Binary compounds of silicon

Binary compounds of silicon are binary chemical compounds containing silicon and one other chemical element.^[1] 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.

Experimental iron-silicon phase diagram

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: $${\displaystyle C_{p}({\ce {M_xSi_y}})=xC_{p}({\ce {M}})+yC_{p}({\ce {Si}})}$$

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, except for silane (SiH₄) whose bonds to hydrogen are covalent. Higher silane homologues are disilane and trisilane. Polysilicon hydride is a two-dimensional polymer network.

Many cluster compounds of lithium silicides are known, such as Li₁₃Si₄, Li₂₂Si₅, Li₇Si₃ and Li₁₂Si₇.^[2] Li_4.4Si is prepared from silicon and lithium metal in high-energy Ball mill process.^[3] Potential uses include electrodes in lithium batteries. Li₁₂Si₇ has a Zintl phase with planar Si₅⁶⁻ rings. Li NMR spectroscopy suggests these rings are aromatic.^[4]

Other group 1 elements also form clusters: sodium silicide can be represented by NaSi, NaSi₂ and Na₁₁Si₃₆^[5] and potassium silicide by K₈Si₄₆. Group 1 silicides are in general high melting, metallic grey, with moderate to poor electrical conductance and prepared by heating the elements. Superconducting properties have been reported for Ba₈Si₄₆.^[6] Several silicon Zintl ions (Si⁴⁻
₄, Si⁴⁻
₉, Si²⁻
₅) are known with group 1 counterions.^[7]

Group 2

Silicides of group 2 elements are also saltlike silicides except for beryllium whose phase diagram with silicon is a simple eutectic (1085 °C @ 60% by weight silicon).^[8] Again there is variation in composition: magnesium silicide is represented by Mg₂Si,^[9] calcium silicide can be represented by Ca₂Si, CaSi, CaSi₂, Ca₅Si₃ and by Ca₁₄Si₁₉,^[10] strontium silicide can be represented by Sr₂Si, SrSi₂ and Sr₅Si₃^[11] and barium silicide can be represented by Ba₂Si, BaSi₂, Ba₅Si₃ and Ba₃Si₄.^[12] 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 °C with 2.3% silicon by weight (18% atom percent) without mutual solubility in the solid state.^[13] Silver forms another eutectic at 835 °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 °C and > 99 atom percent zinc and cadmium at 320 °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.^[14] or grown on a silicon surface^[15][16][17] MnSi_1.73 was investigated as thermoelectric material^[18] and as an optoelectronic thin film.^[19] Single-crystal MnSi_1.73 can form from a tin-lead melt^[20]

In the frontiers of technological research, iron disilicide is becoming more and more relevant to optoelectronics, specially in its crystalline form β-FeSi₂.^[21][22] They are used as thin films or as nanoparticles, obtained by means of epitaxial growth on a silicon substrate.^[23][24]

