If you just want to try fitting the peaks, you could try something like this:

str
        phase_name "Metal_oxide"
        local broad 100 'crys size for hk0 and hkl
        local sharp 2000 'crys size for 00l
        local cs =
                IF (And(H == 0, K == 0, L > 0)) THEN
                        sharp
                ELSE
                        broad
                ENDIF;

        CS_L(cs)
        'insert remainder of structure...



Cheers

Matthew

________________
Matthew Rowles

CSIRO Minerals
Box 312
Clayton South, Victoria
AUSTRALIA 3169

Ph: +61 3 9545 8892
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-----Original Message-----
From: Frank Girgsdies [mailto:[EMAIL PROTECTED]
Sent: Wednesday, 29 October 2008 22:05
To: Rietveld_l@ill.fr
Subject: Anisotropic peak broadening with TOPAS

Dear Topas experts,


C) One could leave the spherical harmonics approach
and go to a user defined model, which refines different
Cry Size parameters for different crystal directions.
In my case, two parameters would probably be sufficient,
one for the c-direction, and a common one for the a- and
b-direction.
The Topas Technical Reference, section 7.6.3. gives a
similar example of a user defined peak broadening function,
depending on the value of l in hkl.
I could probably come up with an analogous solution
which has a 1/cos(theta) dependence and two parameters,
one for the 00l and one for the hk0 case.
My problem with this approach is how to treat the
mixed reflections hkl. I suppose they should be
scaled with a somehow weighted mix of the two
parameters, where the weighting depends on the
angle between the specific hkl and the c-axis.
However, I no idea how a physically reasonable
weighting scheme (and the corresponding Topas syntax)
should look like.

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