Dear Arie,
what you mention is a rather common occurrence, which may depend on
(at least) to factors:
a) the presence of a short axis or a dominant zone (making the whole
powder pattern indexable by a 2D reciprocal lattice)
b) sample morphology (say, needles) leading to a partial sampling
of the
How to pick up the right solution between these high M20 solutions?
No exact solution to that problem related to the needle to
be found in a hay bundle.
The ultimate proof that a solution is the right one is to solve
the structure. Having only 31 hkls, this is not completely impossible
with an
Dear Norberto,
Thanks for your rapid answer.
The obvious way out from this second problem can be a different
preparation of the sample, changing texture coefficients, just
aiming to detect the 'missing informative peaks'.
The sample was in-situ crystallised by putting a drop of the mother
Following up the '2D' indexing..
The sample was in-situ crystallised by putting a drop of the mother
liquid on a silicon substrate and letting evaporate the solvent. This
should favour a random orientation of the crystallites, not?
Unfortunately NOT.
I have seen preferential crystallization
Topas software is very good at solving
such short axis problems. The advantage is that it will look at all
of the peaks you feed it, instead of using just the first twenty or so
to generate candidate solutions (the way that ITO and TREOR work).
If you don't have access to Topas, I
suggest the
Is the fundamental parameter approach better than
mathematical approach used in most of the Rietveld
refinement programs?
Perhaps someone is about to explain the difference is between
fundamental parameters and anything else? I used to think it might
mean convoluting something which was
Nandini
If you're using standard Bragg-Brentano the true fundamental parameters
fitting from first principles will happily fit low angle asymmetry, as the
mathematical basis for it is well known (look for some papers that Alan
Coehlo and Bob Cheary did a while back, in J.Appl.Cryst I think).
Hi all,
Here are the references to these papers:
Cheary RW, Coelho AA (1998a) Axial divergence in a conventional X-ray powder
diffractometer. I. theoretical foundations. J. Appl. Cryst. 31:851-861
Cheary RW, Coelho AA (1998b) Axial divergence in a conventional X-ray powder
diffractometer. II.
Our system has double mirrors and I could never get FCJ to give as good a
fit, but then that may be a peculiarity of these optics. My memory is a bit
hazy so I can't remember what function the simple axial model uses, but I
don't think it's a function of diffractometer characteristics. Topas is
Jilin
As far as I'm aware there is no repository for Topas str files. Bruker does
sell a (fairly) comprehensive set for those without the time or inclination
to make their own. Topas will import CIF files, but the ICSD doesn't always
export CIF files in the same format as Topas is expecting.
In addition, the raytracing fundamental
approach describes at now (planar) transmission geometry and
capillar geometry.
Don't know about the planar transmission (never done it), but I can happily
fit capillary data off my system. I have no quibbles about the
effectiveness of ray-tracing, but
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