Dear Pi Te,

with TEM you have access to the grain size and via XRD peak width you get the crystallite size. They actually differ. A grain may consist of a lot of crystallites. Thus the crystallite size is equal or smaller that the grain size.

You may share what numbers do you get from TEM and what numbers you get from XRD.

There is an enduring discussion on particle, grain and crystallite size here on RG. Please have a look for example at the following link:

https://www.researchgate.net/post/What_is_the_difference_between_crystallite_size_and_grain_size_Is_it_necessary_to_do_both_microstructural_and_XRD_analysis_for_it?_iepl%5BgeneralViewId%5D=QHjY0yL0RxBPj761AMTICQWwoJKiNNt1cMMu&_iepl%5Bcontexts%5D%5B0%5D=searchReact&_iepl%5BviewId%5D=NhB6CZby8GT79B57gN57XS9y5jW689CLnw10&_iepl%5BsearchType%5D=question&_iepl%5Bdata%5D%5BcountMoreThan20%5D=1&_iepl%5Bdata%5D%5BinteractedWithPosition6%5D=1&_iepl%5Bposition%5D=6&_iepl%5BrgKey%5D=PT%3A58f46e01dc332d77bc11a2f9&_iepl%5BtargetEntityId%5D=Q%3A58f46e01dc332d77bc11a2f9&_iepl%5BinteractionType%5D=questionView

In addition to Gerhard Martens answer, I would also suggest to reaffirm with yourself that during the crystallite size analyses with XRD, you correctly took into account any peak broadening from the instrument itself. This will skew your crystallite size determinations if not done (correctly).

Also make sure that there are no effects of you sample preparation for XRD v. TEM on the respective crystallite and crystal sizes.

In addition, you should check whether your sample is radiation sensitive and whether e.g. TEM at high kV and STEM at low kV give the same result (possibly not).