Classification of Ionic Structures

In the following structures, a black circle would denote a cation and a white circle would denote an anion.

In any solid of the type AxBy, the ratio of the coordination number of A to that of B would be y : x.

Rock Salt Structure

Cl is forming a fcc unit cell in which Na+ is in the octahedral voids. The coordination number of Na+ is 6 and therefore that of Cl would also be 6.

Moreover , there are 4 Na+ ions and 4 Cl ions per unit cell. The formula is Na4Cl4 i.e , NaCl.

The other substances having this kind of a structure are halides of all alkali metals except cesium halides and oxides of all alkaline earth metals except beryllium oxide.

Zinc Blende Structure

Sulphide ions are face centered and Zinc is present in alternate tetrahedral voids. Formula is Zn4S4 , i.e, ZnS.

Coordination number of Zn is 4 and that of sulphide is also 4. Other substance that exists in this kind of a structure is BeO.

Fluorite Structures

Calcium ions are face centered and fluoride ions are present in all the tetrahedral voids. There are four calcium ions and eight fluoride ions per unit cell.

Therefore the formula is Ca4F8 , (i.e, CaF2).

The coordination number of fluoride ions is four (tetrahedral voids) and thus the coordination number of calcium ions is eight.

Other substances which exist in this kind of structure are UO2 , and ThO2 .

Anti-Fluorite Structure

Oxide ions are face centered and lithium ions are present in all the tetrahedral voids. There are four oxide ions and eight lithium ions per unit cell.

As it can be seen , this unit cell is just the reverse of Fluorite structure , in the sense that , the positions of cations and anions is interchanged.

Other substances which exist in this kind of a structure are Na2O, K2O and Rb2O.

Cesium Halide Structure

Chloride ions are primitive cubic while the cesium ion occupies the center of the unit cell. There is one chloride ion and one cesium ion per unit cell. Therefore the formula is CsCl.

The coordination number of cesium is eight and that of chloride ions is also eight.

Other substances which exist in this kind of a structure are all halides of cesium.

Corundum Structure

The general formula of compounds crystallizing in corundum structure is Al2O3 . The closest packing is that of anions (oxide) in hexagonal primitive lattice and two-third of the octahedral voids are filled with trivalent cations.

Examples are : Fe2O3 , Al2O3 and Cr2O3.

Pervoskite Structure

The general formula is ABO3 . One of the cation is bivalent and the other is tetravalent.

Example: CaTiO3 , BaTiO3 .

The bivalent ions are present in primitive cubic lattice with oxide ions on the centers of all the six square faces. The tetravalent cation is in the center of the unit cell occupying octahedral void.

Spinel and Inverse Spinel Structure:

Spinel is a mineral (MgAl2O4 ).

Generally they can be represented as M2+M23+O4 , where M2+ is present in one-eighth of tetrahedral voids in a FCC lattice of oxide ions and M3+ ions are present in half of the octahedral voids.

M2+ is usually Mg , Fe , Co , Ni , Zn and Mn; M3+ is generally Al , Fe , Mn , Cr and Rh.

Examples are ZnAl2O4 , Fe3O4 , FeCr2O4 etc.

Many substances of the type also have this structure.

In an inverse spinel the ccp is of oxide ions, M2+ is in one-eight of the tetrahedral voids while M3+would be in one-eight of the tetrahedral voids and one-fourth of the octahedral voids.

Illustration : A solid compound AB has NaCl structure. If the radius of the cation is 95 pm. What is the radius of anion (B ) ?

Solution: Radius into( r+ / r ) for octahedral co-ordination = 0.414

Hence radius of the anion (B) = (95 /0.414) pm = 229.46 pm

Exercise : The unit cell of silver iodide (AgI) has 4 iodine atoms in it. How many silver atoms must be there in the unit cell.

Exercise : The co-ordination number of the barium ions , Ba2+ , in barium chloride (BaF2) is 8. What must be the co-ordination number of the fluoride ions, F .

Also Read :

→ Unit Cells & Crystal
→Bravais Lattices
→Close Packing of Spheres
→ Octahedral and Tetrahedral Voids
→ Radius Ratio Rules
→ Classification of Ionic Structures
→ Imperfections in a Crystal

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