6.13: Chalcogenides of Aluminum, Gallium, and Indium

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The only stable chalcogenides of aluminum are Al₂S₃ (white), Al₂Se₃ (grey), and Al₂Te₃ (dark grey). They are each prepared by the direct reaction of the elements (100 °C) and hydrolyze rapidly in aqueous solution, (6.13.1). All the chalcogenides have a hexagonal ZnS structure in which ²/₃ of the metal sites are occupied.

\[ \text{Al}_3\text{E}_3 \text{ + 6 H}_2\text{O} \rightarrow \text{2 Al(OH)}_3 \text{ + 3 H}_2\text{E}\]

The chalcogenides of gallium and indium are more numerous than those of aluminum, and are listed in Table \(\PageIndex{1}\) and Table \(\PageIndex{2}\) along with selected physical properties.

Table \(\PageIndex{1}\): Stoichiometries, structures and selected physical properties of the crystalline chalcogenides of gallium. ^a dir = direct, ind = indirect, opt = optical.
Compound	Structural type	Crystallographic system	Cell parameters (Å, °)	Band Gap (eV)^a
GaS		Hexagonal	a = 3.587, c = 15.492	3.05 (dir.), 2.593 (ind.)
GaS	ZnS or NaCl	Cubic	a = 5.5	4.0 (opt.)
β-GaSe	GaS	Hexagonal	a = 3.742, c = 15.919	2.103 (dir.), 2.127 (ind.)
γ-GaSe	GaS	Rhombohedral	a = 3.755, c = 23.92
δ-GaSe	GaS	Hexagonal	a = 3.755, c = 31.99
β-GaTe	GaS	Hexagonal	a = 4.06, c = 16.96
GaTe	GaS	Monoclinic	a = 17.44, b = 4.077, c = 10.456, β = 104.4	1.799 (dir.)
α-Ga₂S₃	Wurtzite	Cubic	a = 5.181
α-Ga₂S₃	Wurtzite	Monoclinic	a = 12.637, b = 6.41, c = 7.03, β = 131.08	3.438 (opt.)
β-Ga₂S₃	Defect wurtzite	Hexagonal	a = 3.685, c = 6.028	2.5 - 2.7 (opt.)
α-Ga₂Se₃	Sphalerite	Cubic	a = 5.429	2.1 (dir.), 2.04 (ind.)
α-Ga₂Te₃	Sphalerite	Cubic	a = 5.886	1.22 (opt.)

Table \(\PageIndex{2}\): Stoichiometries, structures and selected physical properties of the crystalline chalcogenides of indium. ^a dir = direct, ind = indirect, opt = optical. ^b High pressure phase.
Compound	Structural type	Crystallographic system	Cell parameters (Å, °)	Band gap (eV)^a
β-InS	GaS	Orthorhombic	a = 3.944, b = 4.447, c = 10.648	2.58 (dir.), 2.067 (ind.)
InS^b	Hg₂Cl₂	Tetragonal
InSe	GaS	Rhombohedral	a = 4.00, c = 25.32	1.3525 (dir.), 1.32 (ind.)
β-InSe	GaS	Hexagonal	a = 4.05, c = 16.93
InTe	TlSe	Tetragonal	a = 8.437, c = 7.139	Metallic
InTe^b	NaCl	Cubic	a = 6.18
α-In₂S₃	γ-Al₂O₃	Cubic	a = 5.36
β-In₂S₃	Spinel	Tetragonal	a = 7.618, c = 32.33	2.03 (dir.), 1.1 (ind.)
α-In₂Se₃	Defect wurtzite	Hexagonal	a = 16.00, c = 19.24
β-In₂Se₃	Defect wurtzite	Rhombohedral	a = 4.025, c = 19.222	1.2 - 1.5 (ind.)
α-In₂Te₃	Sphalerite	Cubic	a = 6.158	0.92 - 1.15 (opt.)
In₆S₇		Monoclinic	a = 9.090, b = 3.887, c = 17.705, β = 108.20	0.89 (dir.), 0.7 (ind.)
In₆Se₇	In₆S₇	Monoclinic	a = 9.430, b = 4.063, c = 18.378, β = 109.34	0.86 (dir.), 0.34 (ind.)
In₄Se₃		Orthorhombic	a = 15.297, b = 12.308, c = 4.081	0.64 (dir.)
In₄Te₃	In₄Se₃	Orthorhombic	a = 15.630, b = 12.756, c = 4.441	0.48 (dir.)

The hexagonal β-form of Ga₂S₃ is isostructural with the aluminum analogue; however, while the α-phase was proposed to be hexagonal it was later shown to be monoclinic. A cubic α-phase has been reported. Cubic Sphalerite structures are found for Ga₂Se₃, Ga₂Te₃, and In₂Te₃, in which the structure is based on a cubic close packing of the chalcogenides and the metal atoms occupying ¹/₃ of the tetrahedral sites. These structures are all formed with rapid crystallization; slow crystallization and/or thermal annealing leads to ordering and the formation of more complex structures. The indium sulfides, and selenides derivatives are spinel (γ-Al₂O₃), and defect Würtzite, respectively.

Unlike the chalcogenides of aluminum, those of gallium and indium also form subvalent compounds, i.e., those in which the metal is formally of an oxidation state less than +3. Of these subvalent chalcogenides the (formally) divalent materials are of the most interest. The thermodynamically stable phase of GaS has a hexagonal layer structure (Figure \(\PageIndex{1}\)) with Ga-Ga bonds (2.48 Å). The compound can, therefore, be considered as an example of Ga(II). Each Ga is coordinated by three sulfur atoms and one gallium, and the sequence of layers along the z-axis is ^...S-Ga-Ga-S^...S-Ga-Ga-S^....

Figure \(\PageIndex{1}\): The ^...S-Ga-Ga-S^...S-Ga-Ga-S^... structure of hexagonal GaS. Gallium atoms are shown shaded.

The structures of β-GaSe, and β-InSe are similar to hexagonal GaS. The layered structure of GaTe is similar in that it consists of ^...TeGaGaTe^... layers, but is monoclinic, while InS is found in both a (high pressure) tetragonal phase (Figure \(\PageIndex{2}\)a) as well as an orthorhombic phase (Figure \(\PageIndex{2}\)b). By contrast to these M-M bonded layered compounds InTe (Figure \(\PageIndex{3}\)) has a structure formalized as In(I)[In(III)Te₂]; each In(III) is tetrahedrally coordinated to four Te and these tetrahedra are linked via shared edges; the In(I) centers lying between these chains.

Figure \(\PageIndex{2}\): Unit cell of (a) tetragonal InS and (b) orthorhombic InS. Indium atoms are shown shaded, and the solid bonds represent the smallest cyclic structural fragment.

Figure \(\PageIndex{3}\): Unit cell of tetragonal InTe. Indium atoms are shown shaded, and the solid bonds represent the [InTe₂]_∞ chains.

Further sub-chalcogenides are known for indium, e.g.; In₄Se₃, which contains [In(III)₃Se₂]⁵⁺ groups (Figure \(\PageIndex{4}\)). While the formally In(I) molecule In₂S has been detected in the gas phase, it is actually a mixture of In and InS in the solid state.

Figure \(\PageIndex{4}\): Structure of [In(III)₃Se₂]⁵⁺ groups in In₄Se₃.

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