[sdpd] Espoir success example for an inorganic

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

Some examples may give you ideas for using Espoir.

How Espoir could succeed when a classic approach fails for
an inorganic compound ?

That example is La2W2O9. An X-ray powder pattern is indexed
in a triclinic cell. The La and W atom sites are easily found by
direct methods in P -1 (2+2 sites). But oxygen atoms are not located.
This may indicate that the structure could be described in P1
instead of P-1, so that 18 oxygen atoms should be found. No
solution is obtained. A neutron pattern is recorded. The resolution
is much lower than for the X-ray pattern, and the weights of all
atoms are similar, so that the direct methods fail. Trying Fourier
syntheses from the known positions of La and W gives nothing,
of course.

An hypothesis of WO4 tetrahedra is used in Espoir. They are
rotated around the fixed W X-ray positions using the neutron data,
whereas the La atoms are kept at fixed positions too. Nothing below 
RF = 25% is obtained. Then the La and W positions are not
allowed to move and the 9 oxygen atoms are searched by
Espoir in a random approach. a R=6% is obtained in P-1 from the
150 first reflections of the neutron powder pattern in (which
contains > 1400 reflections...). Drawing shows that the 2 sites
of W have different coordinations : one 6 (octahedron) and one 5.
These polyhedra are connected by corners, forming isolated rings
of 4 polyhedra. The whole neutron powder pattern is used for
refining that model by the Rietveld method, leading quickly to
RB ~3%. Finally, it is understood that the X-ray pattern had some
preferred orientation in spite of efforts to avoid it, and may be this 
was the reason for not finding clearly the oxygen atoms by
Fourier difference synthesis. To be published.

Using more reflections than the first 150 was unnecessary. In
fact, their high degree of overlapping would not have added more
information, and would have slowed the calculation speed, if not
completely confused the program.

Nice story ?

Armel
http://www.cristal.org/course/
http://www.cristal.org/sdpd/espoir/