Siemens D500 diffractometer
28 mA - 38 KV
Graphite monochromator in the reflected beam
Receiving slit 0.15°
Sample holder loaded vertically
Counting 26 sec. per step
0.02° 2-theta counting step.
Then the Kalpha-2 contribution is estimated and subtracted.
Finally a search for peak positions is done.
The result is shown on the screen
and saved in a file.
Remember that the routine study of this sample led to a positive identification :
Calibration by harmonics
Results are gathered into a file.
For indexing by using TREOR, the 20 first reflections
What would have been proposed by the ITO software ?
Finally, data prepared for DICVOL.
Before to continue, we have to confirm the cell proposition.
Being an expert, you may try the Pawley or Le Bail methods.
FULLPROF is only one of the numerous Rietveld programs which are now able to apply structure factors extraction (a list is available).
The first step will be to estimate visually a background
which will be kept fixed during the process.
Only one parameter is refined at the beginning : the zeropoint. Reliabilities before the first iteration are relatively high :
however at the 20th iteration, the profile R factor Rp is yet very satisfying.
You cannot obtain such a result
if the cell has no relationship with reality.
Since the first step results are encouraging, a second
step will consist in the refinement of all the possible parameters.
Improvement is obtained (with Rp ~ 6.5%) which strengthens us in the idea that the cell is the correct one.
See the structure factors
as they are extracted.
The next figure shows a zoomed view of the observed and calculated patterns as realized by DMPLOT able to read directly the .prf output file from FULLPROF.
Really, we are not yet at the stage of extracting structure factors. This stage should be done after a space group proposition.
Examining the reflections list, looking carefully at the observed and calculated patterns allows to conclude to a P lattice and to suspect that the P21/a space group could characterize the studied compound because of conditions limiting possible reflections for 0k0: k=2n et h0l: h=2n.
Reflection overlapping can lead to ambiguities so that several space groups could remain as possible and should be tested successively.
The ab initio structure determination is even not really decided at this stage because we have a few verifications to perform before to be sure that the structure has never been determined (in this fictitious example).
Indeed, attempts of identification knowing the cell have to be done.
If these attempts were successful, then it would be useless to go further.