John, can you give me a good reference on this? -- Ian
On 21 November 2012 07:32, Jrh <jrhelliw...@gmail.com> wrote: > 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 >> >>> >>> >> >> >
