[Ifeffit] Calculation of the doping level

["Dr. Dariusz A. Zając"]

Dear Bruce, Dear Matt,

thank you for answers.

Short comments to yours:

> I think there is a small problem is what you wrote here.  The cluster
> you describe is consistent with 0.56 at% doping but also with anything
> smaller.  A more pedantically correct way of describing the cluster
> you contrusted is that it is consistent with any physical situation in
> which the Al dopants are, on average, separated by 8 A or more.  That
> necessarily precludes any situation in which there is Al clustering of
> any sort.
yes, actually there is no difference in result between 6A and 8A (and of 
course large too) clusters. Each enlargement of the cluster size causes 
the decrease of the doping level. Actually this was the idea of 
simulation - observation if an effect of clustering can be observed. 
Maybe simply we are on the lowest doping level possible to examine, 
including also the possible detection limits? I can also imagine that 
below some lowest doping level should not be observed any change in 
spectra...

> If you want to consider the effect of clustering on the XANES
> calculation, you need first to consider the convergence of the
> calculation of the host (albeit with the Al absorber) in cluster size.
> That's not quite the same thing as the mean free path, although there
> must be some relationship between the two.
Of course it has been done at the beginning.  The cluster size of 6A for 
ZnO reached already convergence. The trick is that I see difference in 
spectra between this with only 1 ion, as an absorber and this with 1 
additional, but no difference in distance between Al... I expected that 
with increasing of the distance between Al I should observe the change 
of the spectrum toward this with only 1 Al in the centre.
> That's not what that means.  What you have done is to introduce
> clustering.  You are now saying that Al atoms are paired, not that
> they exist in higher concentration.  They may also exist in higher
> concentrarion.  But, strictly speaking, that's not what the addition
> of an Al in that place means.
I see.  You have right - clustering effect can not be recalculated into 
doping level. Instead of that I should randomly distributed Al with the 
respect ration Al ions to all ions for desired concentration.
> OK.  This is the follow-up to the point I made above.  The convergence
> test on the host material sets an upper bound on how far away the
> second Al atom can be and still be visible in the calculation.  You
> are now probing that with an actual placement of the Al atom.
but why I do not observe any change in spectra?
> MFP is relevant, but it is just a way of putting a number to a more
> complicated effect.  The bottom line is that you want to understand
> spectral trends as changes are made to the configuration used in a
> Feff claculation.  To do that, you have to make a lot of Feff
> calculations.
you mean thousands or hundreds thousands? ;)

> Just, to follow up on Bruce's comment:  A single cluster with 1 Al
> absorber and no Al scatterers does not really describe "1%" or even
> "0.5%" doping.  It is "0%" doping.
Dear Matt,
I mixed up effect of clustering with doping, as Bruce explained. Anyway 
I can not agree with you - even one single Al is not 0% doping, almost 0 
= yes...

> By "1% Al doping", you (probably) really mean that each Zn site has a
> 1% chance of being occupied by Al.  I think that's what you have to
> simulate.  That means generated many (several hundred I would expect)
> clusters with each Zn site having 1% Al occupancy, and summing the
> results of those calculations.
Unfortunately you have right - unfortunately because it needs many 
calculations...

Thanks for answers
Darek