Can elaborate more on atomic disorder? If it is structural, are you talking about crystalline or amorphous samples?

However, atomic Pair Distribution Function studies can be employed to estimate chemical and structural disorder. One can get the PDF from XRD patterns measured upto large scattering vector value, typically about 20A(-1) for amorphous samples and more than 30 A(-1) for crystalline samples. You can take a look into the following book

"Underneath the Bragg Peaks, Volume 16: Structural Analysis of Complex Materials (Pergamon Materials Series) By Takeshi Egami and Simon J.L. Billinge

fully agree, pdf analysis is clearly the way to go for the analysis of atomic disorder, but you will need to have large q (2-theta) values if you want to be really precise, so high energy X-rays and high resolution detectors may come in handy to start with...

Dear Sridharan,

Can you please define what is PDF ?

Atomic pair distribution function. First note that it is a function, which is continuous function of scattering vector q, unlike structure factor located only about the Bragg reflections. From its position, FWHM and area one can estimate average bond length, disorder (both static and thermal) and coordination number. You may have a quick look into the book I had mentioned.

As I had asked in my previous post, what are U looking for; structural or chemical disorder (my terminology may bit different)

@Sridharan

I am looking for structural disorder.

Thank you

Well atomic PDF study will do.

@Sidharan

Thank you very much sir.

Dear Lakhan, I need to notify you that the word 'disorder' can mean very different things for crystallographers with different backgrounds. The PDF is a great tool if you want to learn whether the displacements of atoms from their positions in your inorganic structure are correlated or not. If, on the other hand, you want to know how a functional group in your organic molecule is disordered, the PDF study will be utterly pointless.

The disorder in the "partial occupancies" sense can be studied by powder diffraction in a way similar to lousy single-crystal diffraction data.

Could you possibly elaborate what problem you attempt to study?

Dear Ivan,

I am interesting in the disorder between various atomic sites.

Dear Lakhan,

Then you can simply refine these occupancies. It is easier said than done: the refinement of disorder in ONE organic structure from powder data can be difficult enough to be a sole topic of a talk at a high-level conference. Namely, last European powder diffraction conference (EPDIC13) had two such talks, by Martin Schmidt and Peter Stephens.

In inorganic structures, however, the process is relatively straightforward. You need to measure the high-angle data as far as possible (at least to 100 degrees 2theta using copper radiation) with good counting statistics. Then you can refine the occupancies of some atomic sites. Just remember that the thermal displacement parameters are highly correlated with occupancies, take into account the line broadening, and do not forget about the preferred orientation. Try to refine a known structure first to get a hang of it. Silicon will do as a first step to check the consistency of your data and methods — it has significant but known preferred orientation, and only two refineable parameters, namely the Biso of Si and the unit cell parameter a. If you have described everything correctly, the Biso of Si will be close to 0.6 with reasonable difference curve — and hopefully you will be able to refine occupancies from your datasets. The unit cell parameter close to 5.43123(8) A (for pure silicon) will also be an indication of a good instrument alignment, but is not entirely necessary.

Dear Ivan,

Thank you very much for your help. I will try this and will back to you if face any problem.

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