Binary compounds of silicon

Binary compounds of silicon are binary chemical compounds containing silicon and one other chemical element. Technically the term silicide is reserved for any compounds containing silicon bonded to a more electropositive element. Binary silicon compounds can be grouped into several classes. Saltlike silicides are formed with the electropositive s-block metals. Covalent silicides and silicon compounds occur with hydrogen and the elements in groups 10 to 17.

Transition metals form metallic silicides, with the exceptions of silver, gold and the group 12 elements. The general composition is M_nSi or MSi_n with n ranging from 1 to 6 and M standing for metal. Examples are M₅Si, M₃Si (Cu, V, Cr, Mo, Mn, Fe, Pt, U), M₂Si (Zr, Hf, Ta, Ir, Ru, Rh, Co, Ni, Ce), M₃Si₂ (Hf, Th, U), MSi (Ti, Zr, Hf, Fe, Ce, Th, Pu) and MSi₂ (Ti, V, Nb, Ta, Cr, Mo, W, Re).

The Kopp–Neumann law applies; heat capacities are linear in the proportion of silicon:

As a general rule, nonstochiometry implies instability. These intermetallics are in general resistant to hydrolysis, brittle, and melt at a lower temperature than the corresponding carbides or borides. They are electrical conductors. However, some, such as CrSi₂, Mg₂Si, β-FeSi₂ and MnSi_1.7, are semiconductors. Since degenerate semiconductors exhibit some metallic properties, such as luster and electrical conductivity which decreases with temperature, some silicides classified as metals may be semiconductors.

Group 1

Silicides of group 1 elements are saltlike silicides. In general, they melt at high temperatures, appear metallic grey, conduct electricity moderately to poor, and are prepared by heating the elements. The major exception are the silanes, silicon-hydrogen compounds whose bonds are covalent. The parent, silane, is SiH₄; higher homologues are disilane (Si₂H₆) and trisilane (Si₃H₈). Polysilicon hydride is a two-dimensional polymer network.

At the atomic level, group 1 silicides typically form cluster compounds. Several silicon phases include the Zintl ions (, , ). Li₁₂Si₇ has a Zintl phase with planar Si₅^{6−} rings, and Li NMR spectroscopy suggests these rings are aromatic.

Lithium silicides include Li₁₃Si₄, Li₂₂Si₅, Li₇Si₃ and Li₁₂Si₇. Li_4.4Si is prepared from silicon and lithium metal in high-energy Ball mill process. One potential use is lithium battery electrodes.

Sodium silicide can be represented by NaSi, NaSi₂ and Na₁₁Si₃₆, and potassium silicide by K₈Si₄₆.

Group 2

Silicides of group 2 elements are also saltlike silicides except for beryllium whose phase diagram with silicon is a simple eutectic (1085 Ã‚°C @ 60% by weight silicon). Again there is variation in composition: magnesium silicide is represented by Mg₂Si, calcium silicide can be represented by Ca₂Si, CaSi, CaSi₂, Ca₅Si₃ and by Ca₁₄Si₁₉, strontium silicide can be represented by Sr₂Si, SrSi₂ and Sr₅Si₃ and barium silicide can be represented by Ba₂Si, BaSi₂, Ba₅Si₃ and Ba₃Si₄. Superconducting properties have been reported for Ba₈Si₄₆. Mg₂Si, and its solid solutions with Mg₂Ge and Mg₂Sn, are good thermoelectric materials and their figure of merit values are comparable with those of established materials.

Transition and inner transition metals

The transition metals form a wide range of silicon intermetallics with at least one binary crystalline phase. Some exceptions exist. Gold forms a eutectic at 363&nbsp;°C with 2.3% silicon by weight (18% atom percent) without mutual solubility in the solid state. Silver forms another eutectic at 835&nbsp;°C with 11% silicon by weight, again with negligible mutual solid state solubility. In group 12 all elements form a eutectic close to the metal melting point without mutual solid-state solubility: zinc at 419&nbsp;°C and > 99 atom percent zinc and cadmium at 320&nbsp;°C (< 99% Cd).

Commercially relevant intermetallics are group 6 molybdenum disilicide, a commercial ceramic mostly used as an heating element. Tungsten disilicide is also a commercially available ceramic with uses in microelectronics. Platinum silicide is a semiconductor material. Ferrosilicon is an iron alloy that also contains some calcium and aluminium.

