At 14:50 09/07/2004, you wrote: >I was wondering which is the best method for obtaining the structural model when you >have no clue how the structure should look like
I suppose you will be told of the several programs that now exist for solving structures directly from powder data. This is a way to do it "en principe", but it often doesn't work in practice, and to tell beginners they only need a computer programme and then "turn the handle" is perhaps not the best way to teach them. The first thing to do is to try to obtain clues about how the structure might look, and then as Jim Kaduk said, check that similar compounds have not already been solved. (Sounds obvious doesn't it :-) In many, many cases that will be the case, but you really need access to the complete crystal structure databases for that. When you say you have no clues, surely you must know the chemical constituents, and maybe even their proportions ? That may already be enough to get you started on a search of the databases. Then you might need to measure the d-spacings, and again a search of databases might immediately throw up a solution. (You would have to measure d-spacings anyway to apply structure solution programmes). If you still don't have a clue, you will need to index the powder pattern, obtain the unit cell and space group. There are several excellent programs for that, but you do need good data. With the unit cell and space group, the databases have a much better chance of finding a structural model for you. (And you would still have to do that before you could apply structure solution programmes). I hope that I have convinced you that you can easily find clues, and that you will learn a lot more about crystal chemistry this way, and probably find the solution more quickly, than simply applying computer programs. The people who advocate doing this are not exactly beginners. Alan. Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> fax (33) 4.76.20.76.48 (33) 4.76.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm ___________________________________________________________
