What type of particle is present in a sample of aluminum chloride (AlCl3)?

In the sample, aluminum chloride exists as dimer molecules Al₂Cl₆ as shown below.

Aluminum chloride is a compound of a metal and a non-metal, but it is NOT ionic in nature. If the compound were ionic, the Al³⁺ ion would be very small in size with 3 positive charges and thus its high charge density would polarize the Cl⁻ ions, i.e. draw back electrons from Cl⁻ ion to make the Al-Cl bonds covalent.

AlCl₃ molecules are very unstable because the central Al atom has only 6 electrons in its outermost shell. Two AlCl₃ molecules would form a dimer in which the dative Cl→Al bonds would stabilize the two Al atoms by completing their octets.

Aluminum chloride ......

Aluminum chloride has bonds with about 45% ionic character, but it isn't the typical so-called "ionic" substance. It does not ionize when it melts, and therefore, molten aluminum chloride is a poor conductor of electricity.

As a solid, AlCl3, exists in planar sheets of alternating Al and Cl atoms, a 2-dimensional polymer. Envision the layers stacked one on top of the other to give a 3-dimensional structure. In the liquid state the regularity is disrupted and aluminum chloride exists as molecular dimers, Al2Cl6. Chlorine atoms bridge the two AlCl3 units. In the vapor phase at lower temperatures the Al2Cl6 dimer persists, but as the temperature increases it reverts to AlCl3.

The Al2Cl6 dimer found in the liquid and gaseous states.