Crystallographic planes close-packed, metals

The crystal structure of p-alumina may be described as an alternative stacking structure of spinel blocks and conduction planes . The spinel block is constructed by four layers of oxygen in a cubic close packed array, similar to that of spinel. Metal ions occupy a tetrahedral site, Al(2), and octahedral sites, Al(l) and Al(4). The conduction plane is a crystallographic mirror plane. In a unit cell section of the conduction plane... 

Dislocations do not move with the same degree of ease on all erystallographie planes of atoms and in all crystallographic directions. Typically, there is a preferred plane, and in that plane there are specific directions along which dislocation motion occurs. This plane is called the slip plane it follows that the direction of movement is called the slip direction. This combination of the slip plane and the slip direction is termed the slip system. The slip system depends on the crystal structure of the metal and is such that the atomic distortion that accompanies the motion of a dislocation is a minimum. For a particular crystal structure, the slip plane is the plane that has the densest atomic packing—that is, has the greatest planar density. The slip direction corresponds to the direction in this plane that is most closely packed with atoms—that is, has the highest linear density. Planar and linear atomic densities were discussed in Section 3.11. 

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