SDPD Internet Course
Structure completion and refinement by the Rietveld method, part 1.
We are now at the final SDPD step.
When dealing with powder diffraction data, a structure should always be refined by using the Rietveld method. Never present a final refinement using the extracted "|Fobs|".
The reference book is of course :
B3 - The Rietveld Method
The Rietveld Mailing List homepage is a good starting source (CD). The Rietveld mailing list archive (CD) is a mine of information. Note that you have old and recent archives links at this site, both searchable by keywords. At the same place, a list of most prolific authors (CD) using the Rietveld method is given for inorganic materials (ICSD database).
One very recent paper gives hints and
tips about the Rietveld method, written by members of the CPD (Commission
on Powder Diffraction, IUCr)(CD). You should
read it absolutely.
Two very important papers were written at the end of the RRRR (Rietveld Refinement Round Robin) which you should also read :
Hill, R. J. Rietveld refinement round
robin. I. Analysis of standard X-ray and neutron data for PbSO4.
J. Appl. Cryst. 25,
Hill, R. J., Cranswick, L. M. D. International
Union of Crystallography. Commission on Powder Diffraction. Rietveld
You may perform a search at the IUCr journals by using the "Rietveld" keyword.
Finally, read chapter 3.4 (CD) of the SDPD tutorial. Examples are treated showing how to complete and refine a structure by alternating the Rietveld method and Fourier difference syntheses from the "|Fobs|".
Software to download
Updated lists of Rietveld programs (CD) as well as companion programs (for looking at difference plot, making Fourier syntheses, etc) can be found at the CCP14 Web site. A talk on Rietveld software by Lachlan Cranswick gives more. Another list gives those Rietveld programs used in SDPDs (CD). Clearly, the most popular Rietveld programs used by SDPD experts are freeware :
1- Complete the structure determination and refinement of Na5Cr3F14 and Pb2CrF7. Use the data from previous weeks. But in the case of Pb2CrF7, consider that only the Pb and Cr atoms were located before to perform the first Rietveld refinement.
2- A powder diffraction patern of K2TiF6 is recorded (Cu-Kalpha, Bragg-Brentano geometry, back graphite monochromator, sample holder loaded vertically). Produce the best Rietveld refinement you can, starting from the ICSD entry. What is the main problem with this powder pattern ?
In each case, explain briefly your strategy and produce the .sum file if you used FULLPROF, or any equivalent summary of the refinement from another Rietveld program (including all refined parameters, their estimated standard deviation and the reliability factors).
Main programs selected for the correction
FULLPROF, SHELXL, STRUPLO, GFOUR...
The next and last week will be reserved
for the structure completion and refinement by the Rietveld method, part 2.