# Types of Ionic Structures

For any ionic solid of the general formula

*A*in which the molar ratio of cation to anion is_{x}B_{y}*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, Cl

^{â€“ }ions exist in fcc pattern and Na^{+}ions occupy all octahedral voids. There are four effective Na^{+}ions and four effective Cl^{â€“}ions in a unit cell of NaCl. So, the general formula is Na_{4}Cl_{4}or NaCI as per the effective ions in a unit cell. The coordination number of Na^{+}ion is 6 and coordination number of Cl^{â€“}ion is also 6. So, the general formula (using co-ordination number of ions) is Na_{6}Cl_{6}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 CI^{â€“}ion be represented by*r*and_{c}*r*, respectively, and_{a}*a*be the edge length of the unit cell, then r_{c}+ r_{a}= a/2.# Zinc blende (sphalerite) structure

This structure is represented by ZnS. In the zinc blende structure, S

^{2â€“}ions forms fcc lattice and Zn^{2+}ions occupy alternate tetrahedral voids. The effective Zn^{2+}and S^{2â€“}ions in a unit cell are four each, so the general formula is Zn_{4}S_{4}or ZnS. The coordination number of both the ions (Zn^{2+}and S^{2â€“}) is 4, and hence the general formula (using co-ordination number of ions) is Zn_{4}S_{4}or ZnS.The zinc blende structure is also exhibited by BeO. Let the radius of Zn

^{2+}and S^{2â€“}ions be*r*and_{c}*r*, respectively, and the edge length of the unit cell be_{a}*a*, then# Fluorite structure

This structure is exhibited by CaF

_{2}. In the lattice structure of CaF_{2}, Ca^{2+}ions exist as fcc and F^{â€“}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 AB

_{2}such as UO_{2}, ThO_{2}, PbO_{2}, and HgF_{2}.Let

*a*be the edge length of the unit cell, and*r*and_{c}*r*be the radius of Ca_{a}^{2+}and F^{â€“}ions, respectively, then# Antifluorite structure

This structure is shown by Li

_{2}O. In the antifluorite structure, O^{2â€“}ions forms fcc lattice and Li^{+}ions occupy all tetrahedral voids. The effective number of Li^{+}and O^{2â€“}ions in a unit cell is 8 and 4, respectively. So, the general formula is Li_{8}O_{4}or Li_{2}O. 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 Na

_{2}O and K_{2}O.Let

*a*be the edge length of the unit cell, and*r*and_{c}*r*represent the radius of cation and anion, respectively, then_{a}# 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 Cl^{â€“}ions is one in each case. So, the general formula is CsCl. The coordination number of both the ions (Cs^{+}and Cl^{â€“}) is 8, so the general formula using co-ordination number of ions is Cs_{8}Cl_{8}or CsCl.This kind of structure is also exhibited by other halides of cesium and by the halides of ammonium such as NH

_{4}Cl and NH_{4}Br.Let the radius of Cs

^{+}and Cl^{â€“}be*r*and_{c}*r*, respectively, and the edge length of the unit cell be_{a}*a*, then