Aluminium sesqui-sulphide, Al₂S₃

Aluminium sesqui-sulphide, Al₂S₃, was first obtained by Fremy by heating alumina to redness in a current of carbon disulphide vapour. It may be obtained by the action of hydrogen sulphide upon alumina at a red heat or by the action of sulphur on a heated mixture of alumina and carbon.

Aluminium sulphide is readily prepared by heating alumina with galena in a muffle furnace, or by the direct union of its elements. The powdered elements are mixed in a fireclay crucible, heated till the sulphur fuses, and cooled. The reaction is then brought about as in a thermit reaction by means of a firing powder ignited by a piece of magnesium ribbon. The sulphide is purified by sublimation in vacuo at 1100° to 1250°, or in nitrogen at 1500° to 1600°.

According to Biltz and Caspari, aluminium sulphide crystallises in white, asbestos-like needles (which are possibly hexagonal) and forms mixed crystals with alumina. It melts at 1100° ± 10° and its density at 13° is 2.02. Heated to 2100° in an inert atmosphere it loses sulphur and forms aluminium subsulphide, AlS; heated in air or oxygen it is converted into alumina and sulphur dioxide.

Aluminium sesqui-sulphide is rapidly and completely decomposed by water with the formation of aluminium hydroxide and hydrogen sulphide, a behaviour in accordance with the fact that it is the salt of a very weak, practically insoluble base and a very weak acid. The decomposition of the monosulphide by hydrochloric acid is represented by the equation: -

2AlS + 6HCl = 2AlCl₃ + 2H₂S + H₂.

By heating a mixture of alumina and a carbonate to redness in a current of carbon disulphide vapour, the following double sulphides have been obtained: potassium-aluminium sulphide, 3K₂S.Al₂S₃; sodium-aluminium sulphide, 3Na₂S.Al₂S₃; and barium-, strontium-, and calcium-aluminium sulphides. Aluminium-silver sulphide, 4Ag₂S.5Al₂S₃, melts at 1035°.