Calculation of average crystallite size:

Let us consider a large sized crystal as shown in Figure 4.01. This would exhibit a XRD peak occurring only at a particular angle 2θ_B.

Figure 4.01: Schematic of large sized crystal, crystals with reduced size and the corresponding XRD patterns [1].

On the other hand, the width of the diffraction peak increases as the size of the crystal decreases (see Figure 4.01). This allows one to calculate the average crystallite size from the XRD peak broadening.

Ref.: [1]. B. D. Cullity, Elements of X-ray diffraction Addison-Wesley Publishing company, Inc., USA (1956)