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 CaF₂units per unit cell is 4.

Other examples of structure are SrF₂, BaCl₂, BaF₂, PbF₂, CdF₂, HgF₂, CuF₂, SrCl_2, 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.

SUMMARY OF VARIOUS STRUCTURES OF IONIC SOLIDS

Crystal structure		Brief description	Coordination number	No. of atoms per unit cell	Examples
1.	Rock salt (NaCl – type)	Cl- ions in ccp Na+ ions occupy all octahedral voids	Na+ = 6 Cl- = 6	4	Li, Na, KI, and Rb halides NH4Cl, NH₄Br, NH₄I, AgF, AgCl, AgBr, MgO, CaO, TiO, FeO, NiO
2.	Zinc blende (ZnS – types)	S-2 ions in ccp Zn⁺² ions occupy alternate tetrahedral voids	Zn⁺² = 4 S-2 = 4	4	ZnS, BeS, CuCl, CuBr, CuI, AgI, HgS
3.	Wurtzite (ZnS – type)	S-2 ions in hcp Zn⁺² ion occupy alternate tetrahedral voids	Zn⁺² = 4 S-2 = 4	4	ZnS, ZnO, CdS, BeO
4.	Caesium chloride (CsCl type)	Cl- ions in bcc Cs+ ions in the body of cube	Cs⁺² = 8 Cl- = 4	1	CsCl, CsBr, CsI, CsCN,CaS
5.	Fluorite (CaF₂ type)	Ca+2 ions in ccp, F- ions occupy all tetrahedral voids	Ca⁺² = 8 F- = 4	4	CaF₂, SrF₂, BaF₂, BaCl₂, SrCl₂, CdF₂, HgF₂
6.	Anti fluorite (Li₂O – type)	O-2 ions in ccp, Li+ ions occupy all tetrahedral sites	Li+ = 4 O-2 = 8	4	K₂O, Na₂O, K₂S, Na₂S