Hello Zeynab Addayeva

The definition of crystallite size remains the same for layered structures. You must know the logic of the definition of crystallite, grain, and particle. A particle can be made of many grains, which can be made of many crystallites. So, a crystallite is the smallest possible solid formed. Don't misunderstand with unit cell, as unit cell is only an abstract structure.

So, crystallite is the smallest solid formed. A grain can group many crystallites, and a particle can group many grains. Normally, we can not see the crystallite even in a microscope, but there can be a crystallite that is so big that we can see and touch. These are called SINGLE CRYSTAL, a crystallite so big that we can use it to make single crystal X-RAY DIFFRACTION to solve its structure.

Your layered structure is the same as other solids. The difference is that you have a preferred orientation of the atoms.

Feel free to reply if you need help.

Best regards,

Dr. Ricardo Tadeu

Dear Ricardo Tadeu Maia, Thank you very much for your response.

I would appreciate your thoughts on the shape of crystallites .

Additionally, several studies in the literature suggest that the number of stacked layers can influence the calculated crystallite size, especially in materials like graphene. What is your perspective on this? Do you think the change in layer count affects the Scherrer-derived crystallite size or structural coherence length in the c-direction?

You are welcome, Zeynab Addayeva

Crystallite is the smallest solid formed by atoms or molecules packing. When the material is 2D layered, it extends by a 2D plane, then stack one layer on the other, as I show in the picture of cellulose 2D unit cell.

Yes, the crystallite size depends on the numbers of layers, because as much layers you have, the bigger the solid structure. I never made this "geometry" calculation, but you can guess the numbers of layers based on the crystallite size. Look to the unit cell: it has 3 layers in the unit cell in a axis, which is 7.784 angstrom length. Meaning that you have 3 layers in 7.784 angstrom. So, if you have 50 nanometers crystallites, how many layers do you have? I think it's correct to make this assumption.

Please be aware that this cellulose did not properly set c as the stacking direction. Many authors change it to match standardization, so don't worry about this.

About the shapes: We start every refinement considering that the particles are sphere shaped, and we can think that crystallites are aleatory formed with no preferred direction growth. However, if you have like a needle-shaped particles, then you can state that there is a PREFERRED ORIENTATION in which the crystallite grew. This is exactly what happens when you grow a single crystal. A single crystal is a single, big, crystallite with a preferred orientation growth direction. But in powder XRD, preferred orientation is not desired, so we have to avoid it.

I hope it helped. Reply if you need more help.