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@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