Michael Glazer said:
> Rietveld refinement (as opposed to single-crystal refinement) is in fact
> refinement of degraded data (it is one-dimensional instead of
> three-dimensional) and so the errors will be more significant.
While I agree with Mike's list, I can't let him get away with such a
sweeping statement implying that single crystal refinements necessarily
give better structures, simply because "the data is three dimensional".
Errors in crystal refinement arise from all kinds of effects apart from
problems due to overlapping peaks. Extinction is a cause for error that is
usually more serious for single crystals than for powders - especially
with strain associated with a structural transition. Texture is a problem
associated mainly with X-ray powder diffraction etc...
With todays high resolution powder diffractometers and Rietveld
refinement, I don't even believe that overlapping reflections from "one
dimensional data" is the main cause of error. Its true that if peaks are
strongly overlapping at high angles then refining the background may
result in it being overestimated, leading to higher temperature factors.
Refinement of absorption will also strongly correlate with temperature
factors.
That is why you should not keep adding more parameters such as background,
absorption etc simply to obtain a lower R-factor and expect that to equate
to a better structure determination. You should try to actually measure
these other effects and correct for them, not simply refine them away.
______________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
<alan.he...@neutronoptics.com> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
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