In an ideal cubic perovskite (ABO3) compound with SG pm-3m atomic positions are A at 1a (0, 0, 0); B at 1b (0.5, 0.5, 0.5) and O at 3c (0, 0.5, 0.5). In FULLPROF program their occupancies can be given in ratio 1: 1: 3. Now, if atom B is disordered to site 6f (x, 0.5, 0.5) in place of 1b, what would be the occupancy ratios of A:B:O to be given in the FULLPROF pcr file while maintaining same stoichiometry (ABO3) e.g. lead magnesium niobate is such disordered compound? In FullProf manual Occ is defined as chemical occupancy X site multiplicity (can be normalized to general position multiplicity).
Atom Typ X Y Z Biso Occ
Ideal B …. (0.5, 0.5, 0.5) …… 1.0
Disordered B…..(x, 0.5, 0.5) ……. 1.0 or 6.0 or ?
What would be the occupancy number of B?
There are three possibilities for OCC of B i.e. 1, 6 or 1/6. Program accept all three values but how to know "which one is correct?"
In Fullprof you can keep the 1 value if you use the 1 1 3 ratio no matter what the occupation of the site is but I guess since May 8 you already downloaded the Fullprof manual and found out by yourself :-)
Dear Dr. Suescun, thanks for your reply. After may 8, I have done this exercise with Occ of B as 1, 1/6 & 6 . Refinement suggest that it should be 1 (1:1:3 ratio) as you have mentioned. But I am unable to visualize it physically, since there is multiplicity 6 corresponding to 6f positions of B. So, I found RG as platform to clear my queries. Could you please elaborate it more.
The important thing with site occupancies in Fullprof is that they are all proportional to the number of atoms in the unit cell, no mater if you use 1, 1, 3 or 1/48, 1/48, 1/16 that are the number of equivalent atoms in that set of equivalent positions divided by the maximum site multiplicity (multiplicity of the general position) of the space group. Both sets of numbers will contribute different amouts to the scale factor of the refinement but if you keep the relation among them won't change the relative peak intensities. Any set of numbers that are proportional to 1, 1, 3 will work. When you change B-site position from 1b to 6f, then in each of the 6 equivalent x, 1/2, 1/2 positions you shoul find 1/6th of B atom, or equivalently only one of the six equivalent positions is occupied ar the same time in any unit cell. If you calculate the occupancy for this B-site atom at 6f you still have 1 atom per cell so use 1, 1, 3, or if you want to use 1/48 for A site, then for b you are on a multiplicity 6 site, divides by 48 gives 1/8, but you only have one of se si possible sites occupied so you have to divide the site occupancy by 6 or (1/8)/6=1/48. Again yo have the same 1, 1, 3 ratio as 1/48, 1/48, 1/16. You should be able to change one set of numbers by any other keeping the stoichiometric ratio an the only thing you should notice is a change in the scale factor.
I ignore, since I'm not a frequent Fullprof user, if other magnitudes derived from the cell contents (such as density, absorption factor, etc) are affected by the use of one or the other way. In any case a codeword may exist that tells the program what kind of notation is being used or it may internally chech and find the true content ofmthe unit cell.
I hope I made myself clear. This is not such a complicated concept but just a messy one for beginners!!!
Good luck,
Leo
Dear Dr. Leopoldo, thanks for a simple and beautiful explanation. Surely your answer will help many begineers like me.
sir what is the occupancy of ca, cu, ti and O in CaCu3Tio4O12...where Ca at 2a(0,0,0) and Cu at 6b( 0.5, 0.5, 0) , Ti at 6b ( 0.25, 0.25,0) and O at 24 g
9 0.3200, 0.1800, 0),,,please suggest me to clear my doubts,,,
Dear Sandeep, it will be Ca = 1, Cu = 3, Ti = 4 & O = 12. Your structure seems ABO3 actually A4B4O12. Two very good answers regarding occupancy are given above by Dr. Leopoldo above. I am copying following useful lines " The important thing with site occupancies in Fullprof is that they are all proportional to the number of atoms in the unit cell, no mater if you use 1, 1, 3 ........."
Respected sir, i would like to know how to calculate occupancy in rietveld refinement, as m just a beginner and learning the process of refinement. also i would like to know what will be the occupancy for Mn, Fe and O in Manganese Ferrite
What is the occupancy of Li, Cu, Mn, O4 in LiCuxMn2-xO4 required in Full prof software?
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