Electron microscopy
 
Asymmetric Units in Unit Cells
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An asymmetric unit in unit cells is the smallest portion of a crystal structure, which can be duplicated and moved by crystallographic symmetry operations so that the entire unit cell may be created in order to produce the entire crystal. The most common crystal symmetry operations for creating the unit cell include rotations, translations and screw axes.

For instance, the asymmetric unit (red) in Figure 1462 (a) is rotated 180 degrees about a two-fold crystallographic symmetry axis (blue oval) and moved to produce a second copy of the asymmetric unit in Figure 1462 (b). The pair of the opposite asymmetric units forms the unit cell which is translationally repeated in three-dimensional (3-D) to produce the entire crystal in Figure 1462 (c). In practice, the asymmetric unit is also used to refine the coordinates of the structure against the experimental data.

Schematic illustration of formation of an entire crystal with asymmetric units

Figure 1462. Schematic illustration of formation of an entire crystal with asymmetric units: (a) Asymmetric unit, (b) Unit cell, and (c) Entire crystal structure.

 



 

 

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