Aluminium sulfide

Aluminum sulfide 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 aluminum oxides/hydroxides.^[1] This can begin when the sulfide is exposed to the atmosphere. The hydrolysis reaction generates gaseous hydrogen sulfide (H₂S).

Aluminum sulfide

Names
Other names Aluminium sulfide
Identifiers
CAS Number	1302-81-4 Y[ECHA]
3D model (JSmol)	Interactive image
ChemSpider	140154 Y
ECHA InfoCard	100.013.736
EC Number	215-109-0
PubChem CID	16684788
UNII	04PI6P2Z18 Y
CompTox Dashboard (EPA)	DTXSID80893235
InChI InChI=1S/2Al.3S/q2*+3;3*-2 Y Key: COOGPNLGKIHLSK-UHFFFAOYSA-N Y InChI=1/2Al.3S/q2*+3;3*-2 Key: COOGPNLGKIHLSK-UHFFFAOYAY
SMILES [Al+3].[Al+3].[S-2].[S-2].[S-2]
Properties
Chemical formula	Al2S3
Molar mass	150.158 g/mol
Appearance	gray solid
Density	2.02 g/cm3
Melting point	1,100 °C (2,010 °F; 1,370 K)
Boiling point	1,500 °C (2,730 °F; 1,770 K) sublimes
Solubility in water	decomposes
Solubility	insoluble in acetone
Structure
Crystal structure	trigonal
Thermochemistry
Heat capacity (C)	105.1 J/mol K
Std molar entropy (S⦵298)	116.9 J/mol K
Std enthalpy of formation (ΔfH⦵298)	−724 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
NFPA 704 (fire diamond)	4 0 2 W
Safety data sheet (SDS)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Crystal structure

More than six crystalline forms of aluminum 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.^[2][3]

Form	Symmetry	Space group	a (A)	c (A)	ρ (g/cm3)
α	Hexagonal	P61	6.423	17.83	2.32
β	Hexagonal	P63mc	3.579	5.829	2.495
γ	Trigonal		6.47	17.26	2.36
δ	Tetragonal	I41/amd	7.026	29.819	2.71

The β and γ phases are obtained by annealing the most stable α-Al₂S₃ phase at several hundred degrees Celsius.^[4] Compressing aluminum sulfide to 2–65 bar results in the δ phase where vacancies are arranged in a superlattice of tetragonal symmetry.^[5]

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.

Preparation

Aluminum sulfide is readily prepared by ignition of the elements^[6]

2 Al + 3 S → Al₂S₃

This reaction is extremely exothermic and it is not necessary or desirable to heat the whole mass of the sulfur-aluminum mixture; (except possibly for very small amounts of reactants). The product will be created in a fused form; it reaches a temperature greater than 1,100 °C and may melt its way through steel. The cooled product is very hard.