Dear S. Meejoo,
if I have understood it correctly, the six ligands attached to the C6 core are
crystallographically inequivalent, and the mean occupancy of Y is 1/6 and of F
is 5/6.
The problem is, that one cannot restrict a series of occupancies to have a
certain sum, but you have to restrict the
changes of occupancies by adding some sites for several times and putting in
adequate constraints, for occupancies and for atomic positions and displacement
parameters. I will point it out for three sites with 1/3 Y and 2/3 F, try to
make the sixfold case by yourself. The way I show now is possibly not the
shortest, but may be more concise than the shortest solution
First input the ordinary atomic sites and distribute Y and F statistically
# Atom x y z starting value for occupancy
#1 Y x1 y1 z1 0.33333
#2 F x1 y1 z1 0.66667
#3 Y x2 y2 z2 0.33333
#4 F x2 y2 z2 0.66667
#5 Y x3 y3 z3 0.33333
#6 F x3 y3 z3 0.66667
The occupancies of these are held fixed.
Then allow redistribution of Y and F between the sites x1 y1 z1 and x2 y2 z2 by
dublicating some sites
(This is some kind of concentration wave mode. The starting values can be 0 for
the case that the redistribution of F and Y involves no symmetry breaking, they
should end up as, e.g. (see constraint) -.1 .1, .1, -.1).
#7 Y x1 y1 z1 0
#8 F x1 y1 z1 0
#9 Y x2 y2 z2 0
#10 F x2 y2 z2 0
and adding a constraint for occupancies ensuring that x1 y1 z1 and x2 y2 z2 both
remain fully occupied and the overall Y:F ratio is maintained
#Phase variable atom no. coeff
1 frac 7 1
1 frac 8 -1
1 frac 9 -1
1 frac 10 1
And with a next set of four sites the redistribution between sites x1 y1 z1 and
x3 y3 z3
#11 Y x1 y1 z1 0
#12 F x1 y1 z1 0
#13 Y x3 y3 z3 0
#14 F x3 y3 z3 0
with the constraint
#Phase variable atom no. coeff
1 frac 11 1
1 frac 12 -1
1 frac 13 -1
1 frac 14 1
Then the occupancies of #7...#14 have to be refined, and the constraints ensure
that there are only two parameters.
You can then calculate the "real" occupancies of x1 y1 z1, x2 y2 z2 and x3 y3 z3
by summing over the different patial sites
For six sites I think there should be
4+5*4 atomic sites and 5*4 constraints for redistribution between the sites
And of course constraints for the coordinates (If really the coordinates for F
and Y are the same)
A similar problem was treated earlier on the Rietveld list but I did not find
the corresponding entry now. But there a reference was mentioned describing the
procedure for a different case:
Journal of Applied Crystallography, vol.31, pt.3, 1 June 1998, pp.327-32.
I myself applied it similarly, without knowing the above reference in a somewhat
different case (involving vacancies)
(also using GSAS) in J. Alloys Compd. 316 (2001) 21-38.
Is this what you wanted? If this is the case and the procedure does not work,
contact me directly.
Andreas Leineweber
--
Dr. Andreas Leineweber
Max-Planck-Institut fuer Metallforschung
Seestrasse 92
70174 Stuttgart
Germany
Tel. +49 711 2095 527
Fax. +49 711 2095 420
e-mail: [EMAIL PROTECTED] (formerly [EMAIL PROTECTED])
home page of department:
http://finix.mpi-stuttgart.mpg.de/mittemeijer/english/index_english.htm
