luciamaini
Dear all,
I have some doubt about describing atoms in special positions.
I am refining a structure in R3m and I have an atom in the "3m" ( 1/3, 2/3, z)
which I described as:
site Li1 x = -1/3; y =-2/3; z 0.35412 occ Li 1 beq 3
and an atom on the m ( x, -x, z)
site N1 x 0.1017 y -0.1017 z 0.3692 occ N 1 beq 3
I am not really sure about the occupancy. I noticed that if I use occupancy 0.5 for the N1, the rietveld refinement is better. ( while I keep 1 for Li1)
- How topas recnognize the special position?
- How we have do describe the occupancy for atom in special position?
- Do we use always occupancy 1 or as it is done in single crystal refinement ( SHEXL) the occupancy is 1/2 or 1/3 depending on the symmetry elements?
Thanks a lot
Lucia
johnsoevans
What you're doing looks correct.
In Topas you should use occupancy 1.0 for atoms on general or special positions (unless you think that the site is only partially occupied). For atoms on threefold axes you have to use equations like x = 1/3; rather than x 0.3333 as you have done.
If you need to check this type of thing you can ask Topas to e.g. calculate the density for you, or use one of the chemical composition reporting macros.
You can also add num_posns 0 to the site line (e.g. after the site name). After a refinement Topas will report back the multiplicity of the site, which is a useful check that you have things correct.