> 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.
>>>>>
>>>>>
>>>>
>>>
>>
>
>
