Hi Bruce, Actually, these data correspond to Se edge measured on natural pyrite grains; we did not measure Fe edge. There are three possibilities of Se chemical environment in these samples: (1) replacement of S with Se in the pyrite structure, (2) Se is forming its own phase with iron (Se-Fe bonds), (3) it could be the combination of these two. The project AMV-1 contains data for Se edge in a different pyrite grain than the project AMV-2. So, data can be slightly different taking into account the two possibilities above. I found a better fit for AMV-1, which I will really appreciate if you can give me your comments about this project (link is working now). https://docs.google.com/open?id=0B6dBCadbyZwFS0tDTmdDSXlWVVU Kind regards, Adriana -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 26 June 2012 16:14 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Problems with fit in pyrite structure with Se substitution On Tuesday, June 26, 2012 03:51:59 PM Adriana Matamoros Veloza wrote:
I am fitting a crystal pyrite structure with Se substitution in one of the sulphur atoms. I am sharing my projects in artemis. In the AMV1 project, all the parameters are sensible except for the large amplitude. I would like to know if that could be the effect of the substitution.
In the second project AMV-2, I am finding large E0, which I tried to overcome by aligning the spectra with the theory in Athena. However, the fitting afterwards is worse than without such correction.
https://docs.google.com/open?id=0B6dBCadbyZwFS0tDTmdDSXlWVVU (AMV-1)
For me, this one came back as 404 Not found.
https://docs.google.com/open?id=0B6dBCadbyZwFYlc2c3FKT0YtbXc (AMV-2)
This one I downloaded and looked at.
Any ideas to help to solve these problems will be very much appreciated.
I don't think I have quite enough information to offer a specific suggestion, but I have several questions and comments. 1. Sulfer EXAFS is HARD! The big problem is that it can be quite challenging to select E0 sensibly unless the sample is purely of one valence. 2. Have you measured the Fe edge of these samples? The reason I ask is that being able to compare results between the two edge will certainly help. I would expect the Fe data to be easier to deal with than the S, so I suspect that the Fe results would be a useful guide here. 3. It seems plausible that all the Fe might be bound up in your pyrite, but that some S might be in different form. Are you XANES data consistent with a pure sulfide? Or might there be other S species? 4. I downloaded and looked at the second file. You are correct -- that's a poor fit :( The first thing I did was to reset the E0 parameter to 0 and then plot the data along with the three paths. I see that the Fe shell is completely out of phase with the data. That is troubling -- I would expect it to constitute the bulk of the first peak in chi(R). But it is way off. That casts doubt on your starting model. So, is that because the sample is not a pyrite (which could be the case is some portion of the S was not bound to the Fe)? Or is it because E0 in Athena was chosen poorly (which might be the case if there is more than one S valence state)? Either way, a cursory examination of your project file suggests you are off to a unlikely start. Good luck, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit