On Thu, Sep 3, 2009 at 1:33 AM, Eleanor Dodson <[email protected]> wrote:
> Lattice translation is effectively one form of twinning, you can visualise > it as a set of crystals where that lattice are aligned in 2 dimensions but > there is slippage along the third. So each "reflection" is in fact the sum > of two or more intensities and the twinning analyses should be valid. But as > well you have the problem that some classes of reflections are very weak, in > the same way as a pseudo translation affects the data. > And the twinning tests via moments, H test and Britten test are all > distorted by the weak/strong pattern so really the only effective test is > the L test, and that too can be badly distorted by anisotropy and other > defects. > > Apparently it is often possible to recognise a lattice defect by looking at > the images, if you are good at that. Some classes of reflections will be > very streaky ( where there is an overlap between the different crystal > fragments) and others sharp. But once the data is integrated that > information is lost. > This is something I've always been confused by. Could these streaky reflections be caused by something else? Is it possible for lattice translocation to be confused with translational pseudosymmetry? I have seen these streaky reflections (in alternating lattice lines) repeatedly for one crystal form, and although I was able to refine one structure by treating it as pseudotranslation, I'm worried I may have missed something important. (I do not see inexplicable extra density, if that matters.)
