When the symmetry operations, which constitute the space group of a structure, do not include an inversion or a reflection operation, then the structure can exist in two forms, a right-handed and a left-handed one, called enantiomorphs. The presence of the two enantiomorphs coexisting within a sample can be verified in the electron microscope by imaging in dark field in a multi-beam orientation, with the electron beam parallel with a zone axis, along which the crystal does not show a center of symmetry in projection (1). One takes advantage here of violations in Friedel's Law (2), which may cause a difference in background intensity between the two structures. An example is shown in Fig. 1, where an inversion boundary runs through a thick wedge-shaped crystal of lithium ferrite. Ordered lithium ferrite may occur in two enantiomorphic forms P43 32 (right-handed screw axis), and P41 32 (left-handed screw axis). In this paper it will be shown that it is possible to determine uniquely the configuration of the structure using the results of the dynamical theory.
Symmetry elements and symmetry operations