Dear Ahmed,
two different things should be distinguished:
- the physical possibility do the refinement
- the mathematical way do implement the constraints in eg Fullprof
Concerning the first point, as it has already been pointed out, it is
not possible to refine site occupancies of two atoms sharing the same
site from a single X-ray or neutron data set. This is basically because,
from a single dataset, you will get _one _(nuclear or electron) density
from which you cannot get _two _site occupancies (an infinite number of
combinations of the three elements can give the same density). However,
this is possible if you have two datasets eg X-ray+neutron or two X--ray
datasets, one of which using anomalous (or resonant) diffraction. In the
two datasets, the relative contrasts between the elements should be
different to insure higher accuracy.
Concerning the second point, yes it is possible. Let's name the three
elements A, B and C. You may divide the atom A into two lines: A1 and
A2. For the program, it does not matter if you have one A on position
xyz or 0.5 A (A1) on xyz plus 0.5 A (A2) on xyz. Then you can set the
constraints: A1 occupancy +11.00, A2 occupancy +21.00, B occupancy
-11.00, C occupancy -21.00. Please note that it is not a restraint, it
is a constraint maintaining the full occupancy of the site and the
result will be exact. If the mechanism involves the distribution of A,B
and C on different sites, the technique may be applied to constraint the
overall composition of A, B, C. Eg. with two sites, put on the second
site A1 -11.00, A2 -21.00, B +11.00, C +21.00. Of course, the initial
values of the occupancies should be set correctly.
The technique has been described in my paper: J.-M. Joubert, R. Cerný,
M. Latroche, A. Percheron-Guégan, K. Yvon, Site occupancies in LaNi5
three-substituted compound determined by means of multiwavelength X-ray
powder diffraction., J. Appl. Crystallogr. 31 (1998) 327-332.
And it allows to solve very complex substitution schemes such as
refining 3 atoms distributed on 5 sites with overall composition
constraint such as in K. Yaqoob, J.-C. Crivello, J.-M. Joubert,
Comparison of the site occupancies determined by combined Rietveld
refinement and by DFT calculations: the example of the ternary Mo-Ni-Re
s phase, Inorg. Chem. 51 (5) (2012) 3071-3078.
Kind regards.
/Jean-Marc
Le 31/08/2019 à 16:44, Ahmed Subrati a écrit :
Dear all,
I wanted to ask how three atoms, sharing the same xyz position, could
be refined in terms of their occupancies /via /FullProf. We know that
for the case of two atoms, the first atom is set as 11.0 and the
second one as -11.0 so that their sum is unity, but how would be the
case for three atoms.
Thank you very much.
King regards,
Ahmed Subrati
PhD student
NanoBioMedical Centre | Adam Mickiewicz University in Poznan, Poland
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Jean-Marc Joubert
Chimie Métallurgique des Terres Rares
Institut de Chimie et des Matériaux Paris-Est
UMR 7182 CNRS-Université Paris-Est Créteil
2-8 rue Henri Dunant, 94320 Thiais, France
Laboratory's page:http://www.icmpe.cnrs.fr/spip.php?rubrique70
phone/fax: 33 1 49 78 13 44/12 03
Researcher ID: A-1259-2013, ORCID: 0000-0001-7266-1850
email :joub...@icmpe.cnrs.fr
personal page:http://www.icmpe.cnrs.fr/spip.php?article632
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