Definition of a Cubic Crystal Class A cube may he defined as being a regular six faced polyhedron. I t could also be defined by its three four-fold axes a t 90" to one another. However in the crystallographic sense it is defined as a class with four three-fold axes a t 54" 44'. Examination of single crystal X-ray diffraction photographs for some cubic elements and binary compounds, such as copper and sodium chloride, re@ectiveiy, do indicate the property of having three four-fold axes a t 90". However in more complicated compounds t h a t crystallize in the cubic system, this additional symmetry is destroyed.
Figure 1. Oscillat~on photograph Figure 2. Zero lwei Weissenberg . Figure 3. Osc~ilat~onphotograph about the principal (100) axis (A photograph of the principal (100) about the d~agonali l l ? ) axis. (No mirror plane is indicated but no four- axis. There is no four-fold symmetry. mirror plane.) fold symmetry.1
Figure 4. Zero e v e Walssenberg photograph of the princtpal ( 1 1 1 ) a x s . Pattern is repeated every 60indicating a three-fold axis.
A good example was found by examining oscillation and Weissenberg photographs (Figs. 1-4) of 6-hexachlorocyclohexane or 0-benzenehexachloride (1) (Pa3.a= 10.07/A, Z = 4). No four-fold symmetry was found when the crystal was mounted about the principal (100) axis, but when the crystal was remounted about the diaponal ( I l l ) axis, the three-fold symmetry was clearly indicated from the zero level Weissenberg photograph. Hendricks, S. B., and Bilicke, C., J. Arner. Chem Soc., 48,3007 (1926). University of Queensland Brisbane. Qld. 4067. Australia
G . Smith C. H. L. Kennard
Volurne51, Number 72,December 1974
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