If a ternary ABC2 compound with two sublattices, say BC4 and AC8, where B and C are at the center of tetrahedra and octahedra respectively, suppose if A atom is randomly fluctuating inside the AC8 octahedra, which means a fluctuating electron density around A site. Obviously, except A site other atoms will diffract normally, whereas the X-rays seen by the fluctuating electron density at A site will have lesser X-ray diffraction (less electron density to diffract) than the expected diffraction. Can I expect the lattice plane containing A atoms will have lesser intensity as compared to the expected intensity of the XRD pattern?
Dear Sorb Yesudhas ,
your atom A is not absent, but is still there. So, to my opinion, it will still perform elastic scattering. Only its position is moving. So the height of the peaks; to which atom A is contributing to, will decrease a bit in favour of the peak width, which will increase a bit. But the peak integral will remain the same.
Gerhard Martens @Thank you so much sir. This question raised because I couldn't do Rietveld refinement. The calculated peak intensities didn't fit well with the experimental synchrotron XRD pattern. The fitting was too poor for two peaks which are related to the planes containing A atoms.
Gerhard Martens @ I also expect an incommensurate phase formation as BC4 tetrahedra and AC8 octahdra don't synchronize each other
Dear Sorb,
As already pointed out, you have a fluctuation in electron density. You will face a change in peak width for the reflection plane containing the A atom. However, the fluctuation will contribute to changes in the receiving plane intensity as well. Naturally, it depends on the electron density of the central atom. It must be significant to be observable. The reflection on your plane X-ray intensity will show how significant it is. Nevertheless, the exchange velocity plays a role; if too fast compared with your detection time, it will appear as static, may not reflect in a large width.
Best regards,
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