Magnesium silicide

Magnesium silicide, Mg₂Si, is an inorganic compound of magnesium and silicon. As-grown Mg₂Si usually forms black crystals; they are semiconductors with n-type conductivity and have potential applications in thermoelectric generators.

Crystal structure

Mg₂Si crystallizes in the antifluorite structure. In the face-centered cubic lattice, Si centers occupy the corners and face-centered positions of the unit cell, and Mg centers occupy eight tetrahedral sites in the interior of the unit cell. The coordination numbers of Si and Mg are eight and four, respectively.

Synthesis

It can be produced by heating silicon dioxide, SiO₂, found in sand, with excess magnesium. The process first forms elemental silicon and magnesium oxide:
If an excess of Mg is present, Mg₂Si is formed from the reaction of the remaining magnesium with the elemental silicon:
These reactions proceed exothermically, even explosively.

Reactions

Magnesium silicide can be viewed as consisting of Si⁴⁻ ions. As such, it is reactive toward acids. Thus, when magnesium silicide is treated with hydrochloric acid, silane and magnesium chloride are produced:
Sulfuric acid can be used as well. These protonolysis reactions are typical of a group 2 and group 1 silicides. The early development of silicon hydrides relied on this reaction.

Uses

Magnesium silicide is used to create aluminium alloys of the 6000 series, containing up to approximately 1.5% Mg₂Si. An alloy of this group can be age-hardened to form Guinier-Preston zones and a very fine precipitate, resulting in increased alloy strength.
Magnesium silicide is a narrow-gap semiconductor. Its as-grown crystal exhibits n-type conductivity, but it can be changed to p-type by doping with Ag, Ga, Sn and possibly Li. The major potential electronic application of Mg₂Si is in thermoelectric generators.