Correlation parameter (del_r) refinement in difference r-range

[ANUPAM KUMAR SINGH School of Materials Sci. & Tech]

Hi everyone,

I am trying to fit the PDF data [G(r)] in different ranges (r_min to 5, 10, 15Å...r_max) using the same phase.  In this case, I am not sure whether I should keep the correlation parameter (del_r2) to a fixed value (obtained after full range refinement from r_min to r_max) or I could refine the del_r2 for different ranges. Since  del_r2  is highly correlated with ADPs, I may think to keep it fixed but in that case I am wondering how we can model (take care) the 1st (or up to 2nd) nearest neighbours correlation effect in the first two peaks (or so)? 

Also, for different range refinement using the same phase,  should I keep the lattice parameters  fixed to the value obtained from full range refinement or I can refine in the different range? 

I will highly appreciate any help/suggestion about the above queries. 

Thank you in advance!  

Best,

Anupam

[Simon Billinge]

This is a great question.  Depending on the r-ranges this may be a small effect (delta-2 tends to affect only very low-r region).  However, to be on the safe side I think I would refine delta-2 when fitting over 20 A or something like that, then fix it for all the r-dependent boxcar fits.

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Simon Billinge

Professor, 

Department of Applied Physics and Applied Mathematics

Columbia University

[ANUPAM KUMAR SINGH School of Materials Sci. & Tech]

Dear Prof. Billinge,

Many thanks for your suggestion about r-dependent boxcar fitting to handle delta-2.   However, I am still not sure about lattice parameters,  should I keep the lattice parameters fixed to the value obtained from full range refinement (20 A or something as you suggested for delta-2) or I can refine it in the different range?

Best, Anupam

[Simon Billinge]

This depends a bit on what scientific question you are trying to answer.  The lattice parameter shouldn't vary with r but sometimes it does.  As a piece of advice I would be tempted to refine it with the boxcar and look for any variations in r and try and understand them.

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[ANUPAM KUMAR SINGH School of Materials Sci. & Tech., IIT (BHU) Varanasi]

Dear Prof. Billinge,

Thank you very much for your kind suggestions about r-dependent boxcar fitting. I will see it and try to proceed as you per your suggestion. 

With best regards,

     Anupam

-----------------

Anupam Singh

PhD, SMST

Indian Institute of Technology (BHU) 

Varanasi-221005, India

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