Hi Arindam,

you are correct. Smaller crystallite size increases FWHM. 

This topic is very often discussed here in RG. Please see the link on Scherrer equation.

But the Scherrer equation relates to crystallite size, not to grain size..

 Please see the difference of grain and crystallite according to the second link.

You will find examples in the RG links.

https://en.wikipedia.org/wiki/Scherrer_equation

https://en.wikipedia.org/wiki/Grain_size

https://www.researchgate.net/post/How_do_I_calculate_nanocrystallite_size_by_Debye-Scherrer_equation_using_XRD

https://www.researchgate.net/post/May_I_use_the_Williamson_Hall_method_or_Scherrer_equation_to_calculate_the_grain_size_at_GIXRD_diffractogram