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Twinning: Difference between revisions
→Warning Signs for Twinning
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== Refinement == | == Refinement == | ||
== Warning Signs for Twinning == | == '''Warning Signs for Twinning''' == | ||
Experience shows that there are a number of characteristic warning signs of twinning, as given in the following list. Of course not all of them can be present in any particular example, but if one or several apply, the possibility of twinning should be given serious consideration. | |||
a) The metric symmetry is higher than the Laue symmetry. | |||
b) The Rint-value for the higher symmetry Laue group is only slightly higher than for the lower symmetry Laue group. | |||
c) If different crystals of the same compound show significantly different Rint values for the higher symmetry Laue group, this clearly shows that the lower symmetry Laue group is correct and indicates different extents of twinning. | |||
d) The mean value for |E^2-1| is much lower than the expected value of 0.736 for the non-centrosymmetric case. If we have two twin domains and every reflection has contributions from both, it is unlikely that both contributions will have very high or that both will have very low intensities, so the combined intensities are distributed to give fewer extreme values. | |||
e) The space group appears to be trigonal or hexagonal. | |||
f) The apparent systematic absences are not consistent with any known space group. | |||
g) Although the data appear to be in order, the structure cannot be solved. This may of course also happen if the cell is wrong, for example with an halved axis | |||
h) The Patterson function is physically impossible. | |||
The following features are typical of non-merohedral twins, where the reciprocal lattices do not overlap exactly and only some of the reflections are affected by the twinning: | |||
i) There appear to be one or more unusually long axes. | |||
j) There are problems with the unit cell refinement. | |||
k) Some reflections are sharp, others split. | |||
l) K = mean(Fo^2)/mean(Fc^2) is systematically high for reflections with low intensity. This may also indicate a wrong choice of space group in the absence of twinning. | |||
== References == | == References == | ||
Giacovazzo, C. ed. (2002). Fundamentals in Crystallography, I.U.Cr. & O.U.P.: Oxford, UK. | Giacovazzo, C. ed. (2002). Fundamentals in Crystallography, I.U.Cr. & O.U.P.: Oxford, UK. | ||