Dear Luca,
I think ice is not claimed to be turbostratic (see PNAS paper). I think
the disorder in cubic ice case is much closer to the the planar disorder
you can experience in close-packed metals (or in SiC, Laves phases, etc)
than to clays.
Best regards
Andreas
On 07.04.2014 10:18, Luca Lutterotti wrote:
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
--
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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