Dear Luca,
sorry, I don't understand the "mosaic crystal" eplanation, especially
not the "non-centre position" issue. IMHO, when the pattern was
measured at a "powder" Bragg-Brentano configuration, in reflection
mode, axial and equatorial divergence maximum ~ 2 degree, the 00l
series of all misoriented crystal blocks can fulfill the diffraction
condition only within the range of the instruments divergence, and the
parafocusing arrangement would reflect such blocks back very close to
the nominal angular position (as it is the case for a powder as well).
So single crystal reflection of big blocks should only be observable
within the total profile of a "normal" ideal powder peak (the
"instrumental" profile"). This is indeed what we see if coarsely
crystalline materials (e.g. quartz sand) are measured on a typical
powder diffractometer: Such peaks are extremely sharp, sometimes split
or doubled, sometimes with "wrong" alpha1/2 intensity ratio, or may
have small additional maxima and may be "edges" in their slopes,
depending on the number or size of crystals, but all these effects are
very close to the correct maximum position (+- <1 degree). So I can't
imagine that the unknown satellites in the mica pattern are
"misoriented-displaced" Cu Kalpha 00l peaks?
Armel, what type of detector was used for this measurement? If the W L
series was detected, probably no diffracted beam monochromator, and no
Peltier cooled Si drift detector? If so, even more spectral
contamination may come into account. For example, I remember to have
complained about a new Co tube (no company names here) showing
systematic satellites ~ 1 % intensity from standard Si and LaB6 powder
measurements. The wavelength recalculated by Bragg's law was Zn Kalpha
(!!!), confirmed by spectral analysis, the tube Co target was
obviously contaminated. As the satellites in your example in the < 0.x
% magnitude of the Cu K alpha1 intensity, even quite low contamination
of other elements than W may be the source. So it could be
interesting to calculate wavelengths from all positions of the unknown
satellites for the d-spacings of the mica 00l series and see if any
calculated wavelengths do repeat.
Another thought:
Here we have a single crystal of muscovite. As some of the unexplained
peaks appear to be extremely sharp, what about Renninger (multiple
diffraction) hkl peaks? Maybe there are enough lattice planes in the
monoclinic mica structure to give a chance for multiple diffraction to
get registered by a divergent beam (Bragg-Brentano) configuration?
Such peak positions appear to be unsystematic, with no relatiion to
the Cu Kalpha 00l.
However, a take-home message of your example should be: Do never
measure single crystals in parafocusing geometry without safely
monochromatic radiation ;-)
Best regards
Reinhard
Zitat von Luca Lutterotti <luca.luttero...@unitn.it>:
It is call graininess, as Miguel said before, these are mosaic
single crystals and depending on your source (divergence, spot
dimension at different angles etc.) you get each mosaic crystal to
create a diffraction peak that especially at low two-theta angle may
be displaced a lot from its theoretical position because it is
diffracting from a non center position. This is what you get also
when you analyse samples with extremely large grains. If you use a
2D detector instead of scanning with a point or small psd, you will
see all these individual grain or mosaic crystals diffracting around
their ideal spot or if it is a sample with just large grains,
distributed around the Laue circle. More the grain is on a lateral
position respect to the center of the beam, more it is displaced in
two-theta. At higher angle your beam size on the sample is smaller
and there is less displacement for geometrical reasons.
I like to work with 2D detectors (texture, stresses) because it is
easy to see these “figures”.
In addition you have the spectral impurities identified by Frank and
you may get some small grains with different orientation and twins
that will create some of the non 00l peaks.
Analysing these kind of patterns would require a sophisticated
simulation of the grains-crystals distribution and computing like
ray-tracing for the geometrical effects. Not worth it. Single
crystals and sharp textures requires a point beam to avoid these
effects. And a monochromatic one.
Best regards,
Luca
<http://www.unitn.it/>
Luca Lutterotti
Dipartimento di Ingegneria Industriale
Università di Trento
via Sommarive, 9 - 38123 Trento (Italy)
tel. +39 0461 2824-14 (Office), -34 (X-Ray lab)

Maud: http://maud.radiographema.com <http://maud.radiographema/>
On 6 Sep 2023, at 17:43, Le Bail Armel <le-bail.ar...@orange.fr> wrote:
Dear Frank,
Same as you. I have not a complete solution.
Best
Armel
envoyé : 6 septembre 2023 à 16:57
de : Frank Girgsdies <girgs...@fhi-berlin.mpg.de
<mailto:girgs...@fhi-berlin.mpg.de>>
à : Le Bail Armel <le-bail.ar...@orange.fr
<mailto:le-bail.ar...@orange.fr>>
Cc: Rietveld_L <Rietveld_L@ill.fr <mailto:Rietveld_L@ill.fr>>
objet : Re: Step-like basline
Dear Armel,
Thanks for this nice quiz!
However, after identifying the following spectral impurities:
Cu K_beta,
W L_alpha1,2
W L_beta1,2,3,4 and
W L_gamma1,
I am stuck now, leaving bout 1/2 to 1/3 of the tiny extra peaks
unexplained.
Some of them look suspiciously like Cu K_alpha1+2 doublets and might
thus belong to an impurity phase, or differently oriented crystallites
of the main phase, which could lie as dust on the single crystal
surface, but I had no luck trying to identify them.
Furthermore, the irregular high angle tailing of the 00l series (maybe
stacking faults?) makes peak fitting difficult.
Thus, I give up (at least for now), hoping that you might disclose the
solution to the riddle, as far as it is known, for us after a while.
Best wishes,
Frank
On 06.09.2023 10:47, Le Bail Armel wrote:
Hi,
In the same subject.
A special "powder pattern" to play with (try to explain all peaks) :
http://cristal.org/muscovite.pdf <http://cristal.org/muscovite.pdf>
Best
Armel
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