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 ------------------------------------------------------------------- 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. >>>>>> >>>>>> >>>>>> >>>>>> >> >> > > > > >