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|".

Start your lectures this week by first reading the original papers by Hugo Rietveld which are online at his own Web site (CD).Visit completely the site and the links.

The reference book is of course :

B3 - The Rietveld Method 
IUCr Monographs on Crystallography - 5 
R.A. Young, Editor 
Oxford Science Publication, 1993. 

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.
Reference : McCusker, L. B., Von Dreele, R. B., Cox, D. E., Louër, D. & Scardi, P. (1999). J. Appl. Cryst. 32, 36-50.

You should subscribe in order to receive the CPD newsletters (CD), if not already done.

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,
589. 

Hill, R. J., Cranswick, L. M. D. International Union of Crystallography. Commission on Powder Diffraction. Rietveld
refinement round robin. II. Analysis of monoclinic ZrO₂. J. Appl. Cryst. 27, 802. 

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|". 

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 :

• GSAS, the largest piece of Rietveld and related code, so far, including subroutines for drawing, Fourier synthesis, Patterson method, etc. (CD: gsaskit.exe, and CCP14 pages). 
• FULLPROF, avatar of DBW, with a Windows-driven version already downloaded (CD: winplotr.zip), and many friend-programs like GFOUR for Fourier syntheses (CD : gfourier_310.zip) (CD : CCP14 pages) .
• DBWS, distributed by R.A. Young (CD: DBWS.zip), the first really modern program applying the Rietveld method, doing the job in one run (no separated preparation before refinement), with background polynomial representation, evaluation of the list of contributing reflections at each cycle, multiphase, etc, ancestor of many other subsequent programs.

Many tutorials (CD) about some Rietveld (GSAS, FULLPROF,...) codes are available at the CCP14.

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).