[GSAS-II] FW: Issue with LeBail refinement - TOF Data

[Myrsini K.]

Dear GSAS-II users,
  
I am currently analyzing TOF data, at 300 K, of the Lix(C5D5N)yFe2Se2 superconductor, measured at the GEM facility of ISIS.
I've encountered some issues with the LeBail fitting: 
1. I've started a LeBail refinement, using a .cif file (I4/mmm) derived from a previous Rietveld analysis of the same material. The calculated fit is mostly a line somehow following parallel the background function. The fit also includes at different regions (underneath peaks, or not) some wave-like features of negligible intensity. This fit is particularly observed at the first two Banks. At higher Banks, the fit also displays some sawtoothlike features that don’t match the shape of the observed peaks and are still characterized by low intensity. I am wondering why the LeBail fit doesn't even account for the peaks corresponding to the unit cell of the structure.  (Note: I've already verified that the peaks at the low-Q region correspond to our material, and no significant shifts were observed). I've tried also to increase the value of the histogram scale factor, but it didn’t work either.  
2. Also, when attempting to create a simulated pattern of the same phase (I4/mmm) using the .prm file provided by GEM, it does not work. It displayed some sharp squeezed peaks of high intensity in the very low-Q, region that don’t correspond to the unit cell provided. However, when applying to the same phase a .prm file from a synchrotron X-ray source produces the expected simulated pattern. 
3. Also I’ve tried to use the prm file provided by GSAS-II, namely the generic TOF (corresponding to a specific flight path and 2theta), but still, I cannot get a fit in the respective Bank of my data, only a weird linear line again.
4. On the other hand, when I tried to fit some older TOF-data, measured at NOMAD (NOMAD .prm file used), for the parent material FeSe, the Lebail refinement worked well. 

I'm wondering if the issue might be linked to the instrument file parameters or to another fundamental parameter that I haven't yet considered. Or maybe to a characteristic of the measured sample (e.g. nature/evolution/total mass)? Do you have any suggestions on how to address this problem with the GEM data in order to understand the source of the problem?
Which parameters should I focus on modifying/refining to attain a LeBail/Rietveld refinement?
Thank you for your time,
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,
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Office Phone (+30) 2810 391874
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