Rock-salt-like lattice structure

Matter is composed of spherical-like atoms. No two atomic cores—the nuclei plus inner shell electrons—can occupy the same volume of space, and it is impossible for spheres to fill all space completely. Consequently, spherical atoms coalesce into a solid with void spaces called interstices. A mathematical construct known as a space lattice may be envisioned, which is comprised of equidistant lattice points representing the geometric centers of structural motifs. The lattice points are equidistant since a lattice possesses translational invariance. A motif may be a single atom, a collection of atoms, an entire molecule, some fraction of a molecule, or an assembly of molecules. The motif is also referred to as the basis or, sometimes, the asymmetric unit, since it has no symmetry of its own. For example, in rock salt a sodium and chloride ion pair constitutes the asymmetric unit. This ion pair is repeated systematically, using point symmetry and translational symmetry operations, to form the space lattice of the crystal. 

Slip plane behavior is less consistent. In MgO, the separate anion and cation closest-packed planes are 111, but the combined anion and cation closest-packed planes are 100 the observed 110 slip plane is explained in terms of the lower repulsion between like ions during slip on this plane compared with 100 [28]. In more complex structures, it becomes difficult to decide which is the closest-packed plane, and the simple-minded approach loses its value. In MgAl204, for example, stoichiometric crystals prefer the dose-packed 111 anion planes (see Figure 9.1), whereas nonstoichiometric crystals (Mg0 nAl203, > 1) slip on 110 planes, as in rock salt. On the other hand, garnets and rare-earth sesquioxides, in spite of the complexity of their structures, follow the dictates of the underlying bcc lattice and slip on the 110 111) system, as in bcc metals. 

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