> I'm using an INEL CPS-120 (Curved position sensitive-1200 2Theta) to watch
> the development of lattice parameter with time. Unfortunately, I'm not
> getting Rietveld parameters to show the trend I expect.
>
> Does anyone know of either a rietveld program or an analytical function
> that creates the defocusing seen in a 2Theta machine?
>
> thanks,
>
> Charles Witham
>
Dear Charles
I don't know what you are doing exactly in your
experiment, but ordinarily we use a very thin layer of sample on a
silicon 711 cut zero background holder, with the incident beam
striking the sample at an angle of about 3 degrees. The focussing
is set up so that the rear focussing circle is used. In this way the
peak width behaves in a more predictable fashion than with front
focussing. We use GSAS with Bragg-Brentano geometry for analysing our
data. The first reason that we can get away with this is the thin layer of
sample has minimal absorption and therefore we don't need to apply a
correction. Secondly it is very important to ensure that a) the
sample holder never moves between runs and calibration measurements,
b) that the sample layer is smooth and even (and thin).
If a) is not met then the calibration will be invalid. Believe me, the
peak positions are extremely sensitive to any vertical shift of the
sample holder. b) likewise for the latter reason.
I hope this is of use. If you need any more information etc
don't hesitate to contact me.
Yours
Rory.
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.