Dear all,
If, I can to obtain the data from Bragg-Brentano geometry, obtained by very
slowly steps (maybe to 20-30 seconds), Can I make a microstructure analysis
(crystalline sizes and residual tensions) using Rietveld refinament?
Best regards,
Mario Macias
UIS, Colombia
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Sorry for the late answer, I was out of office for a while.
As Luca wrote, a complete texture analysis from a single Brag-Brentano
scan is impossible. The PO correction as mentioned below is only
"seeing" one sample direction, perpendicular to the sample surface. This
is meaningful for powder samples packed in a holder having a PO prallel
to the diffraction vector, but not for any other type of samples.
I don't know anything about Fullprof, sorry.
With best regards
Reinhard Kleeberg
[EMAIL PROTECTED] schrieb:
>>Dear Reinhard,
>>
>>
>
>If I want make a microstructure analysis of data from Bragg-Brentano scan.
>Can I use spherical harmonic model and Rietveld refinament content into
>Fullprof?
>
>Best regars,
>
>Mario Macias
>UIS, Colombia.
>
>
>
>
>Luca,
>
>
>>I understood Gerard's problem to have a measurement of a powder (of
>>unknown particle shape) in Bragg-Brentano geometry, for structure
>>refinement. As you said the graininess problem can be minimized
>>primarily by grinding and to a certain extent by rotation and enhancing
>>the divergence, axial as well as equatorial. It is obvious that overmuch
>>graininess would prohibit any reasonable structure refinement or
>>quantitative phase analysis, with or without the application of a
>>harmonic model for intensity correction.
>>Regarding the PO of powder mounts produced from minerals (big crystals)
>>by grinding (McCrone mill, particle size 1-5 µm) and front- back- or
>>sideloading the powder we must state that it is frequently present, of
>>course strongly dependent from the particle shape (by cleavage) of the
>>crystals. Not only platy or needle-like crystals tend to PO, but also
>>cubes (e.g. halite), octahedra (e.g. fluorite), rhombohedra (e.g.
>>calcite), pinakoids (e.g. feldspars) show PO because such crystals tend
>>to stacking and orientation "face by face", like bricks, in a real
>>powder mount. The variation of the preparation technique (say
>>sideloading instead of frontloading) does induce changes in PO what can
>>be used to identify PO, as I understood Frank's recommendation. This is
>>indeed the "trick" we apply every second week if a sample is suspicious
>>to be not adequately prepared. I remember also that an intentionally
>>enhancement of PO by preparation may also be applied for indexing and to
>>add information in structure refinement (of multiple patterns,
>>McCusker..).
>>Of course the degree of orientation in such prepared powder samples is
>>probably not as high as in your "real textured" samples, but m.r.d.'s in
>>the magnitude of 3 (or down to 0.5) may appear. I believe this is weak
>>texture in the context of texture analysis? For such (in my opinion also
>>real and textured) powder samples, a model similar to the harmonics
>>works fine for PO correction, provided that enough measurable peaks are
>>in the pattern. E.g. the application of a model having 3 parameters to a
>>cubic material showing only 2 different lattice planes in the
>>one-dimensional scan must fail of course. Other problems may arise when
>>two directions (e.g. h00 and 0k0) show strongly overlapping peaks by
>>pseudo-symmetry, or when the intensity of a lot of hkl is very week for
>>structural reasons. In all these cases the one-dimensional powder
>>pattern contains not enough information to apply such a harmonic model,
>>the model must be deactivated, a better (more random) sample must be
>>prepared, the number of peaks must be enhanced... Thus, any PO
>>correction in Rietveld refinement is second-best solution only.
>>But in the daily routine work in Rietveld phase analysis, we can see
>>that in the most cases the calculated PO correction factors are
>>"reasonable" (reflecting the shape of the crystals, biggest faces get
>>highest, directions inbetween lowest...), and the quantitative results
>>are acceptable for such "realisticly oriented" powder samples what can
>>produced by standard techniques. I believe this is an argument to apply
>>these models, even though they are perhaps not able to decribe stronger
>>PO or maybe unable to do a texture analysis (what is anyway impossible
>>from a single Bragg-Brentano scan).
>>Greetings
>>Reinhard
>>
>>Luca Lutterotti schrieb:
>>
>>
>>
>>>Reinhard,
>>>
>>>I stick with what Gerard said:
>>>
>>>"But i have no other information that supports the existence of
>>>preferred orientation"
>>>
>>>so what information give you the confirmation it is the powder mount
>>>responsible of preferred orientation. I work almost exclusively with
>>>image plate detectors and I can assure you that the graininess
>>>problem is appearing more often than the preferred orientation case.
>>>I am working on texture mostly so I am happy when you find them, but
>>>this case is not s frequent as people think and for sure not s
>>>frequent as graininess.
>>>
>>>I wait also confirmation from Gerard that his sample is a powder and
>>>it has plate like or fiber like particles. Otherwise I will
>>>investigate the graininess case that with a proper grinding or a
>>>spinner is easily resolvable.
>>>
>>>Also, for who think that because the harmonic model can fit it is for
>>>sure preferred orientation. I can just suggest to work for a while
>>>with real textured sample and the harmonic and see if there is really
>>>this relationship, you may be surprise by the result.
