Aluminium sulfide
# Aluminium sulfide

Aluminium sulfide is a chemical compound with the formula $Al_2S_3.$ This colorless species has an interesting structural chemistry, existing in several forms. The material is sensitive to moisture, hydrolyzing to hydrated aluminum oxides/hydroxides. This can begin when the sulfide is exposed to the atmosphere. The hydrolysis reaction generates gaseous hydrogen sulfide $(H_2S).$

## Crystal structure

More than six crystalline forms of aluminium sulfide are known and only some are listed below. Most of them have rather similar, wurtzite-like structures, and differ by the arrangement of lattice vacancies, which form ordered or disordered sublattices.

| Form | Symmetry | Space group | a (A) | c (A) | ρ $(g/cm^3)$ |
| - | - | - | - | - | - |
| α | Hexagonal | $P6_1$ | 6.423 | 17.83 | 2.32 |
| β | Hexagonal | $P6_3mc$ | 3.579 | 5.829 | 2.495 |
| γ | Trigonal | | 6.47 | 17.26 | 2.36 |
| δ | Tetragonal | $I4_1/amd$ | 7.026 | 29.819 | 2.71 |

The β and γ phases are obtained by annealing the most stable $α-Al_2S_3$ phase at several hundred degrees Celsius. Compressing aluminium sulfide to 2–65 bar results in the δ phase where vacancies are arranged in a superlattice of tetragonal symmetry.

Unlike $Al_2O_3,$ in which the Al(III) centers occupy octahedral holes, the more expanded framework of $Al_2S_3$ stabilizes the Al(III) centers into one third of the tetrahedral holes of a hexagonally close-packed arrangement of the sulfide anions. At higher temperature, the Al(III) centers become randomized to give a "defect wurtzite" structure. And at still higher temperatures stabilize the $γ-Al_2S_3$ forms, with a structure akin to $γ-Al_2O_3.$

Molecular derivatives of $Al_2S_3$ are not known. Mixed Al-S-Cl compounds are however known. $Al_2Se_3$ and $Al_2Te_3$ are also known.

## Preparation

Aluminium sulfide is readily prepared by ignition of the elements

    2 Al + 3 S → $Al_2S_3$

This reaction is extremely exothermic and it is not necessary or desirable to heat the whole mass of the sulfur-aluminium mixture; (except possibly for very small amounts of reactants). The product will be created in a fused form; it reaches a temperature greater than 1100 °C and may melt its way through steel. The cooled product is very hard.

