2.3- Trying to identify a material knowing the cell

At this stage, one has to be sure of the cell proposition, of course. This is the same position as if the cell had been obtained from a single crystal by a four circle diffractometer or by photographic methods : knowing the cell, should we now record the complete data for a structure determination or has the compound been already characterized ? From powder data, the whole dataset may have been yet registered but not necessarily. For a single crystal study, a classical four circle diffractometer may be immobilized for one day or two weeks depending of the cell complexity so that one should be sure that this is not waste of time and money. Maybe an isostructural compound will be found in databases so that reasons for performing a work presenting a low degree of originality should be found elsewhere (special properties or something else) than in the objective of adding a chapter in a PhD thesis.

The exact formulation of the compound is not always known at this stage. Now that you have the cell, you may divide the volume by nearly 20 and you will not be far from obtaining the number of anions if your material is an oxyde or a fluoride. If you have an estimation of the formula by chemical analysis (possibly by means of electronic microscopy) then you may estimate Z, the number of formula units per cell. You can measure the density if you dispose of a sufficiently large quantity of pure compound and compare it to the calculated one. Thermal analysis (DSC, TGA) is something to do absolutely. Then you dispose of informations allowing you to explore all the databanks, not only the PDF-2 one which is really almost empty if one considers that 200000 compounds or more have been the subject of a structure determination. (cf. introduction in paragraph 2). In principle the JCPDS-ICDD files have yet been examined since this is a powder work. Either your compound is not inside, or there is a doubt about a possible structure determination which would have been already published (PDF-2 is rarely explicit on this point). Nevertheless it is recommended to try an ultimate search in PDF-2 with these new data wich are the cell parameters, the volume and eventually the space group.

With an inorganic compound, the first bank your should consult is ICSD. Possibilities for a query are numerous (the manual has more than 80 pages). The question may consist in providing the chemical elements that you are sure to be included in your sample, without excluding other possibilities by a too much strict query. The crystal system may be given to the search software as well as a cell volume with some tolerance range. The search result may consist in several possibilities which you should examine or try to reduce their number by adding other indications. If the query is too much precise then isostructural compounds may be missed. Use the same approach for an organic or organometallic material with the CSD databank.

If ICSD or CSD consultation are negative, then you should consult CRYSTAL DATA from N.I.S.T.. This is easy because a query consisting in the cell parameters with a tolerance factor may suffice, addind the possibility for finding sub- or supercells. CRYSTAL DATA can allow you to conclude that your compound or an isostructural material exists somewhere in a publication and that its cell at least has been established. You should not conclude that the structure has been determined necessarily until you have seen the atomic coordinates.

WARNING : You will have to examine the 3 or 4 recent years bibliography by using the Current Contents, the Chemical Abstract or the other (commercially) available databanks (PASCAL... see INIST). Do not neglect the more than 16000 indexed periodical iin the Uncover database to which you can access too by Telnet (URL Indeed, searching by keywords is free in this bank, only the order of a publication costs something : an example which should be followed by the other data sources. Last but not least, most of the scientific revues have placed their content together with a summary in a catalog of references searchable by keyword (see Acta Crystallographica and all the IUCr journals for instance indexed since 1983).

If all attempts for identification failed, an ab initio structure determination from powder data is not always the best thing to do :


Copyright © 1997- Armel Le Bail