Space group Fd-3m

[milin...@gmail.com]

I am trying to fit a PDF of a structure belong to the space group
Fd-3m. When I expand the space group, "96g" positions generate atoms
at 96 equal positions, and "192i" generate atoms at 192 equal
positions.

However when I expand (0,0,0) position which is a "16c" position,
PDFFIT generate atoms only at 8 equal positions. Since (0,0,0) is a
"16c" position, shouldn't it generate atoms at 16 equal positions?

Thank you.

Milinda

[Pavol Juhas]

Hi Milinda,

The Fd-3m space group has 2 variants with a different choice of
origin. By default, PDFgui is using representation where the
multiplicity of [0,0,0] is 8. As far as I remember the origin of the
second representation is offset by [1/8, 1/8, 1/8], so you need to set
the space group offset in the "Expand space group" dialog to either
[0.125, 0.125, 0.125] or [-0.125, -0.125, -0.125]. Please, check the
offset in the International Tables, which should have all variants of Fd-3m,
and also make sure that the positions get expanded as expected.

Good luck,

Pavol

[milin...@gmail.com]

Hi Pavol,

Thanks for the reply. Offsetting the space group by [0.125, 0.125,
0.125] generated my structure. I have another question,

Is it possible to do a profile fitting in PDFFIT like the Le Bail
method in GSAS?

Milinda




On Nov 21, 10:53 pm, Pavol Juhas <ju...@pa.msu.edu> wrote:

[Simon]

Please see the new thread on diffpy-users on the topic of Lebail
fitting to the PDF.

S