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Re: [sdpd] overlapping reflections / preferred orientation
Antonio,
You may read this:
http://www.ecole.ensicaen.fr/~chateign/texture/combined.pdf
for preferred orientations it might help, tell me
daniel
> Dear António,
>
>> I am currently trying to solve the structure of an unknown
>> Cu compound using FOX. The problem I am having is that
>> although I am conviced that I should have layers or stacks
>> of Cu (due to the magnetic measurements I have made) all
>> the possible solutions that are proposed show an
>> unchemical copper distribution (with regard to the Cu
>> network and the O coordnation).
>
> Have you imposed the Cu-O coordination by using a description from Cu
> polyhedra ? Or are you using Cu and O atoms ?
> Checking the structure can be real tough for inorganics, due to the high
> symmetry. In the development version of Fox, we added Bond-Valence
> calculations to validate the strucure (and optionnaly minimize also versus
> the total bond valence cost).
>
>> This could be normal if I
>> wasn't having a fit with quite good figures of merit (so I
>> say...). I believe the problem arises because the two
>> highest intensity peaks at 13 and 29 degrees (CuKalfa12)
>> are due to (2) overlapping reflections (this is a
>> triclinic cell with two close lattice parameters) and
>> therefore FOX tries to add electron density to the other
>> HKL plane... The question I have is: can I be right? and
>> is there any way around it? and (this is a FOX question)
>> can I edit the HKL file so I force one reflection at a
>> time to be responsible for the intensity?
>
> Overlapping reflections are not an issue as long as you have a
> sufficiently
> large number of independent reflections observed- i.e. if you have 200
> reflections, overlapped by groups of 2 (100 truly independent
> reflections),
> then the information that can be used is (almost) as good as from 100
> independent, non-overlapping reflections.
> So unless you lack information (powder pattern not going to high
> resolution), the overlap is not a problem.
> You *can* edit the Fox xml file to lower the weight corresponding to
> this
> part of the structure, but I would really not recommend it - it is against
> the principle of powder pattern optimization.
>
> As for preferred orientation (PO), if you believe you have some in your
> sample, I would give the following advice:
> 1) try to get another pattern without PO
> 2) try to get another pattern without PO
> 3) try to get another pattern without PO
> 4) try to get another pattern without PO
> ...
> 99) try to get another pattern without PO
> 100) try to find the solution with the PO
>
> I can't stress that enough-finding the solution *with* PO is very hard.
> You
> can optimize the PO diuring the optimization, but I have had very little
> success with that approach - only very simple structures can be found. At
> the
> very least you should absolutely try to guess the PO parameters.
> --
> Vincent Favre-Nicolin
> Université Joseph Fourier
> http://v.favrenicolin.free.fr
> ObjCryst & Fox : http://objcryst.sourceforge.net
>
>
>
> Yahoo! Groups Links
>
>
>
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>
>
>
--
Daniel Chateigner
Professeur
CRISMAT-ENSICAEN
Bd. M. Juin
14050 Caen, France
http://www.ecole.ensicaen.fr/~chateign/danielc/
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