[Ifeffit] EXAFS of solid-solution alloy

Matthew Marcus mamarcus at lbl.gov
Fri Feb 8 11:37:11 CST 2019

Dear Chongchong.  Regarding yours of Fri, 8 Feb 2019 19:36:24 +0900:

For concreteness, let's pretend that A = Mn and B = Co, which differ by 2 in Z.  The scattering factors (amp+phase) for Co are very close to those of Mn.  Thus, you probably can't
distinguish Co from Mn neighbors.  Instead, if you're doing ab-initio fits, I suggest doing the fits as if all the scattering atoms were Fe, right in the middle.  If you're doing
fits from, say, pure Mn and extracting amp+phase, then you could "correct" to Fe by doing theoretical paths for Mn-Mn and Mn-Fe and taking the ratio of amplitudes and the difference
in phase and applying these to the Mn-Mn experimental factors.  I used to do this all the time back when FEFF wasn't good enough to use without references.  Why ratio of amps
and differences of phase?  You can think of the scattering factor as a complex number Ac = A*exp(i phi), and so Ac(Fe)/Ac(Mn) = (A(Fe)/A(Mn))*exp(i(phi(Fe)-phi(Mn))).

To get an idea of what the error in the 'all Fe' assumption is, you could do the fits pretending that all the scatterers are Mn and again with all the scatterers Co and see how
different the results are.

I imagine that the dsig2 will be smaller for each end-member than in the middle, due to lattice distortion as well as the phase difference between A and B.  Can you do the EXAFS
at both edges?  If so, any difference between these two will be due to the difference in central-atom phase and S0^2 plus any clustering of A with A and B with B.  Again, theoretical
sims will tell you how big the difference in central-atom properties is.  You can simulate a lattice consisting entirely of A and one in which only the absorbing atom is replaced
by a B and apply these differences to the experimental data from each edge.

    Matthew Marcus
> Hi all,
> I have some problems in analyzing the EXAFS of solid-solution alloy, let us define as A0.5B0.5. I have searched the mailbox but cannot find similar discussion yet. So I post the question here and hope to get some suggestion from you.
>  1. A and B are immiscible metals from the phase diagram, which means that I couldn’t get the bulk A0.5B0.5 as a standard reference. In this case, how can I estimate the amp for the K-edge of A or B?
>  2. A and B are very near (atomic number B-A = 2) in the periodic table. So their lattice parameters are quite close to each other. In this case, when I tried to fit the 1st shell use A-A and A-B paths at the same time, the Happiness parameter becomes worse and the coordination number of A-B is not realistic (large errors!).If I fit with only the A-A path or A-B path, I get a good fit. However, atomic resolution STEM-EDX maps show the homogenous distribution of both A and B atoms. The A-A and A-B bond might have an equal ratio. In this case, how can I do the fitting?
> Thank you very much in advance!
> Sincerely,
> Chong
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