Alan, the latest thread on Rietveld and powder diffraction has been most interesting and even thought-provoking. So you should be encouraged to continue with this group. I read it regularly and value it a lot. Some particular comments, if I may:
1. Your point about all structures being triclinic: yes that may be true, but most of what we want to find out about in terms of structure is the average, and that is where one uses higher symmetry as an aid to getting an acceptable solution. 2. Having said that, when I started in this Rietveld business (your fault if you recall!) we accepted that long-range periodicity was what we assumed and that crystal structures for the most part were regular (remember early papers of yours and mine!). However, over the years it has become increasingly clear that it is the departures from regularity that we need to study more. In my own research the gradual realization of this in the PZT series is an example (see http://www.nature.com/ncomms/2014/141024/ncomms6231/abs/ncomms6231.html). In this case, it has become clearer (or at least I think so) that real systems like this are far more complex than thought hitherto. In PZT we now have evidence that the true structure is monoclinic rather than rhombohedral (yes you can call it triclinic if you want but that just introduces too many parameters to be useful), at the unit cell level, but averages into large scale regions that are on average rhombohedral and those that are monoclinic i.e. a mixture of different coherence regions. This was achieved by a combination of neutron Rietveld and PDF. Similar complexities are beginning to be found elsewhere (in the perovskite field for sure). 3. The future? Always dangerous to make predictions (much greater scientists than I have fallen into that trap). In the long run I suspect that the need for crystals in the molecular sciences will become unnecessary. For the most part molecular crystallographers only use crystals as a means of ordering individual molecules so that they can be used in diffraction. They are interested in the molecules and the crystal structures (packing, intermolecular contacts) are of no interest. This is especially the case for macromolecular people for whom having to crystallise molecules is a pain. On the other hand, much of inorganic and related materials have properties that depend on the crystal structures, and so crystals will remain of importance. It may well be the case that the need to do actual powder diffraction for structural studies though will largely go, as techniques evolve to enable individual crystallites to be studied. I guess it will remain necessary though for phase identification studies. Who knows? Except that the whole field of crystallography is undergoing major development right now with the brilliant new techniques appearing in EM, synchrotrons, FEL's, neutron sources, NMR, Raman etc etc. It is in fact an exciting time for crystallography. Mike Glazer
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