Ionic Compound Of The Type AB2

Solid State of Class 12

Ionic Compound Of The Type AB2

Calcium fluoride (Fluorite) structure

The salient features of fluorite structure are

(i) The Ca+2 ions are arranged in ccp arrangement, i.e. these ions occupy all the corners and the centres of each face of the cube

(ii) The F– ions occupy all the tetrahedral holes.

(iii) Since there are two tetrahedral holes for each Ca+2 ion and F− ions occupy all the tetrahedral holes, there will be two F− ions for each Ca+2 ions, thus the stoichiometry of the compound is 1:2.

(iv) Each Ca+2 ion is surrounded by 8F- ions and each F− ions is surrounded by 4Ca+2 ions. The Coordination number of Ca+2 ion is eight and that of F− ion is four, this is called 8:4 Coordination.

Ionic Compound Of The Type AB2

(v) Each unit cell has 4 calcium ions and 8 fluoride ions as explained below

No. of Ca+2 ions = 8(at corners)×1/8 + 6 (at face centres)×1/2

No. of F ions = 8 (within the body)×1 = 8

Thus the number of CaF2 units per unit cell is 4.

Other examples of structure are SrF2, BaCl2, BaF2, PbF2, CdF2, HgF2, CuF2, SrCl2, etc.

Ionic compound of A2B type

The compound having A2B formula are compounds having anti fluorite structure

Anti fluorite structure is having arrangement of cations and anions opposite to the fluorite structure Li2O has an anti fluorite structure.

(i) In the crystal structure of Li2O, the O-2 ions constitute a cubic close packed lattice (fcc structure) and the Li+ ions occupy all the tetrahedral voids

(ii) Each oxide ion, O-2 ion is in contact with 8 Li+ ions and each Li+ ions having contact with 4 oxide ion. Therefore, Li2O has 4:8 coordination

Examples – Na2O, K2O, K2S, Na2S, Rb2O, Rb2S

Ionic Compound Of The Type AB2

Normal spinel structure

Spinel is a mineral MgAl2O4. In it oxide ions are arranged in ccp with Mg+2 ions occupying tetrahedral voids and Al+3 ions in a set of octahedral voids. Many ferrites (such as ZnFe2O4) also possess spinel structure. These are very important magnetic materials and are used in telephone and memory loops in computers.

Structure of Fe3O4 (Magnetite)

In Fe3O4, Fe+2 and Fe+3 ions are present in the ratio 2:1. It may be considered as having composition FeO.Fe2O3. In Fe3O4 Oxide arranged in ccp. Fe+2 ions occupy octahedral voids while Fe+3 ions are equally distributed between octahedral and tetrahedral voids

MgFe2O4 also has structure similar to magnetite. In this Mg+2 ions are present in place of Fe+2 ion in Fe3O4. Magnetite has inverse spinet structure.


Crystal structure

Brief description

Coordination number

No. of atoms per unit cell



Rock salt (NaCl – type)

Cl- ions in ccp Na+ ions occupy all octahedral voids

Na+ = 6

Cl- = 6


Li, Na, KI, and Rb halides

NH4Cl, NH4Br, NH4I, AgF, AgCl, AgBr, MgO, CaO, TiO, FeO, NiO


Zinc blende (ZnS – types)

S-2 ions in ccp Zn+2 ions occupy alternate tetrahedral voids

Zn+2 = 4

S-2 = 4


ZnS, BeS, CuCl, CuBr, CuI, AgI, HgS


Wurtzite (ZnS – type)

S-2 ions in hcp Zn+2 ion occupy alternate tetrahedral voids

Zn+2 = 4

S-2 = 4


ZnS, ZnO, CdS, BeO


Caesium chloride (CsCl type)

Cl- ions in bcc Cs+ ions in the body of cube

Cs+2 = 8

Cl- = 4


CsCl, CsBr, CsI, CsCN,CaS


Fluorite (CaF2 type)

Ca+2 ions in ccp, F- ions occupy all tetrahedral voids

Ca+2 = 8

F- = 4


CaF2, SrF2, BaF2, BaCl2, SrCl2, CdF2, HgF2


Anti fluorite (Li2O – type)

O-2 ions in ccp, Li+ ions occupy all tetrahedral sites

Li+ = 4

O-2 = 8


K2O, Na2O, K2S, Na2S


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