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# Types of Ionic Structures

For any ionic solid of the general formula AxBy in which the molar ratio of cation to anion is x : y, the ratio of co-ordination number of cation to anion would be y : x.

# Rock salt structure

NaCl exhibits this type of structure. In the rock salt structure, Clâ€“ ions exist in fcc pattern and Na+ ions occupy all octahedral voids. There are four effective Na+ ions and four effective Clâ€“ ions in a unit cell of NaCl. So, the general formula is Na4Cl4 or NaCI as per the effective ions in a unit cell. The coordination number of Na+ ion is 6 and coordination number of Clâ€“ ion is also 6. So, the general formula (using co-ordination number of ions) is Na6Cl6 or NaCl.

Halides of all alkali metals except cesium and oxides of all alkaline earth metals except beryllium also represent this structure. AgCI, AgBr, and Agl are also crystallizing in this structure. Other examples include TiO, FeO, NiO, etc. Let the radius of Na+ ion and CIâ€“ion be represented by rc and ra, respectively, and a be the edge length of the unit cell, then rc + ra = a/2.

# Zinc blende (sphalerite) structure

This structure is represented by ZnS. In the zinc blende structure, S2â€“ ions forms fcc lattice and Zn2+ ions occupy alternate tetrahedral voids. The effective Zn2+ and S2â€“ ions in a unit cell are four each, so the general formula is Zn4S4 or ZnS. The coordination number of both the ions (Zn2+ and S2â€“) is 4, and hence the general formula (using co-ordination number of ions) is Zn4S4 or ZnS.

The zinc blende structure is also exhibited by BeO. Let the radius of Zn2+ and S2â€“ions be rc and ra, respectively, and the edge length of the unit cell be a, then

# Fluorite structure

This structure is exhibited by CaF2. In the lattice structure of CaF2, Ca2+ ions exist as fcc and Fâ€“ ions occupy all tetrahedral voids.

The fluorite structure is also represented by other halides of alkaline earth metals and by ionic compounds of the general formula AB2 such as UO2, ThO2, PbO2, and HgF2.

Let a be the edge length of the unit cell, and rc and ra be the radius of Ca2+ and Fâ€“ ions, respectively, then

# Antifluorite structure

This structure is shown by Li2O. In the antifluorite structure, O2â€“ ions forms fcc lattice and Li+ ions occupy all tetrahedral voids. The effective number of Li+ and O2â€“ ions in a unit cell is 8 and 4, respectively. So, the general formula is Li8O4 or Li2O. Antifluorite structure in just the reverse of fluorite structure because the positions of cations and anions are interchanged.

This type of structure is also exhibited by oxides of other alkali metals such as Na2O and K2O.

Let a be the edge length of the unit cell, and rc and ra represent the radius of cation and anion, respectively, then

# Cesium halide structure

This type of structure is exhibited by cesium chloride. Chloride ions exist as primitive cubic and cesium ions occupy body centered position (cubic void). Overall this structure is referred as bcc-like structure. The effective number of Cs+ and Clâ€“ ions is one in each case. So, the general formula is CsCl. The coordination number of both the ions (Cs+ and Clâ€“) is 8, so the general formula using co-ordination number of ions is Cs8Cl8 or CsCl.

This kind of structure is also exhibited by other halides of cesium and by the halides of ammonium such as NH4Cl and NH4Br.

Let the radius of Cs+ and Clâ€“ be rc and ra, respectively, and the edge length of the unit cell be a, then