• topas

  • Non-standard space groups

Hello everyone,

I'm having some troubles with one of my PXRD sample. I want to do a Rietveld refinement on it, but the space group I have to use is the non-standard Pb-3. I have an analogous compound for which I have a crystal structure of with the space group Pb-3. When I compare the PXRD of the two samples (generated from SC-XRD vs measured), they are very much the same. The standard spacegroup #205 is Pa-3 but using that space group understandably generates a completely different powder pattern. Doing a Pawley refinement with the space group Pb-3 works just fine (R = 11.83).

When I try to do a Rietveld refinement, it ends up with the following exception error:
Generating xyz's for Pb-3
Error generating equivalent positions

Is there a way I can fix that error?

Thanks a lot for your help,
Didier
a month later
When I compare the PXRD of the two samples
Hello all,

Problem is solved! The way it was fixed was to switch from the space group Pb-3 to Pa-3 to be able to use the atomic coordinate from my analogous compound in TOPAS.

To do that, I changed the x and y of the atomic coordinates and then regenerated the cif file using Mercury (open the cif file and save it back as a new one).

For some reason, the coordinates in my Pb-3 were really strange (as in really far off from the origin of the cell) and TOPAS would not accept these coordinates for the refinement after I switch to Pa-3 when I was just swapping the x and y (it would result in a fit that is a completely flat line).

Regenerating the cif file with Mercury fixed the problem by resetting the coordinates closer to the origin with my metal center in the appropriate special position. The Rietveld refinement works just fine with Pa-3 and the new coordinates now.

Regards,
Didier