First coordination sphere issues during PDF refinement

[Dimitry Grebenyuk]

Hello everyone, 

I'm trying to run some refinements of nano-CeO2 with diffpy. I use 06-Fit-CdSe-nanoparticle script from ADD 2019 tutorial (https://github.com/diffpy/add2019-diffpy-cmi) more or less as is. 

The problem is that the bulk model doesn't work as it should, as there is simply no peak  at ca. 2.4A on the calculated PDF. It should be there as it corresponds to the shortest Ce-O contact in CeO2. 

The refinements of the same data with the same .cif file using spherical and spheroid shape correction work perfectly fine though, as well as the original CdSe bulk refinement example from the script.

My first idea was that something could be wrong with my .cif file (see attached) but when I run the same bulk refinement with PDFgui, it works fine.

What could cause the problem with the first peak? Thanks in advance for any hints.

Dimitry

[Nils Prinz]

Hi Dimitry,

what are your results of the fit? It looks like the delta parameter could be too high, as I experienced this many times. Maybe try to restrain the delta value as follow:

recipe.restrain("delta2", lb=0, ub = 4, sig=0.001)

Best,

Nils

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[Dimitry Grebenyuk]

Hi Nils,

That was exactly the problem, delta was too high. I added the restrain and it works great now. Thanks! 

Best,

Dimitry