MnSi, known as brownleeite, can be found in outer space. Several Mn silicides form a Nowotny phase. Nanowires based on silicon and manganese can be synthesised from Mn(CO)₅SiCl₃ forming nanowires based on Mn₁₉Si₃₃. or grown on a silicon surface MnSi_1.73 was investigated as thermoelectric material and as an optoelectronic thin film. Single-crystal MnSi_1.73 can form from a tin-lead melt

In the frontiers of technological research, iron disilicide is becoming more and more relevant to optoelectronics, specially in its crystalline form β-FeSi₂. They are used as thin films or as nanoparticles, obtained by means of epitaxial growth on a silicon substrate.

Group 13

In group 13 boron (a metalloid) forms several binary crystalline silicon boride compounds: SiB₃, SiB₆, SiB_n. With aluminium, a post-transition metal, a eutectic is formed (577&nbsp;°C @ 12.2 atom % Al) with maximum solubility of silicon in solid aluminium of 1.5%. Commercially relevant aluminium alloys containing silicon have at least element added. Gallium, also a post-transition metal, forms a eutectic at 29&nbsp;°C with 99.99% Ga without mutual solid-state solubility; indium and thallium behave similarly.

Group 14

Silicon carbide (SiC) is widely used as a ceramic or example in car brakes and bulletproof vests. It is also used in semiconductor electronics. It is manufactured from silicon dioxide and carbon in an Acheson furnace between 1600 and 2500&nbsp;°C. There are 250 known crystalline forms with alpha silicon carbide the most common. Silicon itself is an important semiconductor material used in microchips. It is produced commercially from silica and carbon at 1900&nbsp;°C and crystallizes in a diamond cubic crystal structure. Germanium silicide forms a solid solution and is again a commercially used semiconductor material. The tin–silicon phase diagram is a eutectic and the lead–silicon phase diagram shows a monotectic transition and a small eutectic transition but no solid solubility.

Group 15

Silicon nitride (Si₃N₄) is a ceramic with many commercial high-temperature applications such as engine parts. It can be synthesized from the elements at temperatures between 1300 and 1400&nbsp;°C. Three different crystallographic forms exist. Other binary silicon nitrogen compounds have been proposed (SiN, Si₂N₃, Si₃N) and other SiN compounds have been investigated at cryogenic temperatures (SiN₂, Si(N₂)₂, SiNNSi). Silicon tetraazide is an unstable compound that easily detonates.

The phase diagram with phosphorus shows SiP and SiP₂. A reported silicon phosphide is Si₁₂P₅ (no practical applications), formed by annealing an amorphous Si-P alloy.

The arsenic–silicon phase diagram measured at 40 Bar has two phases: SiAs and SiAs₂. The antimony–silicon system comprises a single eutectic close to the melting point of Sb. The bismuth system is a monotectic.

Group 16

In group 16 silicon dioxide is a very common compound that widely occurs as sand or quartz. SiO₂ is tetrahedral with each silicon atom surrounded by 4 oxygen atoms. Numerous crystalline forms exist with the tetrahedra linked to form a polymeric chain. Examples are tridymite and cristobalite. A less common oxide is silicon monoxide that can be found in outer space. Unconfirmed reports exist for nonequilibrium Si₂O, Si₃O₂, Si₃O₄, Si₂O₃ and Si₃O₅. Silicon sulfide is also a chain compound. Cyclic SiS₂ has been reported to exist in the gas phase. The phase diagram of silicon with selenium has two phases: SiSe₂ and SiSe. Tellurium silicide is a semiconductor with formula TeSi₂ or Te₂Si₃.

Group 17

Binary silicon compounds in group 17 are stable compounds ranging from gaseous silicon fluoride (SiF₄) to the liquids silicon chloride (SiCl₄ and silicon bromide SiBr₄) to the solid silicon iodide (SiI₄). The molecular geometry in these compounds is tetrahedral and the bonding mode covalent. Other known stable fluorides in this group are Si₂F₆, Si₃F₈ (liquid) and polymeric solids known as polysilicon fluorides (SiF₂)_x and (SiF)_x. The other halides form similar binary silicon compounds.

The periodic table of the binary silicon compounds

References