>>>
>>>cheers,
>>> Luca
>>>
>>>On May 8, 2008, at 3:08 PM, Reinhard Kleeberg wrote:
>>>
>>>
>>>
>>>>Luca,
>>>>speaking about powder samples, Frank is right. The PO of powder
>>>>mounts is seldom reproducible and the filling technique is
>>>>responsible for particle orientation, depending on particle shape,
>>>>filling direction, pressure... In practice it is a nice trick to
>>>>repeat the filling of the powder holder with different filling
>>>>techniques to look for PO. Of course, sample graininess may be also
>>>>a reason for not reproducible intensity, but these effects ("rocks
>>>>in the dust") ar mostly hard to correct successfully by spherical
>>>>harmonics as Gerard stated for his problem. In any case, the problem
>>>>sounds to be related to sample preparation.
>>>>Reinhard
>>>>
>>>>Luca Lutterotti schrieb:
>>>>
>>>>
>>>>
>>>>>On May 8, 2008, at 12:30 AM, May, Frank wrote:
>>>>>
>>>>>
>>>>>
>>>>>>You can check for texture effects (preferred orientation) by
>>>>>>obtaining multiple patterns of the material. It's realistic to
>>>>>>expect some differences, but preferred orientation is manifest by
>>>>>>not being able to replicate the pattern.
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>Not true,
>>>>>
>>>>>preferred orientation or texture are perfectly reproducible,
>>>>>provided you use the same sample orientation. What is not
>>>>>reproducible and probably what Frank May is referring to is not
>>>>>preferred orientation but graininess or few big grains that do not
>>>>>guarantee the correct statistic. So if you need to check for
>>>>>graininess, you just move a little your sample, so the beam covers
>>>>>a different area on the sample. If you think you have texturte, to
>>>>>check for it you have to change the sample orientation to see a
>>>>>change. Beware that in a Bragg-Brentano instrument turning around
>>>>>the axis normal to the sample surface is not a valid change in
>>>>>orientation as nothing will change for texture; you have to change
>>>>>the sample inclination instead (omega or chi).
>>>>>
>>>>> Best Regards,
>>>>>
>>>>> Luca Lutterotti
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>That's the simple test. Let us know what you find.
>>>>>>
>>>>>>Another issue for "improper intensities" is when the specimen is
>>>>>>not sufficiently wide enough at low angles (typically below 20-
>>>>>>degrees 2- Theta with copper radiation) and the x-ray beam does
>>>>>>not fully impinge on the specimen. The observed reflections in
>>>>>>the low angle region will be less than calculated by a modelling
>>>>>>program.
>>>>>>
>>>>>>Frank May
>>>>>>Research Investigator
>>>>>>Department of Chemistry and Biochemistry
>>>>>>University of Missouri - St. Louis
>>>>>>One University Boulevard
>>>>>>St. Louis, Missouri 63121-4499
>>>>>>
>>>>>>314-516-5098
>>>>>>
>>>>>>________________________________
>>>>>>
>>>>>>From: Gerard, Garcia S [mailto:[EMAIL PROTECTED]
>>>>>>Sent: Wed 5/7/2008 8:57 AM
>>>>>>To: rietveld_l@ill.fr
>>>>>>Subject: Preferred orientation?
>>>>>>
>>>>>>
>>>>>>
>>>>>>Dear all,
>>>>>>
>>>>>>I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows
>>>>>>intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to
>>>>>>4 Angstroms.
>>>>>>There are overestimated peaks and also underestimated peaks.I
>>>>>>have tried to discard factors that might cause this problem:
>>>>>>
>>>>>>The thermal parameters look sensible. Moreover, the data at high
>>>>>>angle looks ok, so intensity transfer from low angle to high
>>>>>>angle or vice versa does not seem to be the cause.
>>>>>>
>>>>>>Atomic positions also look sensible. And again, data at high
>>>>>>angle looks ok. Is the scattering angle dependence of the atomic
>>>>>>positions the same as for the thermal parameters? (I cannot
>>>>>>remember that, but i am pretty sure it is not).
>>>>>>
>>>>>>Following the advice published in J. Appl. Cryst. 32, 36 (1999),
>>>>>>the other factor that might cause this problem is preferred
>>>>>>orientation:
>>>>>>I have tried to find a hkl dependence in the overestimated and
>>>>>>underestimated peaks but i could not find any. If i try to model
>>>>>>preferred orientation with spherical harmonics the problems
>>>>>>disappears nicely. The problem is how to justify the existence of
>>>>>>preferred orientation. The crystal system is orthorhombic. But i
>>>>>>have no other information that supports the existence of
>>>>>>preferred orientation.
>>>>>>
>>>>>>Is there any other problem that I cannot think of?Is the
>>>>>>preferred orientation correction masking any of these other
>>>>>>problems I cannot think of?
>>>>>>
>>>>>>Regards
>>>>>>
>>>>>>Gerard
>>>>>>
>>>>>>
>>>>>>
>>>>>>________________________________
>>>>>>
>>>>>>Heriot-Watt University is a Scottish charity registered under
>>>>>>charity number SC000278.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>
>>
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