does Diffpy-cmi fit a spherical nanoparticle which is empty inside?

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ali dordaee

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Aug 13, 2021, 10:09:04 AMAug 13
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 Hello everybody,
 I was wondering if Diffpy-cmi could fit a nanoparticle which has a 3D sphere shape but it's core is empty. For example, if the diameter is 100 units of length, the core part with 75 units of length is empty inside. I tried to fit it and get the size, but diffpy gives me a very short size and I guess you know why. Because the biggest peaks happen at the very first of my "r" range and the rest of the peaks are comparatively smaller but they are not negligible at all. Please let me know if I can get the size of this ring-shaped  particle from  pdf fitting. 

 Best regards,
 Ali. 

Mikkel Juelsholt

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Aug 13, 2021, 12:37:18 PMAug 13
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Hi Ali

Diffpy have set of shape functions you can apply. Two of them are shell functions. Try play around with them. 
The functions are described here:
http://www.diffpy.org/diffpy.srfit/api/diffpy.srfit.pdf.html

Another approach is to make the entire hollow particle as a XYZ file and use the Debye-calculator in DIffpy to calculate the PDF:

However I would be careful about concluding that you have a hollow nanoparticle from you PDF. In principle PDF can answer it. In practice it is very very difficult. 
My advice is to make sure you have such a structure in the first place. I would advice you to try some other technique, such as small angle scattering (which Diffpy can also model) or maybe some TEM data. But first ask yourself if it is necessary for you to model this shape in the PDF. What is it you need to know about your material and do you need to model it.

Mikkel

ali dordaee

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Aug 13, 2021, 1:04:26 PMAug 13
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Dear Mikkel,
 Thanks so much for the very good guidance. Could you please also let me know a little more about the second approach(Using Debyer-calculator)? I would be most grateful. 

Mikkel Juelsholt

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Aug 16, 2021, 7:59:56 AMAug 16
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Well there is not a lot to say, since I don't know what you want to do. 

But the approach would be to use a program to manipulate atomic structures, such as Vesta, or use a code, e.g. python, to construct the particle you are interested in. Then you save the atomic coordinates and the atom types in a xyz-file. The xyz file type is standard in most crystal visualization programs. 

The you can use the loadstructure function with the periodic keyword set to false to load in the xyz file as the atomic structure used for the refinement.
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