xdd_diffractometer_radius 173 =20
xdd_divergence_fixed_angle 1.0 =20
xdd_receiving_slit_width 0.2 =20
xdd_receiving_slit_length 12 =20
xdd_sample_length 15 =20
xdd_length_of_tube_filament 12 =20
xdd_primary_soller_angle 5.1 =20
xdd_secondary_soller_angle 5.1
----
While the following opinions may put my head on the chopping block more
than is healthy, the above is required for the new Rietveld program=
Koalariet=20
for Win95 written by Alan Coehlo. This uses the fundamental parameters=20
approach of Coehlo and Cheary. =20
The software is available via anonymous ftp to:
ftp://ftp.minerals.csiro.au/pub/xtallography/koalariet
In trying to find analogies between this Koalariet Rietveld program and=20
others, I have one of the following 3 to yet decide upon:
1) An early jet engine of the 1940's vs piston driven propeller aircraft.
2) An early screw driven ship vs the paddle variety
3) The coming of modern poetry at the start of the 20th century
("But Mr Eliot.....it doesn't rhyme????!!!!")
A personal summary is that the Koalariet software does away with
the typical mathematical peak fits and uses fundamental parameters to model=
=20
the contribution of the diffractometer and other effects. Thus, for=20
instance, in modeling peak width/shape, you have the choice of directly=20
refining on:
Crystallite Size and/or (if necessary)
Gaussian Strain and/or
Leorentzian Strain
All other effects such as low angle peak asymmetry, profile contribution by
the diffractometer, high angle peak asymmetry and peak shifts due to linear=
=20
absorption, etc; are automatically modeled as part of the geometry and=
sample=20
(the diffractometer, linear absorption parameters are refinable/checkable if=
=20
you are in the mood).
Thus there is a much better chance that any misfit of the peaks can
be traced to real physical effects of the sample or diffractometer=20
as all the main parameters profile parameters are defined as physically
meaningful measurements. This compares again to mathematical fits that=20
are physically meaningless(?) and cannot be pursued further if there are
misfits. The Koalariet program can even model rather bizarre peaks shapes=
=20
obtained by removing both incident and diffracted beam Sollers slits -=20
without any requirement for extra mathematical fit parameters.
----
A bonus advantage of this is you can get rid of the diffracted beam=20
Sollers slit, use a coarser receiving slit to double the intensity seen
by the detector and improve particle statistics - the program has
no trouble modeling effects such as the extra peak asymmetry. This=20
compares with the typical 3 width, 3 shape approach and up to a dozen(?)=20
extra parameters to improve the mathematical fit of the peaks(?).
Combined with being completely written anew in C, the program is extremely=
=20
stable. (An example on the ftp site shows that Peak anisotropy on a=20
sample is more likely due to a distribution of unit-cells).
----
Another major feature of the program is an in-built macro/equation editor in=
=20
the input files. Thus any parameter can be refined or linked to any other=
=20
parameter in an arbitrary manner (i.e., link the elemental substitution to=
=20
the unit cell shift for quantitative analysis purposes)
New parameters/definitions/macros can be defined in the input file without
having=20
to get a change in the source code:
(i.e., modeling preferred orientation on multiple vectors
modeling peak anisotropy (presently as mathematical fits) on multiple
vectors
check for peak intensity variations due to non-infinite thickness.
use of automatic divergence slit
modeling data from an INEL 120=B0 PSD, flat plate, fixed theta mode)
The present program only handles fixed count time XRD data from sealed tube
sources.
-----
Obviously, I am a convert to this program and thus not an objective=
reviewer.
A summary is that I believe if in the next few years, if programs don't use=
=20
this (or a better) approach for Rietveld (rather than mathematical fits
of present) they will be placed in the same bag as stone axes and flint=
knives.
Though news of any other programs/papers that try or mention similar things;=
=20
or corrections to the above opinions would be most appreciated.
Cheers,
Lachlan.
Lachlan Cranswick - Melbourne, Australia _--_|\ =20
Phone/Fax : (613) 9455-1345 / \
E-mail : lachlan@melbpc.org.au \_.--._/
Mobile Phone/Voice Mail : 0412-1141-31 v
Crystallographic WWW : http://www.unige.ch/crystal/stxnews/stx/volnteer.htm=
=20
Commission on Powder Diffraction, Quantitative Phase Analysis Round Robin:=
=20
http://www.dl.ac.uk/SRS/XRD/IUCR/QARR/
Society of Crystallogaphers in Australia (SCA): http://www.sca.asn.au/
Australasian Web Publishers Association: (AWPA): http://www.awpa.asn.au/
Australian X-ray Analytical Association (AXAA):=20
http://www.latrobe.edu.au/www/axaa/