[Ifeffit] Fe-Co help

[Dominik Samuelis]

Dear Chachi,

first of all, you need to work on your crystallographic description  
of the system.  I'm sure you are aware of the ordering transistion in  
Fe-Co at around 730 °C. Your description currently uses parts of the  
ordered phase, and other parts of the unordered.

Above 730 °C, your description as a bcc structure is correct, however  
your atomic parameters and the neighbor shells are not. The system  
then has a bcc structure (Im-3m), with mixed Fe/Co occupancy on the  
single crystallographic position 2a. It should be modelled as such,  
with mixed occupancy on the neighboring sites. There's excellent  
tutorials how to deal with mixed sites. However, be aware that the  
scattering properties of iron and cobalt is very similar, you you  
won't really be able to fit your site occupancy.

Below 730 °C, the system starts to order. The ordered phase is NOT  
bcc anymore, but consists of two intersecting primitive cubic  
lattices, shifted by (1/2 1/2 1/2). The structure prototype is often  
called B2, after its Strukturberichte symbol (http://cst- 
www.nrl.navy.mil/lattice/struk/index.html). Now, the rest of your  
crystallographic description matches. The neighbor shells will be  
N1=8Co and N2=6Fe. However, the symmetry is not bcc (Im-3m), but  
primitive (Pm-3m).

Looking at your fit and feff calculation, it seems, that you have  
used the fully ordered phase for modelling your alloy. As you have  
pointed out yourself, it's not clear that the alloy really is fully  
ordered. Your alloy might be partially ordered, as also indicated by  
the Mossbauer data. In this case, you need to mix several feff  
calculations, with each cobalt and iron on the first and second  
neighbor position. You could start by assuming an ordering parameter  
'op'. The first shell would then be N1=8(op Co+(1-op)Fe), the second  
is N2=6(op Fe+(1-op)Co).

Cheers, dominik



>  Dear all
>  I?m trying to deal with exafs data of a Fe-Co alloy sample, using  
> Artemis.
>  The bcc structure is simple to modeling, Fe at 0,0,0 and Co at  
> 1/2,1/2,1/2
> (at Fe-edge, N1= 8Co, N2= 6Fe, theorically),
> But in my fits N1=5.4 and N2=4.9, that is correct?
> amp=0.811293 value I used from Iron calibration data, that is  
> correct too?
> A possibility (I think), is that the sample is inhomogeneous,  
> because, by Mossbauer
> spectroscopy, show two Fe magnetic sites, one attributed to ordered  
> phase, and the
> other to disordered phase.

-- 
Dipl.-Ing. Dominik Samuelis
samuelis at pc.rwth-aachen.de
Physikalische Chemie I
RWTH Aachen University
Phone: +49-241-80-94840
Fax:   +49-241-80-92128
Landoltweg 2
52056 Aachen
Germany