> You might want to have a read of this paper...
Or else you could read the Fullprof manual :-) on
http://www.ill.eu/sites/fullprof/php/tutorials.html
in particular the chapter on "The Rietveld Method in Practice" which is
good advice whatever Rietveld program you use.
To quote: "
Use the best possible starting model: this can be easily done for
background parameters and lattice constants. In some cases, in particular
when the structural model is very crude, it is advisable to analyse first
the pattern with the profile matching method in order to determine
accurately the profile shape function, background and cell parameters
before running the Rietveld method.
Do not start by refining all structural parameters at the same time.
Some of them affect strongly the residuals (they must be refined first)
while others produce only little improvement and should be held fixed till
the latest stages of the analysis.
Before you start, collect all the information available both on your
sample (approximate cell parameters and atomic positions) and on the
diffractometer and experimental conditions of the data measurement:
zero-shift and resolution function of the instrument, for instance. Then a
sensible sequence of refinement of a crystal structure is the following:
1. Scale factor.
2. Scale factor, zero point of detector , 1rst background parameter and
lattice constants. In case of very sloppy background, it may be wise to
actually refine at least two background parameters, or better fix the
background using linear interpolation between a set of fixed points
provided by user.
3. Add the refinement of atomic positions and (eventually) an overall
Debye-Waller factor, especially for high temperature data.
4. Add the peak shape and asymmetry parameters.
5. Add atom occupancies (if required).
6. Turn the overall temperature factor into individual isotropic thermal
parameters.
7. Include additional background parameters (if background is refined).
8. Refine the individual anisotropic thermal parameters if the quality of
the data is good enough.
9. In case of constant wavelength data, the parameters Sycos and/or Sysin
to correct for instrumental or physical aberrations with a COS or SIN
angular dependence. 2θ
10. Microstructural parameters: size and strain effects.
In all cases, it is essential to plot frequently the observed and
experimental patterns. The examination of the difference pattern is a
quick and efficient method to detect blunders in the model or in the input
file controlling the refinement process."
______________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
<[EMAIL PROTECTED]> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
______________________________________________
