[Ifeffit] Alloy EXAFS data fitting

[Matt Newville]

Dear Xianqin,

Anatoly answered much of this, but I'll add a little more:

> It is a two-element alloy system, name AB.  Their fitting results showed
> that A-B coordination number and A-A coordination number did not follow the
> similar trend with the temperature increase. I am just wondering if this
> case is possible?   I had thought if A-A and A-B are from the same compound,
> they should follow the same trend. Maybe I am wrong.

It's hard to say much without more system details (glass, crystal?),  but
it seems strange to see a temperature dependence of a coordination
number.  Most crystals would not have any change in CN with
temperature except at structural phase transitions.   As Anatoly said,
there may be an exchange of one atom for  another, but the total CN =
CN(A-A) + CN(A-B) would be constant.

> Another question is whether it is OK to combine coordination of A-A in A
> compound with CN of A-A in AB alloy if these two A-A bond distances are very
> close? If this case is OK, then combination of A-A CN and CN of A-B could
> follow differnt trend (I guess). But I thought we could not combine CN from
> different materials. Maybe I am wrong again??

I would say it's not OK to do this.  The Ag-Ag coordination number from
pure Ag (ie, 12), would not apply for Ag-Ag coordination in Au0.9Ag0.1,
though the Ag-Ag bond distance would apply.

--Matt