Dear Colleagues, I hope the following detailed review will be of help both with terminology as well as case studies of a wide variety of kinds, including possible approaches for moving forward, regarding the various manifestations of twinning, lattice disorders and multiple crystals. Best wishes, John Prof John R Helliwell
J.R. Helliwell “Macromolecular crystal twinning, lattice disorders and multiple crystals” Crystallography Reviews (2008) 14, 189-250. On Tue, Jun 22, 2010 at 5:35 PM, Ian Tickle <ianj...@gmail.com> wrote: > On Tue, Jun 22, 2010 at 4:32 PM, Colin Nave <colin.n...@diamond.ac.uk> > wrote: > > Secondly, the difference in the cell dimensions (b=123.92 and c=128.89A) > > appears to be quite large and should lead to split spots which (I think) > > corresponds to non merohedral twinning. Did you observe these but > integrated > > them as one? > > The distinction between merohedry (incl. pseudo-merohedry) and > non-merohedral twinning is not whether the spots are split: splitting > to a greater or lesser degree is often observed in the > pseudo-merohedral case since the pseudo-twin law is never perfect. > Rather the defining feature is whether the overlap of the twin-related > lattices occurs in 3 dimensions (i.e. exact overlap for merohedry, or > approximate for pseudo-merohedry) or only 2 in the non-merohedral > case. In the latter case this means that there's no obvious > relationship between the spot positions for the components of the twin > (except possibly in the zone related to the plane of 2-D overlap). > > Cases where 3-D overlap occurs only for some integer fraction of the > spots are often mistakenly termed 'non-merohedral' even though overlap > occurs in 3-D and so there's a clear relationship for the fraction of > spots that are twin-related. The correct term for this case is > 'reticular merohedry' (or 'reticular pseudo-merohedry'). A nice site > where all the twinning terminology is clearly defined is: > > http://www.lcm3b.uhp-nancy.fr/mathcryst/twins.htm > > > (Regarding what to call the twinning I have some sympathies with Humpty > > Dumpty's view "When I use a word... it means just what I choose it to > > mean-neither more nor less" As important a philosopher as Wittgenstein.) > > But it helps a lot if everyone can agree on the terminology (e.g. on > the precise definition of 'non-merohedral') - most of the arguments on > the BB seem to stem from the use of conflicting definitions! > > Cheers > > -- Ian > -- Professor John R Helliwell DSc
