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Re: [sdpd] overlapping reflections / preferred orientation
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
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