Lattice structures for ionic compounds

We saw in the section 2.3.2 Lattice structures that metal atoms are arranged in different crystal lattice structures. The same is the case for many solid ionic compounds. The anions are often much larger than the cations so these anions often constitute the lattice structure and the cations are then located in the holes in the lattice structure. As for the metal atoms we assume that the ions are hard spheres that can be packed together so that they just touch each other. The ratio between the radius of the cations and the anion determines which lattice structure the ionic compound adopts. This ratio is called the r+/r. ratio. In Table 2- 4 you can see which structure the ionic compounds adopt at the different r+/r. ratios. Of course there are some 

The structure of an ionic compound depends on the r+/r. ration. The coordination number tells how many anions each cation touches. 

When the r+/r. ration is between 0.732 and 0.999 the structure is cubic. In this case the cations and anions are not that different is size and the structure corresponds to the simple cubic structure (sc) that we heard about for metals (section 2.3.2 Lattice structures). Anions and cations will be placed in a simple cubic structure so that each cation will be surrounded by eight anions and vice versa. A unit cell for such a structure is show in 

Unit cell for a cubic structure for an ionic compound with a rjr. ration in the interval 0.732 - 0.999. Each cation touches eight anions and each anion touches eight cations. Cesium chloride (CsCl) is an example of an ionic compound with cubic structure. 

Octahedral structure for an ionic compound with a r+/r. ration in the interval 0.414 - 0.732. The cations are placed in the octahedral holes in the anion lattice. Thus each cation touches six anions. This structure is often called sodium chloride structure since the very well known common salt has this structure. 

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