Aluminum sulfide

Aluminum sulfide or aluminium sulphide is a chemical compound with the formula Al₂S₃. This colorless species has an interesting structural chemistry, existing in several forms. The material is sensitive to moisture, hydrolyzing to hydrated aluminium oxides/hydroxides.^[1] This can begin when the sulfide is exposed to the atmosphere. The hydrolysis reaction generates gaseous hydrogen sulfide (H₂S).

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.

The β and γ phases are obtained by annealing the most stable α-Al₂S₃ phase at several hundred degrees Celsius.^[4] Compressing aluminium sulfide to 2–65 kbar results in the δ phase where vacancies are arranged in a superlattice of tetragonal symmetry.

Unlike Al₂O₃, in which the Al(III) centers occupy octahedral holes, the more expanded framework of Al₂S₃ 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-Al₂S₃ forms, with a structure akin to γ-Al₂O₃.

Molecular derivatives of Al₂S₃ are not known. Mixed Al-S-Cl compounds are however known. Al₂Se₃ and Al₂Te₃ are also known.