Dear Bob and Jon,
one reply from the public is: come to share your and to get other ideas to the meeting Size-Strain IV (http://www.xray.cz/s-s4/), a satelite workshop of the EPDIC-9 (http://www.xray.cz/epdic/), end of this summer in Prague.
The thinks can be even more complex: The supperposition of narrow and broad peaks can come not only from the size-strain symmetry lower than Laue symmetry (Andreas's example for the polycrystal), but also from a non-homogeneous distribution of lattice defects (for example dislocations), even in the monocrystal.
See you in Prague
Radovan
Von Dreele, Robert B. a écrit:
Jon, I risk a public reply here. One possibility everyone should be open to is that a real phase change has occured during some experimental manipulation of your sample. Some phase changes are quite subtle and involve only slight (and at first sight) quite odd line broadening. Higher resolution study sometimes reveals a splitting of these peaks which is then taken as a sign of a phase change. However, without this the linebroadening is sometimes well described by various anisotropic models (and sometimes not!). Historically, one only need reflect on the work done over many years on various high Tc superconductors and their relatives to know what I mean. Andreas does have the right idea about random powders but solid polycrystalline materials (e.g. metal bars) are a different matter especially if they have been "worked" because the various crystallites are no longer in "equal" environments. Fortunately, the kind of stuff that happens in metals is generally much less of a problem i! n the other kinds of materials one studies by powder diffraction so models used in Rietveld refinements can be rather simplified. Bob Von Dreele
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From: Jon Wright [mailto:[EMAIL PROTECTED] Sent: Mon 4/26/2004 3:45 AM To: [EMAIL PROTECTED]
>... to answer to your (too) long questions. May be later, OK?
Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise:
Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures).
Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom.
In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't!
Thanks for any advice,
Jon
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Radovan Cerny Laboratoire de Cristallographie 24, quai Ernest-Ansermet CH-1211 Geneva 4, Switzerland Phone : [+[41] 22] 37 964 50, FAX : [+[41] 22] 37 961 08
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