Silicide

A silicide is a compound that has silicon with (usually) more electropositive elements.

Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides.

Similar to borides and carbides, the composition of silicides cannot be easily specified as covalent molecules. The chemical bonds in silicides range from conductive metal-like structures to covalent or ionic. Silicides of all non-transition metals, with exception of beryllium, have been described.

Mercury, thallium, bismuth, and lead are nonmiscible with liquid silicon.

Silicon atoms in silicides can have many possible organizations:

• Isolated silicon atoms: electrically conductive Cu₅Si, (V,Cr,Mn)₃Si, Fe₃Si, Mn₃Si, and nonconductive (Mg,Ge,Sn,Pb)₂Si, (Ca,Ru,Ce,Rh,Ir,Ni)₂Si
• Si₂ pairs: U₃Si₂, Hf and Th silicides
• Si₄ tetrahedra: KSi, RbSi, CsSi
• Si_n chains: USi, (Ti, Zr, Hf, Th, Ce, Pu)Si, CaSi, SrSi, YSi
• Planar hexagonal graphite-like Si layers: β-USi₂, silicides of other lanthanoids and actinoids
• Corrugated hexagonal Si layers: CaSi₂
• Open three-dimensional Si skeletons: SrSi₂, ThSi₂, α-USi₂

A silicide prepared by a self-aligned process is called a salicide. This is a process in which silicide contacts are formed only in those areas in which deposited metal (which after annealing becomes a metal component of the silicide) is in direct contact with silicon, hence, the process is self-aligned. It is commonly implemented in MOS/CMOS processes for ohmic contacts of the source, drain, and poly-Si gate.

Group 1 and 2 silicides e.g. Na₂Si and Ca₂Si react with water to yielding hydrogen and/or silanes. At Consumer Electronics Show (CES) 2012 a safe and eco-friendly 1kW or 3kW mobile phone charger with sodium silicide that runs on water has introduced for 'people who spend time away from the electricity grid'. Any type of water can be used, including salt water and it can even run on puddle water providing it isn't thickened with mud or any other sediment.

The transition metal silicides are, in contrast, usually inert to aqueous solutions of everything with exception of hydrofluoric acid; however, they react with more aggressive agents, e.g. melted potassium hydroxide, or fluorine and chlorine when red-hot.

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) + 4HCl(aq) → SiH₄(g) + 2MgCl₂(s)

SiH₄ + 2O₂ → SiO₂ + 2H₂O

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.

Examples

• Nickel silicide, NiSi
• Sodium silicide, Na₂Si
• Magnesium silicide, Mg₂Si
• Platinum silicide, PtSi (platinum is actually more electronegative than silicon)
• Titanium silicide, TiSi₂
• Tungsten silicide, WSi₂
• Molybdenum disilicide, MoSi₂

See category for a list.

See also

• Binary compounds of silicon