[Ifeffit] Problems with fit in pyrite structure with Se substitution

[Adriana Matamoros Veloza]

Hi Bruce,

In both projects the data corresponds to Se edge (no sulphur).  Selenium data was collected on natural pyrite grains.  When Se associates to pyrite, it is normally inferred that Se substituted sulphur within the pyrite structure.  Both elements are chemically very similar. Looking at selenium as the core atom, we want to see if it is possible to get the chemical environment related to pyrite (Se-S, Se-Fe bonds).  In natural environments if substitution of sulphur with selenium occurs the proportion is rather than low (perhaps maximum 1% of the population of sulphur atoms). So, in that case we are expecting to get data that fit to the pyrite structure taking Se as the core atom.  For that reason, the Feff calculations were made with the crystal structure of pyrite with sulphur as the core atom and used Se edge instead.  Is that making any sense?

These data are also being compared with selenium crystal structures (Se-Fe bonds only), where the abundance of Se atoms is higher.

Thanks a lot for taking your time in looking at my data.

Adriana





-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel
Sent: 02 July 2012 19:02
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Problems with fit in pyrite structure with Se substitution

On Wednesday, June 27, 2012 11:48:27 AM Adriana Matamoros Veloza wrote:
> 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. 

Wait a minute!  Are the data in your first project measured at the Se
edge?  Or the S edge?

In your second project?

You keep talking about Se substitution, but the Feff calculations in
your two project files have no Se atoms in them.  So I am confused --
what you say you want to do is not consistent with what you have
actually done in your project files.  I am not sure how to advise you
because I am not sure which is correct.

B



-- 

 Bruce Ravel  ------------------------------------ bravel at 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
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