I have some confusion about the site occupancy in Heusler Alloys. There are many papers but no single rule on site occupancy.
In a full Heusler Alloys (X2YZ), what the the site occupancies of X, Y and Z atoms. how to veryfy that the given occupancies are correct or wrong?
How to calculate disrder in these alloys?
Generally people calculate it by the ratio of supperlattice and fundamental peaks. but if you have single crystal, you have to measure XRD for each plane separately, how these intensities can be considered same even though the experiment is not performed in one go.
Please clarify these issues.
There are international rules how to handle occupancies as well as substitutions. This is not a big deal during description of crystal structure data and does not only concern Heusler alloy. Therefore, if you write "many papers" then please be a little bit more specific and either cite them or better give some examples added by one or two files with the structure description.
On the other hand, a description of disorder is in act very complicated since all common diffraction models finally are based on a translation symmetry which assumes a perfect ordering (possibly of disordered volumes). But be fair: Do you seriously think that nature cares about that? We measure something and try to press it into a known concept to handles everything as easy as possible. I myself assume that many crystals structures are incorrectly described because of wrong assumptions or an insufficient quality of experimental data. You should be always aware of this.
We are expecting your occupancy descriptions from papers you referred to...
"In a full Heusler Alloys (X2YZ), what the the site occupancies of X, Y and Z atoms. how to veryfy that the given occupancies are correct or wrong?"
Assumed that you have ideal condition of Heusler Alloy X2YZ where X and Y are transition metals and Z is in the p-block. There are 2 system: L21 has ordered Y and Z position (distinguishable) and B2 has disordered Y and Z position (undistinguishable).
L21:
Y 0.00000 0.00000 0.00000 4a
Z 0.50000 0.50000 0.50000 4b
X 0.25000 0.25000 0.25000 8c
B2:
If by definition u cant distinguish them, the measurements cant help.
Anyway since Y and Z elements is from different block of periodic table, where the atomic radius and the unpaired electron is different, assumed the energy system is minimum, you can try to refine by introducing the partial occupancy. And to validate your guess although of course not comparable as what Gert pointed out, DFT simulation is one of the choice.
Since Heusle Alloy usually has magnetic behaviour than neutron diffraction has more advantage than x-ray, or you can combine them to get better structure information.
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