Rock Salt (or NaCl) Type of Structure
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
Total Number of Na+ = 3+1=4
Therefore total NaCl units in one unit cell = 4Other 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
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
Main features
- Arrangement is F.C.C. (or C.C.P) S2- 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 S2- ions per unit cell 8 at corners contributed
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)
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 O2- ions constitute F.C.C. or C.C.P. Na+ ions occupy all the tetrahedral sites.
- Each O2- is surrounded by 8 Na+ ions.
- Each Na+ by four O2- 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.
