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.
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
- Sin 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.
- 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.