This is an archive copy of the IUCr web site dating from 2008. For current content please visit https://www.iucr.org.

II. Polytype symbolism

The Ad-Hoc Committee has examined the systems of structural symbols for polytypic structures already in the literature, which may be divided into two categories:

the first, of indicative symbols , includes systems that can be used without knowledge of the crystal structure; these necessarily make use of observable characteristics such as symmetry, periodicity of layers, interchange of axes, etc.;

the second, of descriptive symbols , includes systems that specify the exact stacking sequence of layers. Such systems allow the reader to deduce the unit cell and space group, and to recognize identical structures described by different axes or unit cells (settings); they facilitate comparison of structural data referring to different polytypes and allow identification by means of calculated intensities.

Most notational systems in the literature have been designed specifically for certain types of structure and, in the opinion of the Ad-Hoc Committee, cannot be applied universally. Several systems that showed promise were examined in more detail and have been further modified.

The Ad-Hoc Committee approves the system recommended in 1977 for the first category, see §II.1, and recommends the system described in §II.2 for the second category.

Copyright © 1984, 1998 International Union of Crystallography