Lecture 7: Systematic Absences - GOODWIN GROUP // …

Crystallography Supplementary SubjectLecture 7: Systematic Absences1 Lecture 7: Systematic AbsencesThe reason that we have focussed on crystal symmetry for the last four lectures is thatsymmetry is going to help us to simplify the interpretation of crystallographic crux of this Lecture is to understand how the various symmetry elements Bravaislattice, translational symmetry and point symmetry affect diffraction patterns. What weare leading towards is being able to use the symmetry of an observed diffraction pattern inreciprocal space to deduce the symmetry about the crystal in real begin by making two simple but important observations.(i) The structure factors at reciprocal space vectorsQand Qare in fact complex con-jugates; (hkl) =F ( h k l). This result, known as Friedel s law , means thatthe corresponding diffraction intensities are equal:I(hkl) =I( h k l).

Crystallography Supplementary Subject Lecture 7: Systematic Absences 3 The first type of systematic absences we will address are those that arise due to lattice

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