TYPES OF IONIC STRUCTURES

[8-8] COORDINATION; THE CsCl STRUCTURE

Of the alkali halides, only CsCl, CsBr and CsI crystalise with this structure.
The structure does not contain CsCl molcules, but is instead one giant molecule with the unit cell repeating in all three dimensions.
The structure resembles that of a body-centred cubic metal, however the lattice is not body-centred cubic because the body centring atom is different to the atoms at the corners of the unit cell. The Bravais lattice is therefore primative cubic, with two ions (one of each element) associated with each of the lattice points.

1 of 7

All of the alkali halides, except for CsCl, CsBr and CsI, have the same structure as NaCl.
In this structure, each ion of one element sits in the centre of an octahedron of ions of the other element.
The are no NaCl molecules in this structure, the whole crystal is a giant molecule.
The structure appears to be primative cubic, however this would not put the same motif at each of the lattice points. The structure is actually face-centred cubic, with each lattice point having 2 ions associated with it - one of each element.

2 of 7

Zinc blende is one of the two possible structures that Zinc can crystalise in; both of these structures have [4,4] coordination.
Each ion is situated in the centre of a tetrahedron made up of 4 ions of the other element.
The Zincblende structure is face-centred cubic, with 2 ions at each lattice point [the co-ordinates of the two ions relative to the lattice point are (0,0,0) and (1/4,1/4,1/4)].
This structure does not occur for the alkali halides.
It should be noted that the bonding in ZnS has a strong covalent character, and so is not purely ionic.

3 of 7

This is the other form of ZnS.
The co-ordination of this structure is the same as for the Zincblende structure and the surroundings of each ion are the same [a tetrahedron formed by ions of the other element], however, the unit cell is primative hexagonal with two ions associated with each lattice point.

4 of 7

Fluorite = CaF2.
The Calcium ions occupy the cube corners and face centres and the Fluoride ions occupy all of the tetrahedral (4-coordinated) interstices.
This structure tends to occur in halides of the type A(2+)X2(-) where the r+/r- ratio is greater than about 0.7, and also in some oxides and sulphides of the type A(4+)X2(2-).

5 of 7

Rutile = TiO2.
The structure is tetragonal, with the titanium ions occupying the cell corners and body centres. Each titanium ion is surrounded by six oxygen ions and each oxygen ion is surrounded by three titanium ions.
This structure tends to occur in halides of the type A(2+)X2(-), with a r+/r- ratio between about 0.7 and 0.3, and also in some oxides and sulphides of the type A(4+)X2(2-).

6 of 7

The structure is named after the high temeprerature form of SiO2. There are 2 other forms of SiO2. All of the structures are [4-2] coordinated, however they all have different crystal structures. The most stable structure at room temperature is alpha-quartz, which has a trigonal structure.
In this structure, the silicon ions occupy the cube corners, face centres and half of the tetrahedral intersticies [i.e. they occupy all of the sites of both the Zn and S ions in the ZIncblende structure] and the oxygen ions are located half way between pairs of silicon ions.
This structure would occur in halides of the type A(2+)X2(-) with a r+/r- ratio of less than about 0.3 (only BeF2 has a small enough ratio to have this structure) and also in GeO2 and SiO2 which are of the type A(4+)X2(2-).
The structure of SiO2 is slightly different from the ideal structure because the oxygen atoms are slightly displaced from the lines joining the pairs of silicon atoms and so the bonds to each of the oxygen ions are not quite collinear.

7 of 7