STRATEGIES IN STRUCTURE DETERMINATION FROM POWDER DATA

- 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 :

• One compound identified in the ICDD databank, given with a cell proposition ; absent from ICSD although compounds classified as organometallic like Ni(CO)₄ are included ; found in CSD : Na₂(COO)₂. This is a simple case chosen as a school case. The study will be by conventional X-ray data.

• [Pd(NH₃)₄]Cr₂O₇, a typical case with heavy atoms for testing the Patterson method (and also, why not, the direct methods). The study will make use of conventional X-ray data. This compound was peviously listed in the ICDD databank with a false cell proposition.

• t-AlF₃, more complex than the two previous ones because the initial model necessary before to begin to refine the structure by the Rietveld method was the whole structure. Indeed, Al³⁺ and F^- are isoelectronic and thus their X-ray diffusion factors are not very different. This is a typical case for applying direct methods : eleven independent sites should be found simultaneously. The study was realized by using conventional X-ray data. This is a beautiful example of the contribution of the powder diffraction methodologies to the advancing of Science and basic knowledge. The result was a crystal structure based on an unknown new way to arrange octahedra exclusively by corner sharing.
• beta-BaAlF₅, examined by neutron diffraction, to show how more difficult is the structure determination from neutron data (remember that determination is not refinement : for a refinement, at least a large part of the structure should be known). In the reality, this structure was determined from X-ray data and the accuracy (on F and Al atomic coordinates) was later improved by refinement from neutron data.
• Cimetidine C₁₀H₁₆N₆S, a famous example, previously known from a single crystal study. This case was examined by synchrotron radiation in order to estimate the feasibility limits by the powder methods several years ago (see R.J. Cernik, A.K. Cheetham, C.K. Prout, D.J. Watkin, A.P. Wilkinson & B.T.M. Willis, J. Appl. Cryst.24 ,1991, 222-226).
• See also the 2 samples of the SDPD Round Robin.