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

[Myrsini K.]

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:
I. 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)
II. 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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PhD Candidate,
Institute of Electronic Structure and Laser (IESL),
Foundation for Research and Technology - Hellas (FORTH),
100 Nikolaou Plastira str, Vassilika Vouton,
700 13 Heraklion, Crete,
GREECE

Office Phone (+30) 2810 391874 
Lab Phone     (+30) 2810 391349
Mobile  (+30) 6945084322

Quantum Materials & Magnetism Lab
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