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