Glad the discussion is opening a bit. Actually the Ice case is a bit different from the point of view of the defects. These are turbostratic planar defects (in the case of close packed alloys you have twins and intrinsic and extrinsic deformation faults). Now it is true that there is not a big difference, but one big is the densities of such defects, that in the case of Ice there, or carbon or clays it reach large values to get such asymmetric profiles (but always in one direction for the nature of such defects). As reported by Andreas, DiffaX and derivates can be used to model. Or a very good alternative, if the disorder is true turbostratic one, is the single layer model of Ufer, that uses the trick described by Jon (the 1D rod in reciprocal space). It was implemented in bgmn and you can check the work of Ufer, Kleeberg, Bergmann etc. on that. I implemented myself on Maud to model clay minerals (http://ammin.geoscienceworld.org/content/95/1/98.abstract) and carbon and I have to say it really works beautifully. The bonus is that you can use the Rietveld framework to refine structure, texture etc. But it cannot be used for the close packed alloys case, the disorder is not turbostratic and it does not account for defects that cause the asymmetry in the other direction and displacements of the peaks (that Leonid model using a lower symmetry description, the trigonal), twin are not accounted. Still if you look carefully, the model of Leonid does not account well for the different in sign of asymmetry between the first and the second. I never tried DiffaX on such alloys to see what you can do, mainly because the Warren model is sufficiently accurate, and (I repeat) the defect density is so low in comparison that the effect on intensities is negligible (< 1%).
Best regards, Luca N.B.: thanks for the Ice reference, I didn't know Thomas was working on that. > Dear all, > > I think Jon is right. In the case of Cu this is the position of a (100) > reflection of a hcp polytype. I have seen this feature in a couple of > cases, and presence of this feature is sometimes taken as a sign for a > two-phase character of the specimen, which need not be the case (other > reflections from the hcp-like polytype generally are then lacking). In > the case of Copper or alpha-Cu alloys a real two-phase character of the > material is very unlikely. Diffax and Diffax_plus are able to model this > feature. > The mentioned ice example is reported in some more depth in PNAS 109 > (2012) 21259 > > Best regards > Andreas > > > On 07.04.2014 09:05, Jonathan WRIGHT wrote: >> >> On 06/04/2014 08:06, Leonid Solovyov wrote: >>> The faulting model in DDM gives nearly perfect agreement with the >>> experiment: >>> http://sites.google.com/site/ddmsuite/home/Copper-DDM.png >>> >> >> It looks a little bit reminiscent of this pattern for ice: >> >> http://www.science24.com/paper/15441 >> >> ...but the little step at about 40.5 degrees doesn't seem to be in the >> model? Isn't that coming from some sort of defects or diffuse >> scattering? Not that I would attempt to model something like that, but >> a 1D "rod" in 3D reciprocal space (coming from 2D defects) gives >> step-like profiles in a 1D powder pattern. Although you generally only >> know that if you've also got data from a single crystal :-) >> >> Cheers, >> >> Jon >> > > > -- > Dr. Andreas Leineweber > Max-Planck-Institut fuer Intelligente Systeme > (ehemals Max-Planck-Institut fuer Metallforschung) > Heisenbergstrasse 3 > 70569 Stuttgart > Germany > Tel. +49 711 689 3365 > Fax. +49 711 689 3312 > e-mail: a.leinewe...@mf.mpg.de > home page of department: http://www.is.mpg.de/de/mittemeijer > >
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