> >P.S. Don't forget to let your 2theta zero rietveld constant to vary
> >with the unit cell parameters since this is how the height
> >sensitivity is expressed in the refinement.
>
>
> Dear Rory
>
> Careful about confusing 2theta zero offset and hight. They are highly
> correlated, but they are not the same. Hight is angular dependent, the other
> is not. This will cause problems when one has a wide 2theta range to refine.
> Most Rietveld programs can deal with the two independantly but I wouldn't let
> them refine at the same time.
>
> Martin
>
>
Yes perhaps I did not make it very clear what I was saying.
If you are using GSAS with continuous wavelength data and Bragg
Brentano geometry as an approximation to describe the fixed flat
plate geometry (reflection) we have with the INEL CPS 120 detector
then, as the observed positions of the diffracted beams are very
sensitive, in the fixed reflection geometry, to vertical shifts this
has to be compensated for somehow in the refinement. All I can say is
that from experience refining the GSAS parameter ZERO along with the
unit cell parameters seems to express this shift quite well.
Certainly without doing it the refinement is much poorer.
I hope this clears this up.
Yours
Rory.