A paper published in science mentioned; "Samples did not show the characteristic diffraction lines from iron oxide, indicating that the Fe2O3 was amorphous or that the crystallites were smaller than 4 nm". Does it mean that if particle size is less than 4 nm we will not get XRD pattern? Can anyone help me clear the confusion?
Particles less than 2 nm are usually referred as the nano clusters which can not be detected by XRD, however, particles in the range of 4 nm should be detected.
Both particle size and concentration are equally important to be detected by XRD. Very low concentrations are often not enough to get response from the XRD.
If u have particle size about 4nm than the FWHM is ~8.2 2theta (SF) at 28.44 2theta (Si 111). For lower symmetry structure, the adjacent peaks will overlaps. This not consider other effect such as defect, surface relaxation etc.
U need other technique called Pair Distribution Function (PDF) which zoom into diffuse pattern.
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