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Figure 2
The crystallographic orbit generated by the position 0, ½, 0 in the same space group of type Cmcm used in Fig. 1[link], seen in projection along the [001] axis. This crystallographic orbit is the only one that belongs to the Wyckoff position 4b. Same conventions as in Fig. 1[link]. All the atoms occur for z = 0 and z = ½; these two coordinates are not shown in the figure because they are the same for all atoms. In (a), the projected atomic positions (in red) fall on the same poisitons as the symbols of 21 screw axes (black). (b) The eigensymmetry of this crystallographic orbit is Cmmm 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, but the c basis vector has half length owing to the presence of atoms every half period in the unit cell of Cmcm.

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