Hi Iraida,

I am a little confused by your question.
GaAs structure has 8 atoms in unit cell with a = 5.653 Ang. If I had a cluster 7 Ang in radius,
that won't give 900 atoms...that is a volume less than 3 x 3 x 3 unit cells (216 atoms).

You can calculate for substitutional and interstitial models where the Mn is isolated in the cluster,
and compare the results to data by combining (linear combination). If the dopants are isolated, this
can give you an estimate of the ratio of substitutional to interstitial. If you know the overall
concentration of Mn, you can say something about concentration of defects (i.e. interstitial defects
versus substitution).

There is only one central atom to the cluster on which the calculation is based.
That central atom is the only absorber for which the calculation is done. If you create a model
that has the desired proportions of substitutional and interstitial absorbers, the calculation is still
only run on one central atom.

Using cluster size to create the average concentration is not what you want to test.
You want to test if the Mn are isolated or clustering near each other.

Clustering changes the local stoichiometry from what one would calculate as an average from known
concentration.

You can test for clustering effects with a model that has the central atom being either substitutional
or interstitial and additional Mn in nearby shells (interstitial or substitutional). For example, you could
calculate for a Mn_Ga with a nearby Mn_Ga, or nearby Mn_i, or both, and similarly for Mn_i. If there
is a pronounced effect on the calculation that you don't see in the data, you can argue that the Mn
are truly isolated. If there is no pronounced effect, then you would have to compare a number of combinations
to see what best approximates with the data.

Does this clarify anything for you?

-R.

On 2022-01-16 1:30 p.m., Iraida N. Demchenko wrote:

Dear All,

Do I understand correctly that using feff calculations it is impossible to calculate the concentration of defects in a sample? I'll try to explain my question with an example. Let us assume that we have a (Ga,Mn)As sample, where Mn is a dopant. Comparing the experimental curve (Mn K edge) with the theoretical spectrum, we see that the experimental one is well described by a linear combination of two theoretical ones, namely, substitutional manganese in the position of gallium (Mn_Ga) and interstitial manganese (Mn_i) in proportions of 65 and 35%, respectively.

Is it possible to recalculate these phases weights (I mean Mn_Ga and Mn_i) to the real concentration of the mentioned above defects in the sample? For example, for the Mn_Ga model, one manganese atom was considered per cluster of 900 atoms (FMS 7A). Does it make sense to recalculate the stoichiometry (using cluster size) and, subsequently, the weights of this model when compared to the experimental curve, which will eventually give the selected defect concentration at the low level (like ~0.2%). Do I understand it correctly?

I will be very grateful for your help in understanding.

Best,

Iraida.
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