The structure of aluminium chloride is often represented as Al2Cl6 under all conditions. It isnt.
At room temperature the solid (r = 2.44 g cm-3) has an ionic layer lattice1, the aluminium being 6-coordinate. It consists of hexagonal close-packed layers of chloride ions in which two-thirds of the octahedral holes between every other pair of Cl planes are occupied by aluminium ions. The pictures at right show the lattice, and two layers of the lattice separated to show the positions of the aluminium ions.
The aluminium chloride lattice
|There are dramatic changes at the melting temperature of 192.4oC (at
1700mmHg). Aluminium becomes 4-coordinate as the covalent molecular dimer Al2Cl6
is formed. The volume of the solid increases dramatically by about 85% (r = 1.31 g cm-3) and the electrical conductivity, which
increases from zero at the ordinary temperature to become quite high just below the
melting temperature, becomes virtually zero. There is thus a considerable change of
bonding and structure at this temperature.
The dimer is also the main species in the gas phase below 200oC, this dissociating increasingly (DHdissoc = + 63 kJ mol-1) into trigonal planar AlCl3 molecules as the temperature is raised 2, 3.
Aluminium bromide and iodide do not share these properties, existing as dimers in the solid phase and not showing such dramatic changes on melting.
The aluminium chloride lattice expanded
1 Laschkarew W. H., Z. Anorg. Chem. 193: 270 (1930).
2 Sidgwick N. V., The Chemical Elements and their Compounds, Oxford University Press, 1950: 424-5.
3 Greenwood N. N. & Earnshaw A., The Chemistry of the Elements, Pergamon Press 1984: 263-4.
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