John Having begun my crystallographic life with small molecules (organic semiconductors) and subsequently moved to PX, and having worked on SOD crystals I stand in both camps (i.e. both meanings: site-occupancy disorder and superoxide dismutase!). It seems to me that static disorder is the appropriate description of any situation where the time-averaged unit cells are not all identical and the variations are more or less random throughout the lattice. This would then apply both to SOD and the more common (at least in MX) positional disorder.
But I'm puzzled where you say "where there is disorder surely such a chemical moiety would be invisible". Surely if there is static disorder such that a fraction x of the sites are randomly occupied, with the remaining fraction 1-x vacant the moeity in question will be perfectly visible, just with reduced occupancy x. In fact I had an example of this: a 9-methyl anthracene molecule sitting on an inversion centre with the Me group randomly occupied with half occupancy. The disordered Me was certainly visible in the map, just with reduced density compared with the other C atoms. -- Ian On 20 November 2012 17:58, Jrh <jrhelliw...@gmail.com> wrote: > Nomenclature hazard warning:- > > Ian, Thankyou for drawing attention to the nomenclature school:- > Partial occupancy disorder > Which I prefer to refer to as > Partial occupancy order. > > Outside our MX field static disorder refers to what we call split > occupancy order. I like the latter and dislike the former. Ie where there > is disorder surely such a chemical moiety would be invisible, let alone > allowing us to be able to determine its occupancy from Bragg intensities. > > I once tried to propose an amendment to the IUCr Nomenclature Committee to > replace static disorder terminology with split occupancy order terminology. > The forces to which you refer were too strong. Static disorder remains the > term in approved use. > > > Prof John R Helliwell DSc > > > > On 20 Nov 2012, at 15:49, Ian Tickle <ianj...@gmail.com> wrote: > > PS: Partial occupancy is not the same as disorder. You can have >> well-ordered different occupancies that manifest themselves then in >> superstructure patterns. Common in small molecule/materials. >> > > Hello Bernhard > > Agree with everything you said up till this point, but I think the owners > of the "site occupancy disorder" websites below would disagree that partial > occupancy is not the same as disorder! > > http://www.ucl.ac.uk/~uccargr/sod.htm > > > http://www.tcm.phy.cam.ac.uk/castep/documentation/WebHelp/Html/thCastepDisorder.htm > > There are also many research papers on partial occupancy disorder of > superlattice materials in the solid state, eg: > > > http://www.researchgate.net/publication/226559734_Order-disorder_behavior_in_KNbO3_and_KNbO3KTaO3_solid_solutions_and_superlattices_by_molecular-dynamics_simulation > > Cheers > > -- Ian > >> >> > >
