[GSAS-II] Generalized-Model-Negative-Parameter

[Von Dreele, Robert B.]

Dear Myrsini,
A negative S220 is perhaps “improbable” and fixing it at zero (or maybe 1.0) isn’t a bad idea. The mustrain surface plot will show, by a strong indentation in the surface, the effect of a small S220. For whatever its worth, the implication is that the structure is very “stiff” in that direction/plane. You can also try setting it to a small positive value, and then repeating the refinement. Sometimes, the least squares will push a too large initial value negative on a refinement cycle and then not be able to return it to be above zero. Starting it small & positive then allows the LS to find a better value that is otherwise close to zero. I’ve seen this when the isotropic mustrain is apparently very small & starting at the default (1000) yields a negative mustrain on the 1st cycle; starting at 100 yields a more sensible result.
Best,
Bob

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From: GSAS-II <gsas-ii-bounces at aps.anl.gov> on behalf of Myrsini K. via GSAS-II <gsas-ii at aps.anl.gov>
Sent: Tuesday, November 1, 2022 8:31:57 AM
To: gsas-ii at aps.anl.gov <gsas-ii at aps.anl.gov>
Subject: [GSAS-II] Generalized-Model-Negative-Parameter


Dear GSAS-II users,



I am refining a synchrotron’s pattern of intercalated FeSe sample named LiPyFeSe, with Pnma space group. I’ve  tried to use the generalized microstrain model which well-fits couple of the high-Q peaks which the isotropic model doesn’t. BUT, one of the parameters (S220) of the generalized model becomes negative. I’ve tried not to refine this specific parameter and the pattern is well fitted again.



My questions are:

  1.  What could be the meaning of the negative S220 parameter? (note: I have calculated instrument parameters from calibrant and also I’ve investigated the family of 220 planes in the patterns and I didn’t find something weird eg. Bkgr, noise)
  2.  Is it obligatory to refine all the parameters of the generalized model, coming from the selected space group?



Thank you in advance!



Best regards,

KAITATZI Myrsini

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