Dear Luca,
Now I see the principal problem of your faulting model. You do not take into account the influence of faults on the peak INTENSITIES, which is definitely wrong. This explains the discrepancies in the QPA results and Biso. In the faulted crystal some layers of atoms are displaced from the ideal sites of the cubic close-packed lattice. These displacements give rise to both the peak broadening and the intensity alterations. In my model I take it into account by the inclusion of the additional atomic pseudo-position. In your model you use an idealized cubic FCC structure and, thus, idealized intensities which differ from the real ones due to the faulting. So, my quantification should not go closer to yours. Best regards, Leonid ******************************************************* Leonid A. Solovyov Institute of Chemistry and Chemical Technology 660036, Akademgorodok 50/24, Krasnoyarsk, Russia http://sites.google.com/site/solovyovleonid ******************************************************* ----- Original Message ----- From: Luca Lutterotti <luca.luttero...@ensicaen.fr> To: Leonid Solovyov <l_solov...@yahoo.com> Cc: Sent: Thursday, April 3, 2014 6:33 PM Subject: Re: Stacking faults and antiphase boundary Dear Leonid, if you wish to know, using the trigonal space group and populating the 0,0,1/3 site (even keeping the same number of atoms in the cell) it affects the intensities. Try yourself by simulation with low broadening to see it better. This is what make your quantification wrong. You can try as a counterexample to fit the bbm48bis using the FCC model. Do not care if the two first peaks are displaced, focus on the intensities. You quantification will go closer to mine. Best regards, Luca On Apr 3, 2014, at 7:16, Leonid Solovyov <l_solov...@yahoo.com> wrote: > Dear Luca, > > I'm glad to see your interest to the problem even after such a delay. > >> So in the end, your model for faulting as you describe: >> "A more general model [J Appl Cryst (2000) 338] is included in DDM:" was to >> use a trigonal >> cell with hexagonal axis to allow refining the anisotropic shift of the >> peaks caused by the planar defects? That is a nice trick, you can try to >> justify it by reasoning that one effect of the planar defect is to get >> same stacking sequence of the hexagonal (we all know that), but what about >> the FCC stacking now? > > I chose the trigonal setting since the presence of layered faults decreases > the symmetry of the > material making it intermediate between cubic close-packed and hexagonal > close-packed. This is > an approximation of the real complex structure, of course, but I find it > applicable to most cases > of close-packed faulted materials I've dealt with. In some cases one has to > choose more > sophisticated models in order to account for peculiar diffraction features > due to correlations > between faults, see, for instance Fig. 2 in [O. Ersen, J. Parmentier, L. A. > Solovyov, M. Drillon, > C. Pham-Huu, J. Werckmann, P. Schultz, Direct Observation of Stacking Faults > and Pore > Connections in Ordered Cage-Type Mesoporous Silica FDU-12 by Electron > Tomography. > J. Am. Chem. Soc. (2008) 16800] > >> To make the audience aware, just changing the cell was not sufficient, you >> have to reproduce the intensities. So in the structure a couple of Cu, Zn >> was set in 0,0,0 and another in 0,0,1/3 and the occupancies refined (to >> values as 1.10392 for the first position and 0.39608 for the second, but >> look like the second is calculated from the first) to adjust the >> intensities and the density. > > Yes, the additional (0,0,1/3) position is included in the model to account > for the partial > displacement of atoms from ideal CCP sites due to the faulting. It allows > accounting for the > influence of faults on both the intensities and the peak broadening according > to the methodology > described in J Appl Cryst (2000) 338. > >> But actually this didn't work out completely >> as the resulting quantitative phase analysis is completely wrong. > > Why do you think that the QPA is wrong? Just because it differs from what you > have from > Maud??? > >> And where are the crystallite sizes? Planar defect densities? So what kind >> of results did you get from the material science point of view? > > I can't determine the crystallite size from this pattern since I don't have > the instrumental > broadening information. As for the faulting probability, it can be derived > form eq. 5 of J Appl > Cryst (2000) 338. For the alpha-brass phase, the fraction C of defective > cells (atoms displaced by > faults) is given by the total occupancy of the pseudo-position at (0,0,1/3) > that is refined to 0.132. > The reciprocal values (1/t1 - 1/t) are listed in the column hkl of the > DDM-output reflections > listing. For the faulting direction [003] this value is 64.2A. Thus, the > fault probability: > p = 2Cd(001)(1/t1 - 1/t) = 2*0.132*2.1464/64.2 = 0.0088 > This value, however, may be biased as I don't have the instrumental > broadening parameters. > I > must also note that for such low-quality data one can hardly expect > highly-reliable > microstructural characteristics. > > Best regards, > Leonid > > ******************************************************* > Leonid A. Solovyov > Institute of Chemistry and Chemical Technology > 660036, Akademgorodok 50/24, Krasnoyarsk, Russia > http://sites.google.com/site/solovyovleonid > ******************************************************* > > > ----- Original Message ----- > From: Luca Lutterotti <luca.luttero...@ing.unitn.it> > To: rietveld_l@ill.fr > Cc: > Sent: Thursday, April 3, 2014 3:08 AM > Subject: Re: Stacking faults and antiphase boundary > > Dear Leonid, > > sorry to come back to an old thread, but there is something to be know > that you didn't tell completely. > So finally I came across this morning to a picture of the fit by ddm of > this bbm48bis sample with planar defects (coming from Maud examples, > picture and example now also on the ddm web site). I didn't see it at that > time as it was not included in the zip of Leonid and I didn't have Windows > to install ddm and check it (nor the time). > Now that I saw in the picture that Leonid did not use a cubic cell to fit > the alpha-brass, I took the time to download ddm, and check the solution. > > So in the end, your model for faulting as you describe: "A more general > model [J Appl Cryst (2000) 338] is included in DDM:" was to use a trigonal > cell with hexagonal axis to allow refining the anisotropic shift of the > peaks caused by the planar defects? That is a nice trick, you can try to > justify it by reasoning that one effect of the planar defect is to get > same stacking sequence of the hexagonal (we all know that), but what about > the FCC stacking now? > To make the audience aware, just changing the cell was not sufficient, you > have to reproduce the intensities. So in the structure a couple of Cu, Zn > was set in 0,0,0 and another in 0,0,1/3 and the occupancies refined (to > values as 1.10392 for the first position and 0.39608 for the second, but > look like the second is calculated from the first) to adjust the > intensities and the density. But actually this didn't work out completely > as the resulting quantitative phase analysis is completely wrong. Not to > mention a quite high B factor (2.17 compared to the near 0.6 value of Maud > and the other BCC phase) for the atoms in the alpha phase, probably to > compensate for the wrong structure and kill the two high peaks at high > angle resulting from the trigonal structure. > > Well this kind of trick may be able in certain cases to fit the pattern > (here just because peaks are broad and you don't notice the split on the > first peak and others of the alpha), but the results you get speak for > themselves. > And where are the crystallite sizes? Planar defect densities? So what kind > of results did you get from the material science point of view? > You could have used just a full profile pattern fitting at this point, and > at least get the crystallite sizes. So now I am asking myself why I don't > use always a triclinic cell to describe my phases, I will have the > flexibility to fit everything without resorting to complex "physical" > models. > > Best regards, > > Luca >
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
