I am doing Rietveld refinement of high pressure data for a Thallium based ternary system. I am not sure about the refinement for the occupancy values of all the three atoms. Also I want to know is there any problem if I refine all the occupancies together.
In principle, the strategy of refinement should finally end up with refinement of all structural and stoichiometric parameters; all together. As far as the occupancy is concerned we start refining the heaviest ions first. They will have the greatest effect on the refinement results. Then refine for the smaller ions. The final step is to refine all occupancy values together. This will guarantee that the refinement is a convergent process.
I only want to know what you meant by high-pressure XRD data?
If you refine all occupancies together, you get 100% correlation with the scaling factor. Ever reduction of all occupation factors can be offset by a corresponding increase in scaling factor. Even if your software catches this and stops the numbers from running into the hills, the results will be meaningless. You will have to fix the occupation for at least one crystallographic site, or fix the total occupation.
If you really need to determine the occupation of all sites, e.g. because you expect vacancies on all sites, you must do so experimentally. Either by extremely careful diffraction experiments with control of the primary beam intensity and amount of sample in the beam. Or by determining the density of the compound by another method and comparing this result with the x-ray-density.
In principle, the strategy of refinement should finally end up with refinement of all structural and stoichiometric parameters; all together. As far as the occupancy is concerned we start refining the heaviest ions first. They will have the greatest effect on the refinement results. Then refine for the smaller ions. The final step is to refine all occupancy values together. This will guarantee that the refinement is a convergent process.
I only want to know what you meant by high-pressure XRD data?
Dear Ahmed A. Al-Tabbakh ·sir,
Thank you very much for answering my question. You asked a question that what do you mean high pressure XRD data. The answer is the following.
Like temperature, pressure is also a thermodynamical parameter. It is possible to apply pressure on a sample even more than the pressure inside the earth (core) using diamond anvil cell. The in-stu measurements like XRD, Rman, Brilouin etc can be studied on the sample under pressure. From this study we will get the informations such as structural phase transitions, lattice dynamics, insulator-metal transitions, pressure induced amorphization, bulk modulus, electronic topological transition, topological quantum phase transitions etc. High pressure resaerch is an active area of research.
Dear Mr. Sorb Yesudhas .. This is really a very interesting field. I think it also involves high technology. Tell me please, do you have positions offered for foreign scientists to work in your institute?
Dear Dr. Ahmed A. Al-Tabbakh
Right now I feel no positions are available in our institute. Lot of post doctoral positions in this field are frequently coming, why don't you try that ?
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