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Magnesium silicide

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Magnesium silicide

Magnesium silicide
Sample of magnesium silicide as powder
Identifiers
CAS number  YesY
PubChem
ChemSpider  YesY
EC number
Jmol-3D images Image 1
Properties
Molecular formula Mg2Si
Molar mass 76.70 g mol−1
Density 1.988 g cm-3
Melting point 1,102 °C (2,016 °F; 1,375 K)
Hazards
EU Index Not listed
R-phrases R23, R24, R25, R34
Main hazards reacts with water to give silane
Related compounds
Other cations Calcium silicide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

Magnesium silicide, Mg2Si, is an magnesium and silicon. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide, SiO2, found in sand and glass, when heated with magnesium forms magnesium oxide, and, if an excess of magnesium is used, magnesium silicide is formed. The first product in this reaction is silicon, which then reacts further with magnesium to produce Mg2Si. The stoichiometry of these reactions are such that with a 2:1 Mg:SiO2 molar ratio, MgO is formed:

2 Mg + SiO2 → 2 MgO + Si

If an excess of Mg is present, Mg2Si is formed from the reaction of the remaining magnesium with the silicon via:

2 Mg + Si → Mg2Si

Hence, the overall reaction for the formation of magnesium silicide from a 4:1 Mg:SiO2 molar ratio may be represented as:

4 Mg + SiO2 → 2 MgO + Mg2Si

These reactions proceed violently, producing a great amount of heat.

Magnesium silicide can also be formed by the reaction between magnesium hydride and silicon with the evolution of hydrogen gas at temperatures above 250°C:

2 MgH2 + Si → Mg2Si + 2 H2

This reaction releases 5 wt.% hydrogen and has been considered for hydrogen storage. However, its reversibility has yet to be demonstrated.

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, both resulting in increased strength of the alloy.[1]

When magnesium silicide is placed into hydrochloric acid, HCl(aq), the gas silane, SiH4, is produced. This gas is the silicon analogue of methane, CH4, but is more reactive. Silane is pyrophoric, that is, due to the presence of oxygen, it spontaneously combusts in air:

Mg2Si(s) + 4 HCl(aq) → SiH4(g) + 2 MgCl2(s)
SiH4 + 2 O2 → SiO2 + 2 H2O

These reactions are typical of a Group 2 silicide. Mg2Si reacts similarly with sulfuric acid. Group 1 silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate, Na2SiO3, and hydrogen gas.

Crystal Structure

Mg2Si crystallizes in a face-centered cubic lattice. It possesses the antifluorite structure with Si4- ions occupying the corners and face-centered positions of the unit cell and Mg2+ ions occupying eight tetrahedral sites in the interior of the unit cell. There are also four equivalent interstitial sites.[2] Hence, the structure contains isolated Si4- ions. This feature makes the Si4 - ion of Mg2Si a good Brønsted–Lowry base (proton acceptor).

References

  1. ^ ASM Handbook, 10th Ed., Vol. 1, Properties and Selection: Non-ferrous Alloys and Special Purpose Materials, 1990, ASM International, Materials Park, Ohio.
  2. ^ A. Kato et al. J. Phys: Condens. Matter 21 (2009) 205801.
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