Structures of Simple Ionic Compounds

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Rock Salt (or NaCl) Type of Structure

fig 2.24

Main features

• It has a face centred arrangement (or CCP). Cl^- ions occupy the corners and face centers, Na⁺ occupy body centre and edge centers.

• Each Na⁺ is surrounded by six Cl^- and each Cl^- is surrounded by 6 Na⁺. Therefore it has 6:6 coordination.

• Number of NaCl Units per unit cell.

Cl

^- ions

Total number of Cl^- ions = 4

Na

⁺ ions

One at the body center contributes fully = 1 x 1 = 1

Total Number of Na⁺ = 3+1=4

Therefore total NaCl units in one unit cell = 4

Other examples of NaCl structure

• Halides of alkali metals (except caesium) and that of ammonium

• Oxides and sulphides of alkaline earth metals (except BeS)

• Halides of silver (except silver iodide).

Caesium Chloride (CsCl) Type of Structure

fig 2.25

fig 2.26 - Structure of CsCl

Main features

• It has body centred cubic (bcc) arrangement, with Cs⁺ at the center and Cl^- at the corners.

• Each Cs⁺ ion is surrounded by 8 Cl^- ions and each Cl^- ions is surrounded by 8 Cs⁺ ions. Therefore the crystal has 8:8 co-ordination.

• It has one CsCl per unit cell because,

Atoms at the centre contribute fully i.e.,1.

Other examples : CsBr, CsI, CsCN, TiCl, TiBr, TiCN etc.

Zinc Blende (ZnS) Structure

fig 2.27 - ZnS structure

Main features

• Arrangement is F.C.C. (or C.C.P) S^2- ions are present at the corners and face centers.

• Zn²⁺ ions are present in alternate tetrahedral voids.

• Each Zn²⁺ ion is surrounded tetrahedrally by S^-2 ions each and S^-2 ion is surrounded tetrahedrally by Zn²⁺ ions. Therefore this structure has 4:4 coordination.

• Number of S^2- ions per unit cell 8 at corners contributed

6 at the face centre contribute

Total number of S^-2 per unit cell = 4.

Since Zn²⁺ ions occupy half the tetrahedral sites, the number of Zn²⁺ ions per unit cell will be 4.

Total number of ZnS per unit cell = 4.

Other examples : CuCl, CuBr, CuI, AgI, BeS.

Fluorite Structure (CaF₂)

fig 2.28 - CaF₂ structure

Main features

• Each Ca²⁺ ion is present at the corners and the face centers (F.C.C. or C.C.P.).

• F^- ions are present in all the tetrahedral voids.

• Each Ca²⁺ is surrounded by 8F^- where as each F^- is surrounded by 4Ca²⁺ so it has 8:4 co-ordination.

• Number of Ca²⁺ per unit cell is 4. Number of F^- is 8 (because all the tetrahedral voids are occupied by F^- ions). Therefore 4CaF₂ units per unit cell.

• Other examples of fluorite structure: BaF₂, BaCl₂, SrF₂, SrCl₂.

Antifluorite Structure (Na₂O)

Main features

• Each O^2- ions constitute F.C.C. or C.C.P. Na⁺ ions occupy all the tetrahedral sites.

• Each O^2- is surrounded by 8 Na⁺ ions.

• Each Na⁺ by four O^2- ions. Na₂O has 4:8 co-ordination. Number of Na₂O per unit cell is 4.

Summary of Main Characteristics of Ionic Solids

Crystal structure	Examples	Co-ordination number	Number of formula units per unit cell
Rock Salt (NaCl) Type	Halides of Li, Na, K, Rb, AgF, AgBr	Na + = 6 Cl - = 6	4
Caesium Chloride type	CsCl, CsBr, CsI, TiCl, TiBr, Til	Cs + = 8 Cl - = 8	1
Zinc Chloride type (ZnS)	CuCl, CuBr, CuI, AgI	Zn+ 2 = 4 S 2 - = 4	4
Fluorite (CaF2) Type	BaF2, BaCl2, SrF2, SrC12, CaF2, PbF2	Ca2 + = 8 F - = 4	4
Anti Fluorite (Na2O) type	Na2O	Na+ = 4 O2 - = 8	4

 

Effect of Temperature and Pressure on the Crystal Structure

At ordinary temperatures and pressures, chlorides, bromides and iodides of lithium, sodium and rubidium and some halides of silver possess the NaCl structure with 6:6 co-ordination.

On application of high pressure they transform to the CsCl structure with 8:8 co-ordination.

On the other hand CsCl on heating transforms to the NaCl structure with 6:6 co-ordination.