[Ifeffit] Nearest neighbor statistics in a random alloy

[Qingying Jia]

Hi Felix,


Your assumptions are not correct because the model you use is not a
representative unit cluster model. Basically the following three equations
are always correct for foil:

 NPt-Pt+NPt-Co=12

NCo-Co+NCo-Pt=12

NCo-Pt/NPt-Co=9

And the specific values for the coordination number can be determined if
you know the orderness of the structure. As for the random distribution of
Pt and Co in foil,

NPt-Pt/NPt-Co=9

or equivalently,

NCo-Pt/NCo-Co=9,

Then you can get

NPt-Pt=10.8, and NPt-Co=1.2

The results will be different if the structure is ordered.


  Regards,

On Fri, Jun 23, 2017 at 9:02 AM, Felix E. Feiten <feiten at cat.hokudai.ac.jp>
wrote:

> Dear all,
>
> I am fitting a Pt9Co1 foil sample.
> I assume the sample to be basically fcc platinum with every 10th atom
> randomly replaced by cobalt.
>
> Here are the assumptions I make:
>
> 1. every atom has 12 nearest neighbors
>
> 2. every Pt atom has (on average) 10.7 Pt nearest neighbors and 1.3 Co
> nearest neighbors
>
> 3. every Co atom has (on average) 11.7 Pt nearest neighbors and 0.3 Co
> nearest neighbors
>
> I have been told that assumptions 2 and 3 are not precise. Here is what
> I'm confused about:
>
> In order to determine the number of Pt or Co nearest neighbors I looked at
> an ensemble of 13 atoms:
> 1 absorber and 12 scatterers/nearest neighbors.
> I assume that the Pt/Co ratio in this ensemble is 9/1 and therefor,
> statistically, 1.3 out of those 13 atoms should be Co. That means that if
> Co is the central atom only 0.3 Co atoms are remaining for the 12 nearest
> neighbors. On the other hand, if Pt is the central atom, (all) 1.3 Co atoms
> are remaining for the 12 nearest neighbors.
>
> I have been told that the ratio of Pt/Co should be 9/1 in the 12 nearest
> neighbors, not in the 12 atoms including the central atom (quote: "because
> we have a bulk sample").
>
> But I do not understand why.
>
> Can someone please explain the correct answer to me?
>
> With kind regards,
>
> Felix
>
>
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