Recently we synthesized some CdS particles. Prepared samples were characterized by X-ray diffraction. The ayanlysis result shows a hexagonal structure (ICSD 43599) in space-group of P 63 m c (186). Its cell parameters are a=4.1400 Å and c=6.7150 Å.
I use diamond3.2 opened the ICSD 43599.cif and created several planes on the crystal. I found that in the planes of (100), (101) and (103) there had no atoms presence. Yet in the X-ray diffraction results these three planes all showed diffraction peaks.
My question was: Must the corresponding crystal planes of the X-ray characteristic peaks have atoms present? If it is, why in the crystal models (ICSD 43599.cif) on the (100), (101) and (103) planes I created have no atoms presence; If it is not, why these three planes showed diffraction peaks?
Further more, the question is: did diffraction planes (which showed diffraction peaks) must have atoms presence? And the reasons.
Thanks!
Kun Li
Hi Kun Li,
yes, in diffraction peaks there are atoms. If these planes must have not present, may be there is another phase in your sample. The synthesis can be not finalized.
Description of the X-ray diffraction as reflection from the planes is a nice simplification that allows to get a quick image of the Bragg-Brentano geometry but is not the real physics of the process. The crystal does not have any mirrors inside and does not know anything about the choice of the origin that the scientist uses. If you want to continue using this image, you can easily move your set of planes parallel to itself until you have atoms on them. Then you need to sum up all contributions from all possible positions of this set and this will give the intensity. Sometimes the intensity will become zero because the contribution of the different positions exactly negate each other - so called systematic extinctions.
Crystals consists of plane of atoms that are spaced a distance d apart, so in diffraction peaks, there are atoms.
Thanks for the explanation of S. R. de Lazaro and Alexander B. Missyul’. Both of your answers are useful for me to further understand the growth mechanism of CdS particles during preparation. Since we would want to know the preferentially grown planes and the atoms distributed on it, then we can judge the charges of each plane. Combining the pH condition of solution, we might able to deduce the effect of pH value on the preferentially grown planes, so as to the morphology of final CdS particles.
Thanks again for your explanation and concern. : )
Kun Li
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