2- Materials unidentified by the routine approach with PDF-2

If the search/match failed by using the PDF-2 databank, one should try more. Other databanks contain crystallography related informations :

- ICSD (Inorganic Crystal Structure Databank), on CD-ROM, contains nearly about 42000 phases of which the structures were determined from single crystal or powder diffraction data. The atomic coordinates are generally given (not for 3000 entries) together with the full reference and some other details. The 1992 version was quasi-complete (?) up to 1988, however the CD-ROM contained only 2 entries for 1990, 105 for1989, 924 for1988, 1203 for1987 ..etc. One can observe the presence of multiple entries for one compound in this bank too. There are also a huge quantity of isotypical compounds (unavoidable). The user should be very careful and make use of his common critical sense with any databank, including this one. It is not exhaustive, unfortunately.

- CSD (Cambridge Structural Databank) on CD-ROM contains about 160000 entries for small organic molecules and organometallic compounds. This number increases by 10000 each year !

- N.I.S.T. CRYSTAL DATA, on CD-ROM, contains about 200000 entries corresponding to 60000 single phases (inorganic/organic). Are gathered a large number of published propositions of indexation. These could be false or the structure (which is the final proof that a cell is correct) was not necessarily determined. There are numeorous multiple entries and isotypical compounds in this bank.

- CRYSMET is a compilation of data concerning metallic compounds.

To be able to use these databanks, complementary of the powder diffraction databank, one information which may suffice is the sample composition. However the best way is to add the cell knowledge. Several methods giving access to the cell are to be considered depending on the presence or not of large enough single crystals in your preparation.

The PDB (Protein Data Bank of Brookhaven, 5400 entries) has been excluded from this discussion. Indeed, you have little chance to succeed in a protein structure determination from standard powder diffraction data (unless it is a small protein or you are working with a high resolution diffractometer using synchrotron radiation, eventually at low temperature). A discussion about the limits of what you can expect to do from powder data will be given later (chapter 3.1).

The scenario will continue with the study of five real cases :

The various (standard) strategies used in structure determinations from powder diffraction data will be examined at each step of the whole process for one or several of the examples above, successively in the next parts of this tutorial. The corresponding data are made available (together with other data) in the SDPD-D (Structure Determination from Powder Diffraction - Database).

Copyright © 1997- Armel Le Bail