Question regarding NN correlation with PDFgui

[Tomas Northam]

Dear Diffpy users

 

I am a PhD working in the UK on disorder materials. I am currently doing some fitting to PDF data using your PDFgui program.

The sample that I am studying is the pyrochlore Pr2ScNbO7. It has the following occupancy

A-site 100% Pr

B-site 50% Sc and 50% Nb

O-site 100 % O

 

I wanted to know how, if possible, to extract the Sc-Nb correlation. So if I have a Sc at a B-site what is the probability that the nearest neighbour B-site will be a Sc or a Nb. Is there an extra routine, like a .py program, that I need to write to extract this information? Or are there any inbuilt PDFgui routines that can do this.

 

Kind regards,

Tom.

[Simon Billinge]

Hi Tom,

You will have to build a supercell to do this so you have more than one B site site.  Then allow the different B sites in the supercell to refine occupancies separately (but sensibly). Whether or not this succeeds will depend on whether there is enough info in your data to see it.If the data are x-ray then there is a fighting chance because Bn and Sc are somewhat far apart in atomic number.  I didn't check the neutron scattering lengths.

If you need to you can break local symmetry by allowing neighboring oxygen atomos to relax around the different B sites if you like.   To avoid introducing too many new variables make sensible constraints so that it is a breathing mode (oxygens move together) for example.  You will have to figure out the constraint equations to make this wok, for example  O_top (z) = @1 O_bottom = -@1, O_left = @1, O_right = -@1 and  so on .

S

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Simon Billinge

Professor, Columbia University

Physicist, Brookhaven National Laboratory