[uvw], and the four‐index Weber notation, [UVTW]
Some indices of directions specified in both ways are given in Figure 2.14. In relating planes and zone axes using Eq. (1.6), it is usually best to work entirely in the three‐index notation for both planes and directions and to translate the three‐index notation for a direction into the four‐index system at the end of the calculation. It is also useful to note that the condition needed for a four‐index vector [UVTW] to lie in a four‐index plane (hkil) is:
(2.5)
Other point groups besides 6/mmm in the hexagonal system are shown in Figure 2.6. We note that
It is apparent from the stereogram in Figure 2.12b that stereograms with 0001 at the centre showing {hki0} poles are straightforward to plot. To plot more general poles on a stereogram with 0001 at the centre, it is apparent that the c/a ratio has to be used. Thus, for example, this has to be used to determine the angle between faces such as (0001) and (hh
From Figure 2.15, the angle θ between the (0001) pole and the (hh
(2.6)
Figure 2.15 Geometry to determine the angle θ between the (0001) pole and the (
Similarly, the angle θ between (0001) and (h0
(2.7)
An example of a stereogram centred at (0001) with poles of the forms {11
2.6 Trigonal System
This crystal system is defined by the possession of a single triad axis. It is closely related to the hexagonal system. The possession of a single triad axis by a crystal does not, by itself, indicate whether the lattice considered as a set of points is truly hexagonal, or whether it is based on the staggered stacking of triequiangular nets. When the lattice is rhombohedral, a cell of the shape of Figure 1.19k can be used. The cell in Figure 1.19k is a rhombohedron and the angle α (< 120°) is characteristic of the substance. When the lattice of a trigonal crystal is hexagonal, it is not appropriate to use a rhombohedral unit cell.
The symmetry elements in the holosymmetric class
A stereogram of a trigonal crystal indexed according to a rhombohedral unit cell is shown in Figure 2.16. The value of α is 98°. The x‐, y‐ and z‐axes are taken to lie in the mirror planes and the inverse triad is a body diagonal of the cell, therefore lying along the direction [111], which, from the geometry of the rhombohedral unit cell, is also parallel to the normal to the (111) plane. It is clear that the x‐, y‐ and z‐axes – that is, the directions [100], [010] and [001] – do not lie normal to the (100), (010) and (001) planes, respectively. However, these directions are easily located. For example, the z‐axis, [001], is the pole of the zone containing (
