The empirical PO corrections like March-Dollase or Rietveld-Toraya can
correct only for fibre like PO, i.e. symmetrical around one axis. If it
is not your case, you can make it fibre like by spinning, if you spin
around the PO vector. If not than see J. Appl. Cryst. 28(1995)247-253.
However, there are few other limitations, which are usually completly
ignored:
The distribution of crystallites around any pole hkl must be axially
symmetrical either, so-called disc- or rod-shape like behavior. No way
to do it by spinning ...
Most of implementations of these corrections in the Rietveld codes also
assume another condition: The texture axis is parallel (or
perpendicular) to the diffraction vector for all measured hkls. This
condition is fullfilled only in Bragg-Brentano or Debye-Scherrer geometries.
Surprising that it still works ...
Radovan
Mibeck, Blaise a écrit :
I am learning QPA and am worried about PO.
I wonder why sample spinning isn’t discussed more.
I am luckily able to barrow time on a newer diffractometer with a sample
spinner.
Does this reduce PO? Or completely eliminate it? There is a set up
variable in GSAS for whether you are spinning the sample – are there
guidelines for how many revolutions to spin the sample if you just want
to reduce PO?
Thanks to all for there help!!!
Blaise
* * * * * * * * * * * * *
Blaise Mibeck
Research Scientist
Energy & Environmental Research Center
University of North Dakota
15 North 23rd Street, Stop 9018
Grand Forks, ND 58202-9018
Phone: (701) 777-5077
Email: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
-----Original Message-----
*From:* David L. Bish [mailto:[EMAIL PROTECTED]
*Sent:* Tuesday, October 28, 2008 9:09 AM
*To:* Martin; [EMAIL PROTECTED]; rietveld_l@ill.fr
*Subject:* RE: Quantitative analysis
I agree that it's always best to avoid preferred orientation, but that
is easier said than done on a routine basis. I have personally had good
luck with the M-D PO correction on many known samples, as long as the PO
was not severe.
However, I imagine that Mario's problems are related to microabsorption
in this case. Mario, if you can re-measure your data with a Co or Fe
tube, it would be a good test of this.
Dave Bish
At 08:51 AM 10/28/2008 -0400, Martin wrote:
Sorry to disagree. Experience tells me otherwise - the March-Dollase
correction has nearly always led to poor quant results for me. It most
certainly cannot be applied safely.
Martin
------------------------------------------
M Vickers
Dept of Chemistry
UCL
------------------------------------------------------------------------
Subject: Re: Quantitative analysis
Date: Tue, 28 Oct 2008 02:53:20 -0700
From: [EMAIL PROTECTED]
To: Rietveld_l@ill.fr
Dear Mario,
One more possible problem of applying preferred orientation
corrections in QPA is that not all of them are normalized. For example,
the March-Dollase correction is normalized and can be applied safely,
but the Rietveld-Toraya correction is inapplicable to QPA as it does not
preserve the scale normalization.
Best regards,
Leonid
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