Another way to check or convince yourself of preferred orientation is to take note what is happening when grinding the sample.
Some crystallite samples will fracture along a particular plane upon grinding and this summed up for the whole sample may produce preferred orientation. Alternatively, by not grinding up the sample, you may not achieve a uniform crystallite size, many crystals grow along one or two preferred axes and grinding will help in this case. To convince yourself the crystallite morphology is giving rise to preferred orientation, begin by grinding your sample to oblivion (not so much to make it appear amorphous), you will have broaden peaks, and expected intense peaks should reduce in intensity relative to other peaks. Then you work up from there, decreasing the amount of grinding until no grinding and you should see the relative peak intensity increase (or decrease if the case may be) more for some peaks than you would expect from differing crystallite sizes. Then from the having determined the correct degree of grinding (to ensure a good FWHM) the preferred orientation correction can be put in place with greater confidence. An aside - mismatches in crystallite sizes or small crystallites will result in peak broadening, though when the crystallite size approaches single crystal size (using small amount of sample), then coherent interference will breakdown for the sample as a whole and on a powder diffractometer this will be identified with a few incredibly large intense peaks. The technique of mounting samples will also reduce or increase preferred orientation. Back filling is a good skill to learn, it creates a more random distribution of crystallite orientation. Acetone slurries on the other hand will enhance the ability of the crystallites to layer themselves in a preferred direction. Regards William > 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. >> >> > > -- William Bisson Administrator and Webmaster http://www.ccp14.ac.uk http://bca.chem.ucl.ac.uk
