Before I waffle on - one thing I should note is that
if a diffractometer is moderately/slightly out of alignment -
Fundamental Parameters peak fitting can tell you about this.
This has caused some angst/annoyed users trying out XFIT in
the past using Fundamental Parameters fitting because
misalignment of a diffractometer can be conveniently ignored
to some extent with empirical peak fitting. Also many users
have no control over the instrument and have to use it as is.
When fundamental parameters is defined as a standardless method -
this is not exactly true - in that you would normally use a
standard to check that the system is as expected. While, in
theory you can measure directly values such as Receiving
slit width and height, it is not easy to measure the actual
divergence angle of the Soller "when" it is inside the
diffractometer. As explained below - there is some slop
with inserting diffracted beam Philips 1050 type Sollers slits.
Though with due care is taken inserting the Soller Slit - the
refined angle should match the ideal angle near enough.
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What are the official diffractometer settings that were used to run
the sample. I.e.,
xdd_diffractometer_radius 173
xdd_length_of_tube_filament 12
xdd_sample_length 15
xdd_receiving_slit_length 12
xdd_receiving_slit_width .2
xdd_primary_soller_angle 5.1
xdd_secondary_soller_angle 5.1
xdd_divergence_fixed_angle 1
Normally it is the receiving slit(width), divergence slit
(normally 1 degree on a Philips) and either 5.1 degree or no
diffracted beam Sollers slit that can vary from day to
day on a Philps system.
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Also, what type of material is it? The CCP14 based tutorial
refers to heavy absorbing Y2O3 and Lithium Titanate. If you
have a light element sample - you may also have to look into
linear absorption which affects the peak profile - mainly
high angle asymmetry - i.e., as in corundum.
(Can be refined on via File Details, Specimen)
Normally, if you are not fitting assymmetry correctly on
a known sample - it can imply the diffracted beam Sollers
slit parameter may require refinement. When inserting the
diffracted beam Sollers, not doing this carefully can result
in it being slightly out of whack. This is very noticable
when doing intensity/X-ray Tube reference measurements
where counts can fluctuate due to slight change in the
Soller orientation. (for routine phase ID the diffracted
Sollers would not be inserted so as to increase the
intensity)
To refine on the Sollers Slit angle, this is done via:
File Details,
Axial Divergence,
Codes - set the Scnd. Sol. to freely refine. It should
vary between 3 to 9 degrees.
Be wary that refining on the diffractometer geometry is best
done with a known sample or standard such as Y2O3, etc (which
you know is a pure material). If the machine is known and well
defined with respect to the geometry parameters - misfits are
due to the sample (i.e., impurity, a form of strain causing
a different peak shape from that modelled, degradation of
the sample, etc)
Lachlan.
PS: The Lithium Titanate used in one of the XFIT tutorials
at CCP14 was slightly decomposed which was causing misfits
as it quite visible in this tutorial compared to the Y2O3
standard tutorial).
http://www.ccp14.ac.uk/tutorial/xfit-95/liti/rwfunpar.htm
http://www.ccp14.ac.uk/tutorial/xfit-95/fun1.htm - Y2O3
(There is no decomposition macro/model describing what this
for of sample decomposition is doing to the profile. But
the misfit is telling you something is happening to your sample -
in this case decomposition. Again, this information might be
hidden using an empirical peak fitting method using Pseudo
Voight or Split Pearson)
Thus one of the trace peaks selected and fitted (not identified in
the tutorial) is actually due to sample degradation. You would
hopefully find this out when trying to index from the peak list.
(It does say "Real World" example on the title)
-- Lachlan M. D. CranswickCollaborative Computational Project No 14 (CCP14) for Single Crystal and Powder DiffractionDaresbury Laboratory, Warrington, WA4 4AD U.KTel: +44-1925-603703 Fax: +44-1925-603124 E-mail: l.cranswick@dl.ac.uk Ext: 3703 Room C14NEW CCP14 Web Domain (Under heavy construction): http://www.ccp14.ac.uk