Dear Fatima
The constant of proportionality, K (the Scherrer constant) depends on the how the width is determined, the shape of the crystal, and the size distribution. the most common values for K are:
0.94 for FWHM of spherical crystals with cubic symmetry
0.89 for integral breadth of spherical crystals w/ cubic symmetry 1, because 0.94 and 0.89 both round up to 1
K actually varies from 0.62 to 2.08.
if you are going to calculate crystallite size of graphene using XRD, you may calculate but it is not right measurement abt graphene. you should go for TEM.
And yes, K depends on width, crystal shape and size.
Mr. Basker is right : 0.94 for FWHM of spherical crystals with cubic symmetry
0.89 for integral breadth of spherical crystals w/ cubic symmetry 1,
K varies from 0.62 to 2.08.
Moreover, in detail K is different for different hkl-directions (as body diagonal = sqrt(3) * edge of cube ). You can compare distribution of peak intensity between experimental and calculated patterns. If they are the same you can no take into account K-constant for determination of size crystal
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