[Ifeffit] Nearest neighbor statistics in a random alloy

[Felix E. Feiten]

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