I very much agree with Luca in that graininess is not given the importance it actually has. Older textbooks like Klug-Alexander or Peiser considered graininess to some depth, and simple estimations show that in a usual BB sample, the number of grains in Bragg condition may be as low as 1 for a grain diameter of 10 Ã…!
For Kurt: try Debye Scherrer (or better Gandolfi) if you can accept the peak broadening. best miguel ------- Forwarded message follows ------- Date sent: Thu, 08 May 2008 19:17:53 +0200 From: Luca Lutterotti <[EMAIL PROTECTED]> Subject: Re: Preferred orientation? To: [EMAIL PROTECTED] Copies to: rietveld_l@ill.fr [ Double-click this line for list subscription options ] 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. >>> >>> >> > ------- End of forwarded message --------- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email [EMAIL PROTECTED]