Atomic number	Name	Symbol	Group	Period	Block	Phases
21	Scandium	Sc	3	4	d	Sc5Si3, ScSi, Sc2Si3,[25][26][27][28]
22	Titanium	Ti	4	4	d	Ti5Si3, TiSi, TiSi2, TiSi3, Ti6Si4[25]
23	Vanadium	V	5	4	d	V3Si, V5Si3, V6Si5, VSi2, V6Si5[25][29]
24	Chromium	Cr	6	4	d	Cr3Si, Cr5Si3, CrSi, CrSi2[25][30]
25	Manganese	Mn	7	4	d	MnSi, Mn9Si2, Mn3Si, Mn5Si3, Mn11Si9[25]
26	Iron	Fe	8	4	d	FeSi2, FeSi[31][32] Fe5Si3, Fe2Si, Fe3Si
27	Cobalt	Co	9	4	d	CoSi, CoSi2, Co2Si, Co2Si, Co3Si[33][34]
28	Nickel	Ni	10	4	d	Ni3Si, Ni31Si12, Ni2Si, Ni3Si2, NiSi (Nickel monosilicide), NiSi2[25][35]
29	Copper	Cu	11	4	d	Cu17Si3, Cu56Si11,Cu5Si, Cu33Si7, Cu4Si, Cu19Si6,Cu3Si,Cu87Si13[25][36]
30	Zinc	Zn	12	4	d	eutectic[37]
39	Yttrium	Y	3	4	d	Y5Si3, Y5Si4, YSi, Y3Si5,[38][39] YSi1.4.[40]
40	Zirconium	Zr	4	5	d	Zr5Si3, Zr5Si4, ZrSi, ZrSi2,[25] Zr3Si2, Zr2Si, Zr3Si[41]
41	Niobium	Nb	5	5	d	Nb5Si3, Nb4Si[25]
42	Molybdenum	Mo	6	5	d	Mo3Si, Mo5Si3, MoSi2[25]
43	Technetium	Tc	7	5	d	Tc4Si7 (proposed)[42]
44	Ruthenium	Ru	8	5	d	Ru2Si, Ru4Si3, RuSi, Ru2Si3[43][44]
45	Rhodium	Rh	9	5	d	RhSi,[45] Rh2Si, Rh5Si3, Rh3Si2, Rh20Si13[46]
46	Palladium	Pd	10	5	d	Pd5Si, Pd9Si2, Pd3Si, Pd2Si, PdSi[47]
47	Silver	Ag	11	5	d	eutectic[48]
48	Cadmium	Cd	12	5	d	eutectic[49]
57	Lanthanum	La		6	f	La5Si3, La3Si2, La5Si4, LaSi, LaSi2[50]
58	Cerium	Ce		6	f	Ce5Si3, Ce3Si2, Ce5Si4, CeSi,[51] Ce3Si5, CeSi2[52]
59	Praseodymium	Pr		6	f	Pr5Si3, Pr3Si2, Pr5Si4, PrSi, PrSi2[53]
60	Neodymium	Nd		6	f	Nd5Si3, Nd5Si4, Nd5Si3,NdSi, Nd3Si4, Nd2Si3, NdSix[54]
61	Promethium	Pm		6	f
62	Samarium	Sm		6	f	Sm5Si4, Sm5Si3, SmSi, Sm3Si5, SmSi2[55]
63	Europium	Eu		6	f
64	Gadolinium	Gd		6	f	Gd5Si3, Gd5Si4, GdSi, GdSi2[56]
65	Terbium	Tb		6	f	Si2Tb (terbium silicide), SiTb, Si4Tb5, Si3Tb5[57]
66	Dysprosium	Dy		6	f	Dy5Si5, DySi, DySi2[58]
67	Holmium	Ho		6	f	Ho5Si3,Ho5Si4,HoSi,Ho4Si5,HoSi2[59]
68	Erbium	Er		6	f	Er5Si3, Er5Si4, ErSi, ErSi2[60]
69	Thulium	Tm		6	f
70	Ytterbium	Yb		6	f	Si1.8Yb,Si5Yb3,Si4Yb3, SiYb, Si4Yb5, Si3Yb5[61]
71	Lutetium	Lu	3	6	d	Lu5Si3[62]
72	Hafnium	Hf	4	6	d	Hf2Si, Hf3Si2, HfSi, Hf5Si4, HfSi2[25][63]
73	Tantalum	Ta	5	6	d	Ta9Si2, Ta3Si, Ta5Si3[25]
74	Tungsten	W	6	6	d	W5Si3, WSi2[64]
75	Rhenium	Re	7	6	d	Re2Si, ReSi, ReSi1.8[65] Re5Si3[25]
76	Osmium	Os	8	6	d	OsSi, Os2Si3, OsSi2[66]
77	Iridium	Ir	9	6	d	IrSi, Ir4Si5, Ir3Si4, Ir3Si5, IrSi3. Ir2Si3, Ir4Si7, IrSi2[67][68]
78	Platinum	Pt	10	6	d	Pt25Si7, Pt17Si8, Pt6Si5, Pt5Si2, Pt3Si, Pt2Si, PtSi[69]
79	Gold	Au	11	6	d	Eutectic diagram at link[70]
80	Mercury	Hg	12	6	d	eutectic[71]
89	Actinium	Ac		7	f
90	Thorium	Th		7	f	Th3Si2, ThSi, Th3Si5, and ThSi2−x[72]
91	Protactinium	Pa		7	f
92	Uranium	U		7	f	U3Si, U3Si2, USi, U3Si5, USi2−x, USi2 and USi3[73]
93	Neptunium	Np		7	f	NpSi3, Np3Si2, and NpSi[74]
94	Plutonium	Pu		7	f	Pu5Si3, Pu3Si2, PuSi, Pu3Si5 and PuSi2[75]
95	Americium	Am		7	f	AmSi, AmSi2[76]
96	Curium	Cm		7	f	CmSi, Cm2Si3, CmSi2[77]
97	Berkelium	Bk		7	f
98	Californium	Cf		7	f
99	Einsteinium	Es		7	f
100	Fermium	Fm		7	f
101	Mendelevium	Md		7	f
102	Nobelium	No		7	f
103	Lawrencium	Lr	3	7	d
104	Rutherfordium	Rf	4	7	d
105	Dubnium	Db	5	7	d
106	Seaborgium	Sg	6	7	d
107	Bohrium	Bh	7	7	d
108	Hassium	Hs	8	7	d
109	Meitnerium	Mt	9	7	d
110	Darmstadtium	Ds	10	7	d
111	Roentgenium	Rg	11	7	d
112	Copernicium	Cn	12	7	d

