Structures of Simple Ionic Compounds

Structures of Simple Ionic Compounds:

Compounds of the type AB or AB₂ represent simple ionic compounds where A and B represent the positively and negatively charged ions, respectively.

Structure of Ionic Compounds of the type AB:

These compounds have any one of the following three types of structures-

• Sodium Chloride or Rock Salt (NaCl) Type Structure.
• Caesium Chloride (CsCl) Type Structure.
• Zinc Blende (ZnS) Type Structure.

Sodium Chloride or Rock Salt (NaCl) Structure:

This structure is formed of Na⁺ and Cl^– ions. The radius ratio in NaCl-

This value of radius ratio corresponds to coordination number six with an octahedral shape which is further confirmed by X-ray studies. The main features of this structure are-

• Cl^– ions have cubic close-packed (ccp) arrangement i.e. face-centred cubic (fcc) packing with Na⁺ ions in all the octahedral voids.
• Each Na⁺ ion in NaCl crystal is surrounded by six Cl^– ions and similarly each Cl^– ion is surrounded by six Na⁺ ions lying at the six corners of a regular octahedron. This indicates that the NaCl structure has 6 : 6 co-ordination.
• The number of NaCl units per unit cell is four which can be calculated as follows-

Thus, a unit cell of NaCl consists of 4 Na⁺ ions and 4 Cl^– ions. Examples of compounds having NaCl structure are-

• Halides of alkali metals (except Cs) and that of Ammonium.
• Oxides and sulfides of Alkaline Earth Metals (example- BeS).
• Halides of Silver (except AgI).

Caesium Chloride (CsCl) Structure:

Caesium chloride is formed of Cs⁺ and Cl^– ions. The radius ratio in CsCl-

This value of radius ratio corresponds to coordination number eight with body centred cubic structure which is confirmed by X-ray studies. The main features of this structure are-

• It has body centred cubic (bcc) arrangement i.e. Cl^– ions are at the corners of the cube whereas Cs⁺ ion at the centre of the cube or vice versa.
• Each Cs⁺ ion is surrounded by 8 Cl^– ions and each Cl^– ion is surrounded by 8 Cs⁺ ions i.e. this structure has 8 : 8 co-ordination.
• The packing of Cl^– ions slightly opens up to accommodate Cs⁺ ion because of their larger radius ratio (0.933) than 0.732.
• A unit cell of CsCl consists of only one unit of CsCl i.e. one Cs⁺ ion and one Cl^– ion as calculated below-

Ionic compounds have CsCl structure if their cations and anions are comparable in size. Example- CsBr, CsI, CsCN, TlCl, TlBr, TlI and TlCN.

Zinc Blende (ZnS) Structure:

ZnS is formed of Zn²⁺ and S^2- ions in which radius ratio is-

This value of radius ratio corresponds to coordination number four with tetrahedral structure. The main features of this structure are-

• The arrangement possessed by ZnS is cubic close packing (ccp) in which S^-2 ions form a fcc arrangement and Zn⁺² ions occupy the tetrahedral voids.
• There are two tetrahedral sites for each S^-2 ion and so there will be eight tetrahedral sites for four S^-2 ions out of which only four are occupied by four Zn⁺² ions while the remaining four are vacant. Thus, each Zn⁺² ion is tetrahedrally surrounded by four S^-2 ions and each S^-2 ion is tetrahedrally surrounded by four Zn⁺² ions. This structure has 4 : 4 co-ordination.
• A unit cell of ZnS consists of four units of ZnS i.e. four Zn⁺² ions and four S^-2 ions as calculated below-

Some ionic solids having zinc blende structure are CuCl, CuBr, CuI, AgI.

Structure of Ionic Compounds of the type AB₂:

These are the ionic compounds having cations and anions in the ratio 1 : 2. Most of these compounds have a fluorite (Calcium Fluoride, CaF₂) structure. In CaF₂, the radius ratio, rCa⁺²/rF^– = 0.737 suggest the co-ordination number of Ca⁺² ion to be 8 i.e. each Ca⁺² ion in fluorite structure is surrounded by 8 F- ions. The Ca⁺² ions are assumed to be in a cubic close-packed arrangement occupying all the corners and central position of each face of the cube and the F^– ions in all the tetrahedral voids in the lattice of Ca²⁺ ions. In this type of structure, each Ca²⁺ ion is in contact with eight F^– ions and each F^– ion is in contact with four Ca²⁺ ions. Thus, this structure has 8 : 4 co-ordination. The number of CaF₂ units per unit cell is four i.e. CaF₂ unit consist of four Ca²⁺ ions and 8 F^– ions as calculated below-

Effect of Temperature and Pressure on Crystal Structure:

• Under ordinary temperature and pressure, chlorides, bromides and iodides of Li⁺, Na⁺, K⁺, Rb⁺ and Ag⁺ have a sodium chloride (NaCl) type structure, (6 : 6 coordination).
• Chloride, bromides and iodides of caesium have a caesium chloride (CsCl) structure (8 : 8 coordination) under ordinary temperature and pressure.
• Under high pressure, compounds having NaCl structure transform into the CsCl structure.

• When CsCl is sublimed on an amorphous surface, it changes to NaCl type structure.

Thus, an increase in pressure increases the coordination number and an increase in temperature lowers the coordination number.