SDPD Internet Course
Advanced methods for extracting structure factors, part 2.
Read the conference "Standard and special strategies in structure determination from powder data" (CD). Become conscious of the main reasons for failure in SDPD. One is that preparing a pure sample can be sometimes impossible. You cannot pick a single crystal in a mixture, like this is frequently done for single crystal structure determination techniques. So that one should be able to cope with a multiphase powder pattern. Sometimes several unknowns are mixed together. This is certainly the most complex situation and success was rarely obtained (but see one case shown at conference "New Developments in Microstructure Analysis via Rietveld Refinement", slide 13 - (CD)). Syntheses by varying the component concentrations often show that a sample is a mixture and allow to attribute some reflections to one phase and other reflections to another phase, due to intensity variations from one preparation to the other. Anyway, it is not always possible to completely eliminate a parasite phase, for instance when the process is hydrothermal synthesis. Fortunately, the parasite phase may be already known and will serve as internal standard for indexing. But at the stage of structure factors extraction, what should be the strategy, since a pattern of the pure unknown is unavailable ? The best is always to take account of the known phase with structure constraint while the "|Fobs|" are extracted for the unknown. We are not touching exactly to the Rietveld method since we will not refine any structure this week, but we are now very close to it.
Investigations of the PbF2/CrF3 in aqueous HF hydrothermal conditions revealed a new phase which could not be obtained as large enough single crystal. Moreover, this new phase was never pure. The impurity was soon identified as being the starting CrF3 material (rhomboedral form).
Two patterns are delivered for this exercice which will allow you to make a complete review of the knowledge acquired during the previous weeks of the SDPD Internet Course (identification, indexing, structure factor extraction with cell confirmation and space group determination as by-products). One pattern corresponds to the starting mixture 3PbF2/CrF3 (PbCrF1.raw, PbCrF1.uxd, PbCrF1.udf). The second pattern corresponds to the starting mixture 2PbF2/CrF3 (PbCrF2.raw, PbCrF2.uxd, PbCrF2.udf). Both pattern were measured on a D8 Advance Bruker diffractometer at CuKalpha radiation, they show different concentrations of the unknown and of CrF3.
- Find crystal data about CrF3
in ICSD and ICDD databases, or elsewhere (see the FAQ,
maybe (CD)). You may possibly
face a small problem : rhomboedral phases are generally described in hexagonal
setting only by many crystallography software. You may have to transform
rhomboedral coordinates to hexagonal ones, Cryscon is a good software for
that (CD : CrysconI.exe).
The next week (or when you will be ready)
will be reserved for