Crystallographic consequences of pseudosymmetry in crystal structures

The term pseudo-symmetry means a spatial arrangement that feigns a symmetry without fulfilling it. In crystal structures pseudo-symmetry is a more common feature than often recognized. In case of small deviations a variety of phenomena results: polytypism and stacking faults, disorder and twinning, commensurate and incommensurate super-structures, phase transitions and unusual reflection conditions. Most of these effects complicate crystal structure determination considerably. A series of examples with focus on the new class of ternary bismuth subhalides illustrates the different crystallographic consequences of pseudo-symmetry in crystal structures.