Ian, yes absolutely but your very description of where the unit cells are not identical is NOT the situation where we see fractional occupancy moieties. In such cases a large fraction of the unit cells ARE ordered. QED. John
Prof John R Helliwell DSc FInstP CPhys FRSC CChem F Soc Biol. Chair School of Chemistry, University of Manchester, Athena Swan Team. http://www.chemistry.manchester.ac.uk/aboutus/athena/index.html On 20 Nov 2012, at 17:33, Ian Tickle <ianj...@gmail.com> wrote: > 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 >> >> >> >
