2.2.2- "Automatic" indexing

Maybe your teacher said you that a simple slide rule will suffice for indexing a powder pattern. That's true only if the problem is trivial. Serious problems need a computer and telling that it is "automatic" is truly a lie. You, in person, will have to make the final choice, frequently among multiple propositions. Figures of merit will help you in your choice and also your common sense. You should never have any full confidence in a cell proposed by an indexing program. The ultimate proof that the cell is correct is obtained when the structure has been determined and refined. Even at this point, you may realize that the cell is in fact larger with a higher symmetry or that you were in a wrong space group and so on.

Don't try any indexing program before to be sure that your data are reliable. Never use only one indexing program. The most recommended actually which have proven their hability in dozens of structure determinations from powder data are TREOR, ITO and DICVOL (you can find elsewhere other indexing programs which were not listed in the SDPD-D because this databank gathers only successful experimental cases), each of them has qualities and deficiencies. You will have to read their manual and to test them on the cases included in the packages (see links for obtaining programs in the introduction). Test the program on some powder data of your own, corresponding to known materials, so that you will obtain some proof that you are ready to fight with a really unknown material. Gain some experience. Why these advices ? The indexing step is certainly one of the most difficult !

The strategy applied to the samples in this scenario was generally as explained below :

We will see the results of this general strategy as applied to the experimental cases retained in this scenario (warning : read the manuals of the programs mentioned, they will not be fully restored here) :

Copyright © 1997- Armel Le Bail