Research & Publications
X-ray intensity corrections for crystals containing lattice-translocation defects
Lattice translocation defects are caused by the random translocation of some layers by a fixed constant within the stacked layers of a crystal. When such events result in the fragmentation of the crystal into smaller mosaic blocks, the observed intensities are simply additive from each block and independent of the translocation vector. When such events occur within one single coherent mosaic block, interference in X-ray diffraction is observed with the intensities modulated by a factor that is a function of this translocation vector and the fractions of the translocated and un-translocated layers (Wang et al., 2005). This phenomenon was first observed nearly 50 years ago (Bragg and Howells 1954; Cochran and Howells 1954; Howells and Perutz 1954). The atomic structures of crystals containing the lattice-translocation defects were considered to be unsolvable (Glauser and Rossmann 1966; Pickersgill 1987). Recently, an equation for the modulation factor caused by these defects was formulated and the observed intensities become correctable using this factor (Wang et al., 2005).
Because of lattice translocation defects, two identical but translated lattices can co-exist as a single coherent mosaic block in a crystal. The observed structure in such cases is a weighted sum of two identical but translated structures, one from each lattice; the observed structure factors are a weighted vector sum of the structure factors with identical unit amplitudes but shifted phases.