Need help with PDF analysis

[preeti verma]

Greetings to everyone, 

My name is Preeti Verma, PhD student at National Institute of Technology, Rourkela, India in the department of Chemistry. I am trying to look at the change in the local structure of NaYF4 based upconversion materials upon Li+ doping. I have obtained data from Synchrotron facility (RRCAT) in India with the Qmax -11.94. This is the first time I am trying PDF. I did Rietveld refinement of my material using full prof software and found the exact phase matching my material. I have installed PDFGui and tried fitting the data following the manual but I couldn't get the perfect fit. I don't know where I am going wrong, I am attaching my refinement for your reference the  lowest weighted R I could achieve was around 35%. Should I refine the occupancy or atomic positions? Or have I started with the wrong constraints?

Thanks in advance

[Priyank Shyam]

Hi Preeti,

It seems that you have the correct phase and the peaks lining up at right bond distances - so the overall fit seems quite reasonable!  

Is the Qmax for your measurements 11.94 Å-1? That's a bit on the lower end for PDF analysis with sufficient resolution in real space. In my opinion, decent r-space resolution only starts building up after ~20 Å-1 . The lower Qmax might be one reason that you don't get a perfect fit and a weighted R of approx 35%.

As for choosing what parameters to refine and what constraints to place, this would have to be decided based on existing literature and physical reasonability of different models for your system. What is the question that you are trying to answer with the PDF analysis and what additional info do you wish to obtain as compared to the Rietveld refinements? Also, consider whether the effect that you are trying to investigate will actually show up in a PDF with the resolution of your measurements. 

Hope this helps!

Best,

Priyank

[preeti verma]

Thanks for the reply Priyank. 

I agree that I have lower Qmax value for my samples but that's the max I could achieve from the instrument, so I am trying if I can get something useful out of it. With PDF analysis, I am trying to find the local distortion in NaYF4 upconversion materials with Li+ doping and also if I can find something related to lattice strain. 

With few suggestions given by Marcos, I could actually get a better refinement and the Rw reduced to 14%. I am further trying to reduce the Rw, since this is the first time I am trying PDF so getting confused and stuck at some point. I am attaching my new refinement for the reference.

Any other valuable suggestion is most welcome.

Thanks

 

Preeti Verma

PhD Research Scholar

DST Women Scientist Scheme (WOS-A)

Nanomaterials Laboratory

Department of Chemistry

National Institute of Technology Rourkela

Odisha-769008

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