Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 89

The principal axis in the space group I4₁, Fig. 1.66, is a 4₁ screw axis parallel to z. There are four such axes which intersect the unit cell at x = , y = ; x = , y = ; x = , y = and x = , y = . The operation of a 4₁ screw axis involves translation of combined with rotation by 90° about the axis. The positions 1 to 4 are related to each other by the 4₁ axis at s, and it can be seen that these positions lie on a spiral about s. The symbols of screw axes s and t are reversed because their direction of rotation is different; s involves clockwise rotation and t an anticlockwise rotation (e.g. the sequence of positions 7′, 2, 5, 4′). The body‐centring, I, relates positions 1–4 to 5–8 and the cell contains eight general equivalent positions. Several 2‐fold axes parallel to z are also generated.

Figure 1.66 Tetragonal space group I41 (No 80); coordinates of equivalent positions 8(b):

Special positions with point symmetry 2, 4(a): 0, 0, z; 0, ½, + z; ½, ½, ½ + z; ½, 0, + z.