Re: [sdpd] structure and structureless lattice determination

[Armel Le Bail <alb...@cristal.org>]

Hi,

>I'm interested in cell lattice determination using both Le Bail and 
>Rietveld analysis with structure refinement, using the same data. 
>Specifically, where the cell dimensions obtained are not the same.

Whole profile refinement with cell constraint but without the structure
constraint, as practiced with the Pawley or Le Bail methods, is able
to produce better fits than with structure constraint (Rietveld method).
This is not surprising, but it is not necessary to publish any comparison
for guessing that this better fit does not necessarily produce better
cell parameters (removing constraints always produce better fits).

I think that, if not properly applied, or if overlapping is really large,
the Pawley and Le Bail methods will produce worse cell parameters
than by using the Rietveld method. The structure constraint
force the correct calculated peaks to match with the observed ones
not only in position but also in intensity. I would never believe
any paper concluding that the Pawley or Le Bail methods produce
better cell parameters than the Rietveld ultimate fit. Pawley
and Le Bail methods are the penultimate test, not the ultimate.

However, the fact that structures can be determined from
intensities extracted by the Pawley and Le Bail methods is
certainly a good point favouring some correctness of the cell
parameters refined by these methods - in many cases. The
advice is that you should not make the economy of the final
Rietveld fit (unless you study compounds as simple as cubic
perovskite in which case the cell parameters with or without
the structure constraints will not change that much - the more
the structure is complex, increasing overlapping, and the more
the Pawley and Le Bail methods can produce slightly wrong
cell parameters).

Best,

Armel
   


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