Another potential reason for this behaviour is that, depending on your slit
arrangement and sample size, for a flat plate specimen at low angles, the
beam spreads wider than the sample. This means the incident intensity is
less at low angles giving observed intensities that are much smaller than
the calculated intensities. To check which it is, compare higher orders of
the same reflection (200), (300) etc. If they are all too small then it is
preferred orientation. If only the low angle peaks are affected then it is
beam spread. Manufacturers usually supply minimum angles for each slit size
below which this effect will be observed or alternatively you can calculate
them from the divergence angle and diffractometer geometry. The only way out
is to only use data above this cut off angle. To maximise the angular range,
use narrow slits.
Oh yes, and if you are operating in transmission geometry, then what you
observe may be genuine absorbsion which can be corrected in some Rietveld
programs.
Hope this helps.
Erich Kisi
>
>I'm trying to refine a LaB6 XRD pattern to get my instrument function.
>Right now I'm using FullProf. The problem I have is that the calculated
>intensity for the low angle peak, (100) at 24.87° for Co Ka rad., is way
>too big (100%). So that throws off all my intensities (which I imagine are
>also not correct) and I can't get reasonable R's. This happens whether I
>use an Inel CPS-120 or a Siemens D500.
>
>Is this a result of micro-absorption and can I correct for it using the
>Brindley correction? I've found a few articles using this (just ordered
>them), by Taylor, JC, et al. and Collazo J, et al. Are there any others
>that would help to understand this phenomenon?
>
>thanks,
>
>chuck
>
>Charles Witham
>
>Graduate Research Assistant, Materials Science
>California Institute of Technology
>Mail Stop 138-78 ckw@hyperfine.caltech.edu
>Pasadena, CA 91125 (626) 395-3627
> fax -2940
> JPL lab (818) 393-4209
>
>
>