adopts three structures, depending on the temperature and the state (solid, liquid, gas). Solid  has a sheet-like layered structure with cubic close-packed chloride ions. In this framework, the Al centres exhibit octahedral coordination geometry. Yttrium(III) chloride adopts the same structure, as do a range of other compounds. When aluminium trichloride is in its melted state, it exists as the dimer , with tetracoordinate aluminium. This change in structure is related to the lower density of the liquid phase (1.78 g/cm) versus solid aluminium trichloride (2.48 g/cm).  dimers are also found in the vapour phase. At higher temperatures, the  dimers dissociate into trigonal planar  monomer, which is structurally analogous to boron trifluoride|. The melt conducts electricity poorly, unlike more ionic halides such as sodium chloride.
Aluminium chloride monomer belongs to the point group D in its monomeric form and D in its dimeric form.