[GSAS-II] GSAS-II Rigid Body Refinement with Partial Disorder in a CF3COO Group

[Dayane Vieira Nascimento dos Santos]

Hi,

I am trying to refine a CF3COO group as a rigid body in a crystal with space group P-1. The atoms in the group occupy sites with 1 symmetry. The CF3 moiety is disordered over two positions with a 70:30 occupancy ratio.

When I define the entire CF3COO group as a rigid body and keep the occupancy fixed, the refinement quality deteriorates significantly: the Rw increases from about 5% to 20%, and the fit becomes much worse. However, if I allow the occupancy to refine, the refinement converges well and the fit remains good (Rw ≈ 5%). The problem is that the resulting structure assigns fractional occupancies (0.5 or lower) not only to the disordered CF₃ moiety but also to the COO group, which was not disordered in the original model.

My question is: how can I refine a rigid body in which only part of the group has fractional occupancy? More specifically, is there a way in GSAS-II to model a rigid body where the CF3 atoms are disordered, but the COO atoms remain fully occupied?

I am a little puzzled because GSAS-I/EXPGUI handled this situation without any apparent difficulty, so I suspect there must be a way to do it in GSAS-II as well.

Any suggestions or guidance would be greatly appreciated.

Thank you,

Dayane V. N. Dos Santos


Graduate Teaching Assistant

Department of Chemistry

Wayne State University

Detroit, MI 48202

Email: dayane.santos at wayne.edu
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