The SDPDRR Homepage is at http://www.cristal.org/SDPDRR/
Results for Sample 1
Indexing could be obtained from either TREOR, ITO or DICVOL.
Nothing was found when searching in the JCPDS-ICDD data base nor in
ICSD. Due to clear strong preferred orientation effects on patterns zhu1
and zhu2, the zhu3 pattern was selected for structure factor extraction.
The background was estimated manually with the DMPLOT program. The program
FULLPROF was used (Le Bail method) selecting P21/m space group
(which is unimportant at this stage). Links are given to files renamed
as .txt files :
R-Factors (%) were : Rp = 4.65, Rwp = 6.33 (background
included) and Rp = 7.59, Rwp = 9.41 (background subtracted). The fit,
as shown by DMPLOT, seems acceptable.
Six data files were prepared for use by SHELXS-97. The first data contains
the whole data, with 780 hkl, (zhu0.hkl) and
the OVERLAP program eliminated reflections having a neighbouring one at
less than :
Note that OVERLAP does not care with the estimated standard deviations
and lowers the values in order to keep the weak reflections in the data
set prepared for SHELXS. The weight of the Co atom is not enough for expecting
a Patterson map to disclose it so easily, because of the presence of ~15
other non-hydrogen atoms (O, N, C). Trying with direct methods was thus
preferred in a first step. SHELXS-97 produces 12 output files from the
6 data sets above, 6 in P21/m and 6 in P21 space
All these files were examined having in mind the fact that only one independent
Co atoms should be found in the cell (in twofold position) and that distances
with N/O atoms are expected to be of the order of 2.0-2.1 Angstroms. Seing
one CO3 and one NO3 group would also be fine and
one isolated water molecule. Without extra information about possible acentricity,
it was preferable to consider both space groups. Statistics in SHELXS-97
seems to indicate acentricity, however, this is powder diffraction data...
The zhu2, zhunc2, zhunc3 and zhunc4 SHELXS-97 output give all the same
proposition for the Co coordinates. The P21/m spacegroup could
not lead to any coherent result. None of the 3 above propositions in the
P21 space group led to the complete structure (remember the
FWHM minimal is 0.25° 2-theta !). However, using the zhu5.hkl as reduced
data set for testing the 3 propositions, it was possible to obtain R ~
30% (with SHELX-76), keeping ~ten of the proposed atoms. Then, several
cycles of Rietveld refinement and Fourier syntheses, revealed the whole
structure. The final refinement was made either with or without soft constraints.
Hydrogen atoms were not located. At least the positions of the water hydrogen
atoms may be guessed from an evident O-H...O bonding scheme.
To be noted is the anisotropic broadening which affects particularly
the patterns with best resolution (zhu1.txt and zhu2.txt). The 0k0 reflections
are the narrower and were fitted separately by introducing a pseudo two-phases
The preferred orientation in zhu1 and zhu2 patterns seems to correspond
to several orientations including 100 and 010. The best fit with structure
constraint (although with problems), is obtained from the choice of the
001 direction leading to a March-Dollase parameter value >> 1 (indicating
that the preferred orientation is all but the 001 one, may be the microcrystal
shape correspond to needles with 001 as long axis). The zhu1 and zhu2 patterns
were provided for possible use by methods able to cope with preferred orientation,
and even to take advantage of it (their resolution is two times better
than that of the zhu3 pattern).
Thanks for your participation.
Copyright © 1998
L.M.D Cranswick & A.
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