STRATEGIES IN STRUCTURE DETERMINATION FROM POWDER DATA

1.3- Using the PDF-2 database (ICDD)

PDF-2 is a database (on CD-ROM or other support) gathering approximately 65000 powder diffraction patterns. The bank contains the pattern of your sample, maybe. There are two ways for using the bank. It is not always necessary to make use of it if you know yet the system you are studying. Sometimes you may visually recognize immediately which sample you have prepared.

Softwares exist which are able to examine in a few seconds the 65000 powder diffraction patterns of the database (at least on a PC Pentium 100 MHz or better) and to propose a list of patterns supposed to fit with your experimental data. The final decision is yours : you will have to compare each pattern proposed by the software and to accept or not the identification attempt. An example of automatic search for the sample chosen in the scenario is shown on the figures 8 and 9. The software is EVA 2 from the Socabim Inc. Very easily, the background was subtracted (figure 8) and then the default options in the search/match process were applied (figure 9), the result is a list of possible compounds which may correspond to the scenario sample. Examining all the propositions, it becomes clear that the beginning of the scenario is based on the study of a known material which is KAlF₄. It was placed at the head of the list proposed by the EVA 2 program.

Preferably, don't have a full confidence in an automatic search when it is negative. Usually you know what chemical elements are in your sample and so you should sort and examine the complete list of JCPDS-ICDD cards corresponding to various combinations of these elements. This is a laborious work, it is decisive time to time. You should even include in the list the patterns corresponding to some permutations of chemical elements for some well known isostructural substitutions. For instance, in fluorides, you could have synthesized an aluminum-based material, previously unknown, which could be isostructural with a compound in which the Al atoms would be replaced by Cr, V, Ga or Fe. This latter known material could be present inside the database with neighbouring cell parameters, too much different however for an automatic recognition by the search/match process.

A manual search by the Hanawalt method (using a classification according to the most intense reflections) gives sometimes ideas for isostructural compounds, sometimes it gives a positive identification when the automatic search fails.

Continuing this scenario, we will consider powders of unknown materials. Nevertheless a failure to identify a compound by means of PDF-2 is not a sufficient proof that it is really unknown or that an evident isostructural compound does not exist, one should be conscious of that. Indeed, powder patterns from 3 different origins are found in the PDF-2 bank :

• copy of d_obs and I_obs in publications,
• patterns sent directly to the bank by a chemist or equivalent (private communication, graciously or under "Grant-in-Aid" contract),
• patterns asked by ICDD to the authors of a publication mentioning powder work but not giving d_obs and I_obs. In this case, people may send sometimes patterns calculated from the crystal structure (for instance with the well known LAZY-PULVERIX software).

Critics can be formulated against the ICDD politics. The theoretical patterns that anybody could easily calculate from published crystal structures, determined from single crystal diffraction data, are not systematically included in the PDF-2 !!! On another hand, multiple works concerning the same compound are inside the bank, some JCPDS-ICDD cards carry the mention "Deleted" meaning invariably that a more recent card, corresponding to more accurate data, is in the bank. However, the old "deleted" card is still there, contributing to the 65000 of which is proud ICDD. The quality of numerous cards is really dubious (some carry the mention "questionable") and less than half the cards inside the bank propose a cell indexation. Some cards do not mention if the crystal structure has been determined in a subsequent or even previous work which made not use of powder diffraction methodology. This is the case of the sample chosen at the beginning of the scenario. The recent 40-0549 card about KAlF₄ is given for 1986 and presents a false cell, a false spacegroup, a very bad figure of merit characterizing the cell parameters refinement. This is not understandable because the structure was published in 1981 par J. Nouet, J. Pannetier and J.L. Fourquet , Acta Crystallographica, B37, pp 32-34)... Actualization is a problem : very few cards are more recent than 1995. My opinion is that there are no more than 15000 cards which are correct in the bank. Unfortunately ICDD has a monopolistic position in this domain. This point of view is mine, not that of my employer, of course. It is surprising to observe an increase of 10000 new entries per year in the Cambridge (CSD) databank of organic and organometallic compounds whereas PDF-2 increases of approximately 2000 cards of which 25% are organic. I suggest that the ICDD should quickly :

• actualize cards which are not mentioning structure determination for the concerned compound ;
• introduce systematically the patterns calculated according to the published crystal structure data.

As viewed from outside the ICDD, it seems to be exactly the contrary which is done. Having discussed with some responsibles, I heard the following argument : if compounds were not observed as pure compounds so that an experimental powder pattern could not be measured, then they should not be included in the bank as calculated patterns ! Seems that powder specialist leaders at ICDD estimate that single crystal data are not reliable ! Try to explain that to a researcher who will prepare a compound (pure or as a mixture, why to make such a distinction ?) and will waste a lot of time at the identification stage because it is known but not in the bank (I have yet wasted a lot of time, believe me)... The ICDD decisioners have probably not tried by themselves to identify anything for a long time.