Miguel I have seen low angle peaks that are void or have very little axial divergence asymmetry. As an example, the low angle peak of Brucite in a multiphase pattern often exhibits far less asymmetry that surrounding peaks from different phases. This I am pretty sure is due to the preferred orientation of the Brucite phase; do you have another explanation. Also maybe the choice of word "spotty" is not accurate eqither but again do you have a better adjective; how about incomplete cones.
>Refinement gave H/L=S/L=0.027 for the equatorial and axial >divergences stated on the fig. By the way the FCJ corrects for axial divergence; not equitorial divergence. It will as you pointed out fit to low angle peaks if its two parameters were to be refined. These same two parameters fixed at their refined low angle values will not fit to higher angle peaks. This conclusion was reached from comparision with experimental data, you may want to look at Fig. 5 of Cheary & Coelho (1998b)J. Appl. Cryst., 31, 862-868. alan -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Monday, 5 June 2006 8:37 AM To: rietveld_l@ill.fr (Larry) > Would it be possible for you to generate the data points for the same > set of parameters with peaks at 5 and 10 degrees? That way the > asymmetry will be really pronounced and the convolution method will be > even more stressed. I don't expect it to make any difference, but I'd > like to see it on the screen. As a complement to calculated vs calculated peak shapes it might be interesting to compare calculated vs observed peak shapes for a material with low angle reflections (~8 deg 2theta, see attached figure). I used this MFI zeolite material to calibrate H/L and S/L for our BB diffractometer, although it is probably not the best choice due to peak overlap at higher angles, it simply was the best crystallized zeolite sample I had at hand. Refinement gave H/L=S/L=0.027 for the equatorial and axial divergences stated on the fig. Note that the axial div was 0.04 rad (as specified by Philips), maybe just the full opening as opposed to half opening (0.02 rad) specified by Larry earlier in this discussion. Anyway, the fig shows clearly that the FCJ correction gives the proper peak shape at low angles, and in the case of MFI it was by far the most important contribution to get the quite low error indices given in the figure. (Alan) > - Because of the speed dynamic analysis of things like preferred > orientation effects on axial divergence is possible. These effects are > manifested as spotty cones leading to a peak dependent axial > divergence. (Pamela) axialdivergence is concerned, I believe that's what Soller slits are usually used for! Unless you're unlucky, poor particle statistics are far more likely to be a seriousheadache (one of my particular favourite soapbox subjects :-). In a sample prepared for BB geometry, preferred orientation is not normally supposed to give spotty cones, I think. I agree with Pamela that spottiness is more typically due to problems with graininess of the sample. This effect is normally not even recognized in BB diffraction patterns, but it contributes, of course, to asymmetries of the peak shape and intensity enhancement, fortunately in a more or less random way in both cases. So if things look funny, the first thing to check is graininess of the sample: this issue cannot be overemphasized! best miguel
