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Structures of Ionic Solids

It has earlier been discussed that ionic solids consists consist of positive and negative ions  arranged in a manner so as to acquire minimum potential energy. This can be achieved by decreasing the cation-anions distance to a minimum and reducing anion-anion repulsions. The structures which these solids adopt can be described n terms of the large anions/cations forming a close-packed arrangement and the small cations/anions occupying one or the other type of interracial sites. A large number of ionic solids exhibit one of these five structures which are discussed here:
(a)    Sodium chloride (NaCI)
(b)    Zine blende (ZnS)
(c)    Wurtzite (ZnS)
(d)    Fluroite (CaF2)
(e)    Cesium chloride (CsCI).

Before taking up the initial structural arrangements, it mat be motioned that we do not draw the entire crystal lattice but draw a unit cell only. The smallest distinguishable repeating astern (motir) of ions is called the unit cell. If this pattern, called unit cell, is repeated in three dimensions, a crystalline lattice is generated. Thus, a unit cell is the smallest network of an entire crystal lattice. The unit cell must fulfill the following conditions:
(i)    It should possess the same symmetry as the crystal structure.

(ii)    If there is a choice between more than one repeating arrangements, the one with the smallest number of atoms is chosen as the unit cell. Such a unit cell is often labeled as the primitive or simple unit cell representation.

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