[Ifeffit] atoms and the alloys

Anatoly Frenkel frenkel at bnl.gov
Sun Nov 13 16:42:39 CST 2005


You should modify feff.inp by hand. No automatic procedure generating a
single cluster that will be characterized by statistical RDF of AB, AA
and BB correlations exists and here is why. In any individual cluster
the radial distribution of neighbors Rho_{AB}(r)= dN_{AB}/dr, where dN
is the number of A-B bonds in interval dr, is not representative of an
ensemble since the requirement that the number of B type neighbors to
the A type atom in each "shell" is equal to the bulk concentration of B
can be satisfied only for the central A atom (that has coordinates
0,0,0). There is an interesting procedure of simulating a random alloy
cluster that was published by J. Moonen, J. Slot, L. Lefferts, D. Bazin,
H. Dexpert in Physica B, 208 & 209 (1995) 689-690. They populate the
cluster 5000 times, truly randomizing the "average" cluster, but each of
their individual clusters has RDF deviating from the average.

It is besides the point though that to simulate the EXAFS of an alloy it
may not be important to generate such a statistically random cluster
because the single scattering or multiple-scattering paths
{A<->NN<-->NN<-->A) depend only on the types of neighbors in the
linkages and not so much on the surrounding. It is due to the
transferability of photoelectron amplitude and phase that makes it not
too sensitive.

Anatoly


-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Mauro
Rovezzi
Sent: Sunday, November 13, 2005 5:19 PM
To: IFEFFIT
Subject: [Ifeffit] atoms and the alloys

Hi to all,

I have noticed that Atoms fails to generate a correct "feff.inp" file
in the case of alloys. For example, let's take the Ni_xCu_{1-x},
x=0.5, an fcc substitutional. If I put Ni as central atom and give 0.5
occupancy on the shared Cu-Ni site (0,0,0.5) the result is a normal Ni
fcc without Cu.

At the moment I modify my feff.inp by hand to simulate alloys, but
maybe I'm just not using Atoms properly. If this is the case, it could
be useful to insert your answer in the FAQ.

Cheers,
Mauro

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