Re: [Ifeffit] Fe K-edge second shell problems

2011-08-03 Thread Andrea L Foster
Paul, have you considered explicitly multiple scattering from within the 
tetrahedron?  The short distance you describe for the second shell might 
be consistent with that.

Its pretty easy to write an expression in the fit routine that makes the 
MS path distances explicitly dependent on the SS paths (no new variables 
in the fit)--the number of MS paths is constrained by the geometry 
(tetrahedral).  The order of scattering is probably just 3- and 4- legged 
MS paths, no higher (would give longer distances)

I study arsenic, and so I've seen this approach described in the following 
paper on As: 

Voegelin et al (2007) Geochimica et Cosmochimica Acta v. 71, pp. 
5804-5820.  Look in the section 2.6 for the details.

sorry if someone already answered this,


~~
Andrea L. Foster, Ph.D.
U.S.G.S. WMESC
345 Middlfield Rd., MS 901
Menlo Park, CA 94025
T: 650-329-5437
F: 650-329-5491

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[Ifeffit] appropriatness of feff model when central atom is not embedded (or only 1 layer embedded)

2012-03-07 Thread Andrea L Foster
Hello All,  can someone please help with this:

(1) feff requires that the adsorbing atom be embedded  (that is, 
surrounded by) other atoms in order to calculate potentials correctly, but 
how deeply does the atom need to be embedded? 

For example, if  I construct a model of an adsorbate on a mineral surface, 
where that adsorbate has a full complement of 1st shell coordinating OH, 
O, and H2O groups as appropriate, but if I do not include all the 
uncoordinated f water atoms that would be nearby (so that I essentially 
have a surface in vacuum), will the potentials for my central atom be 
calculated correctly (or correctly enough for use in IFEFFIT)?

a more specific example: the AsO4 tetrahedron bonded to the goethite 
surface, which is made of octahedrally-coordinated iron.  The arsenate 
bonds to the goethite  by sharing 1 edge of the tetrahedron with the 
apices of 2 adjacent Fe octahedra.

a related question that comes to mind: if I just wanted to see the signal 
from the 2 adjacent Fe atoms, could I create a perfectly useable FEFF 
model (for IFEFFIT) by limiting my calculations to the small cluster of 1 
arsenate tetrahedron and 2 Fe octahedra?

thanks, andrea

~~
Andrea L. Foster, Ph.D.
U.S.G.S. WMESC
345 Middlfield Rd., MS 901
Menlo Park, CA 94025
T: 650-329-5437
F: 650-329-5491

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