It has taken me a bit of time to respond to the XFIT questions of a week
ago. I have worked my way through CRYS, UNITCELL and even played around
with ABSEN a bit. The results were encouraging, but also frustrating.
I'm still not convinced that the indexing results for most of the tests
were real.
I had a chance to work on the diffractometer settings and think I have
the basics resolved. At least I'm getting a decent fit to my
knowns/unknowns. If I had about 26 hours in the day, I could really get
something done!
You wrote previously:
>About possible misfits when using XFIT:
>
>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.
I've done everything I can to make certain of alignment and controllable
factors like internal standards and sample randomization.
>
>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.
I went down and measured everything and then checked to see that all the
slits were properly inserted and aligned (at least as well as I could)
>
>----
>
>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 12
> xdd_receiving_slit_length 12
> xdd_receiving_slit_width .2
> xdd_primary_soller_angle 5.1
> xdd_secondary_soller_angle 9.1
> 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.
The revised settings above seem to give the best results and are as near
the "real" values as I can determine. We have a constant divergence slit
that will vary from near zero at 2 degrees 2TH to somewhere around 2-3
degrees at 90 degrees 2TH. The idea being to illuminate 12mm of sample
at all angles. I see similar problems in the wings for the INEL systems!
>----
>
>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)
I typically run Si as a standard, but alternatively use low quartz (2-3
micron tripolitic novaculite - 98% quartz).
>
>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)
As I have a chance to become comfortable with the software, I'll try
checking things using this method. Thanks.
Henry
>