Re: [sdpd] background

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

Sean X. Ouyang wrote :

>Some pratical questions.
>How many fix background points/ refinable background terms are reasonable?
>GSAS can let you use up to 99 fix background points and 36 refinable background
>terms.
>The examples in the tutorial use about 4-6  (There is no polynomial type
>function in GSAS.
>I have been suggested to use cosine fourier series for in house data.)
>Are there articles comparing different background functions?

No less than 60 emails in the old Rietveld mailing list archive about 
background :
http://www.cristal.org/forum/rietveld_l/
and 50 in the new archive :
http://www.mail-archive.com/rietveld_l...@ill.fr/

To resume, background is one of the most difficult part to
estimate when reflections spread all over the diffractogram.

When using fixed background points, visually or automatically 
estimated, I would recommend to look accurately at the values
(which can be  > 50) so as to have a final smooth variation.
I generally remove the values which are between two smallest
values (small ripples should not occur). At the end of that
process, you may keep 10 to 25  fixed background points -
probably all false...;-). Anyway how to do that better ? Using
a zero-background sample holder may help a lot. When
producing a final refinement, with Fullprof for instance, you
should always try the polynomial background option. Unless
the background is a very special one, this should considerably
improve your Rp and Rwp factors, if compared with the results
corresponding to a fixed background. Anyway, that polynomial
background is also probably false ;-).

Fourier background has chances to be also false ;-).

So, you have to choose the method giving the less false
background, an unsolved question...

Need for a Round Robin on background estimation??

About references, there is a chapter on the background in
the book : "The Rietveld method" :
http://www.oup.co.uk/isbn/0-19-855912-7

Recent references :

Brückner, S. (2000). Estimation of the background in powder
diffraction patterns through a robust smoothing procedure.
J. Appl. Cryst. 33, 977-979.

Riello, P., Fagherazzi, G., Clemente, D. & Canton, P. (1995). 
X-ray Rietveld Analysis with a Physically Based
Background. J. Appl. Cryst. 28, 115-120.

Best,

Armel Le Bail
http://www.cristal.org/course/
  

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