Caesium chloride 3: the unit cell of caesium chloride

Other compounds that adopt this structure: TlCl, TlBr, TlI, TlSb, LiHg, LiTl and FeAl.

The unit cell of caesium chloride is shown to the left. The black wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond. The chloride ions are shown in green and the caesium ions in gold. The chloride ions adopt a simple cubic arrangement (not close packing), ie the ions occupy just the eight corners of the cubic unit cell. The caesium ions occupy the body centre of the cube. So altogether there are 9 ions involved with the unit cell, but the cell occupancy is less than this since many of the ions are only partially inside the cell.

Cell occupancy:

The cell occupancy of chloride ions = (8 x 1/8) = 1.

The cell occupancy of caesium ions = 1.

The equal cell occupancy reflects the 1:1 stoichiometry of caesium and chlorine in the formula of caesium chloride.

Coordination numbers:

The coordination number of caesium is 8, ie it has eight nearest neighbours. The coordination number of chloride is also 8. This is less easy to observe in the unit cell but can be appreciated if you consider that each corner-occupying chloride ion is shared by eight neighbouring unit cells, and in each of these unit cells it is in contact with a central caesium ion. Note that the nearest neighbours of an ion are the counter ions of opposite change. This is how the structure maximises the attractive ionic forces between ions of opposite charge.

The following pages take a more advanced approach.

Go to page 4 to consider the equivalence of the caesium and chloride positions in the lattice.

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