM demonstrate the actual crystallite size. 

I fully agree with Arvind Kumar and in addition to his comment, I could say that the FWHM that you use in the Scherrer equation depends on the microstrains and instrumental profile. The  K (the Scherrer constant) depends on how the width is determined, the shape of the crystal (the most applications of Scherrer analysis assume spherical crystalline shapes), and the size distribution. That's why the crystallite size could differ from the particle size.

Best regards,

Maria

Thank you so much everyone

Crystallite size calculated from powder XRD data is inversly related to the line width of the diffraction peak. How distortion effects this line width will be reflected in crystal size.

Yes, I also calculated the same in my results. so Mr. Arvind and Dr. Christian are absolutely right.