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Figure 1
The crystallographic orbit generated by the position 0, ¼, ¼ in a space group of type Cmcm, seen in projection along the [001] axis. This crystallographic orbit is one of the infinitely many that belong to the Wyckoff position 4c (0, y, ¼); the particular orbit shown has y = ¼. (a) Projection of the symmetry elements and of the atomic positions, in grey. One unit cell is shown. (b) Projection of four unit cells, from which the symmetry elements have been removed. The atoms occur alternately with z = ¼ and z = ¾, as shown in the figure. The eigensymmetry of this crystallographic orbit is Fmmm and its unit cell is shown in red, with the origin moved onto one atom for ease of visualization. The basis vectors are parallel to those of the space group and have the same length, but the unit cell is F centred.

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