Group 13

In group 13 boron (a metalloid) forms several binary crystalline silicon boride compounds: SiB₃, SiB₆, SiB_n.^[78] With aluminium, a post-transition metal, a eutectic is formed (577 °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.^[79] Gallium, also a post-transition metal, forms a eutectic at 29 °C with 99.99% Ga without mutual solid-state solubility;^[80] indium^[81] and thallium^[82] 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 °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 °C and crystallizes in a diamond cubic crystal structure. Germanium silicide forms a solid solution and is again a commercially used semiconductor material.^[83] The tin–silicon phase diagram is a eutectic^[84] and the lead–silicon phase diagram shows a monotectic transition and a small eutectic transition but no solid solubility.^[85]

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 °C. Three different crystallographic forms exist. Other binary silicon nitrogen compounds have been proposed (SiN, Si₂N₃, Si₃N)^[86] and other SiN compounds have been investigated at cryogenic temperatures (SiN₂, Si(N₂)₂, SiNNSi).^[87] Silicon tetraazide is an unstable compound that easily detonates.

The phase diagram with phosphorus shows SiP and SiP₂.^[88] A reported silicon phosphide is Si₁₂P₅ (no practical applications),^[89][90] formed by annealing an amorphous Si-P alloy.

The arsenic–silicon phase diagram measured at 40 Bar has two phases: SiAs and SiAs₂.^[91] The antimony–silicon system comprises a single eutectic close to the melting point of Sb.^[92] The bismuth system is a monotectic.^[93]

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.^[94] Silicon sulfide is also a chain compound. Cyclic SiS₂ has been reported to exist in the gas phase.^[95] The phase diagram of silicon with selenium has two phases: SiSe₂ and SiSe.^[96] Tellurium silicide is a semiconductor with formula TeSi₂ or Te₂Si₃.^[97]

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.^[98]

The periodic table of the binary silicon compounds

SiH4

He

LiSi

Be

SiB3

SiC

Si3N4

SiO2

SiF4

Ne

NaSi

Mg2Si

Al

Si

SiP

SiS2

SiCl4

Ar

KSi

CaSi2

ScSi

TiSi

V5Si3

Cr5Si3

MnSi

FeSi

CoSi

NiSi

Cu5Si

Zn

Ga

Si1−xGex

SiAs

SiSe2

SiBr4

Kr

RbSi

Sr2Si

YSi

ZrSi

Nb5Si3

Mo5Si3

Tc

RuSi

RhSi

PdSi

Ag

Cd

In

Sn

Sb

TeSi2

SiI4

Xe

CsSi

Ba2Si

LuSi

HfSi

Ta5Si3

W5Si3

ReSi2

OsSi

IrSi

PtSi

Au

Hg

Tl

Pb

Bi

Po

At

Rn

Fr

Ra

Lr

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Cn

Nh

Fl

Mc

Lv

Ts

Og

↓

LaSi

CeSi

PrSi

NdSi

Pm

SmSi

EuSi

GdSi

TbSi

DySi

HoSi

ErSi

Tm

YbSi

Ac

ThSi

Pa

USi

NpSi

PuSi

AmSi

CmSi

Bk

Cf

Es

Fm

Md

No

Binary compounds of silicon

Covalent silicon compounds	metallic silicides.
Ionic silicides	Do not exist
Eutectic / monotectic / solid solution	Unknown / Not assessed