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Silicon monoxide
Silicon monoxide
Silicon monoxide
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Silicon monoxide
Names
Preferred IUPAC name
Silicon monoxide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.030.198 Edit this at Wikidata
EC Number
  • 233-232-8
382
MeSH Silicon+monoxide
UNII
  • InChI=1S/H3OSi/c1-2/h2H3 ☒N
    Key: UXMAWJKSGBRJKV-UHFFFAOYSA-N ☒N
  • InChI=1/OSi/c1-2
    Key: LIVNPJMFVYWSIS-UHFFFAOYAO
  • InChI=1S/OSi/c1-2
    Key: LIVNPJMFVYWSIS-UHFFFAOYSA-N
  • [O+]#[Si-]
Properties
SiO
Molar mass 44.08 g/mol
Appearance brown-black glassy solid
Density 2.13 g/cm3
Melting point 1,702 °C (3,096 °F; 1,975 K)
Boiling point 1,880 °C (3,420 °F; 2,150 K)
insoluble
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
Flash point Non-flammable
Related compounds
Other anions
 Silicon sulfide
Silicon selenide
Silicon telluride
Other cations
 Carbon monoxide
Germanium(II) oxide
Tin(II) oxide
Lead(II) oxide
Related silicon oxides
 Silicon dioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Silicon monoxide is the chemical compound with the formula SiO where silicon is present in the oxidation state +2. In the vapour phase, it is a diatomic molecule.[1] It has been detected in stellar objects[2] and has been described as the most common oxide of silicon in the universe.[3]

Solid form

[edit]

When SiO gas is cooled rapidly, it condenses to form a brown/black polymeric glassy material, (SiO)n, which is available commercially and used to deposit films of SiO. Glassy (SiO)n is air and moisture sensitive.[clarification needed][citation needed]

Oxidation

[edit]

Its surface readily oxidizes in air at room temperature, giving an SiO2 surface layer that protects the material from further oxidation. However, (SiO)n irreversibly disproportionates into SiO2 and Si in a few hours between 400 °C and 800 °C and very rapidly between 1,000 °C and 1,440 °C, although the reaction does not go to completion.[4]

Production

[edit]

The first precise report on the formation of SiO was in 1887[5] by the chemist Charles F. Maybery (1850–1927) at the Case School of Applied Science in Cleveland. Maybery claimed that SiO formed as an amorphous greenish-yellow substance with a vitreous luster when silica was reduced with charcoal in the absence of metals in an electric furnace.[6] The substance was always found at the interface between the charcoal and silica particles.

By investigating some of the chemical properties of the substance, its specific gravity, and a combustion analysis, Maybery deduced that the substance must be SiO. The equation representing the partial chemical reduction of SiO2 with C can be represented as:

SiO
2 + C ⇌ SiO + CO

Complete reduction of SiO2 with twice the amount of carbon yields elemental silicon and twice the amount of carbon monoxide. In 1890, the German chemist Clemens Winkler (the discoverer of germanium) was the first to attempt to synthesize SiO by heating silicon dioxide with silicon in a combustion furnace.[7]

SiO
2 + Si ⇌ 2 SiO

However, Winkler was not able to produce the monoxide since the temperature of the mixture was only around 1000 °C. The experiment was repeated in 1905 by Henry Noel Potter (1869–1942), a Westinghouse engineer. Using an electric furnace, Potter was able to attain a temperature of 1700 °C and observe the generation of SiO.[5] Potter also investigated the properties and applications of the solid form of SiO.[8][9]

Gaseous form

[edit]

Because of the volatility of SiO, silica can be removed from ores or minerals by heating them with silicon to produce gaseous SiO in this manner.[1] However, due to the difficulties associated with accurately measuring its vapor pressure, and because of the dependency on the specifics of the experimental design, various values have been reported in the literature for the vapor pressure of SiO (g). For the pSiO above molten silicon in a quartz (SiO2) crucible at the melting point of silicon, one study yielded a value of 0.002 atm.[10] For the direct vaporization of pure, amorphous SiO solid, 0.001 atm has been reported.[11] For a coating system, at the phase boundary between SiO2 and a silicide, 0.01 atm was reported.[12]

Silica itself, or refractories containing SiO2, can be reduced with H2 or CO at high temperatures, e.g.:[13]

SiO
2(s) + H
2(g) ⇌ SiO(g) + H
2O(g)

As the SiO product volatilizes off (is removed), the equilibrium shifts to the right, resulting in the continued consumption of SiO2. Based on the dependence of the rate of silica weight loss on the gas flow rate normal to the interface, the rate of this reduction appears to be controlled by convective diffusion or mass transfer from the reacting surface.[14][15]

Gaseous (molecular) form

[edit]

Silicon monoxide molecules have been trapped in an argon matrix cooled by helium. In these conditions, the SiO bond length is between 148.9 pm[3] and 151 pm.[16] This bond length is similar to the length of Si=O double bonds (148 pm) in the matrix-isolated linear molecule SiO
2 (O=Si=O), suggestive of the absence of a triple bond as in carbon monoxide.[3] However, the SiO triple bond has a calculated bond length of 150 pm and a bond energy of 794 kJ/mol, which are also very close to those reported for SiO.[16] In the carbon analogues the formal double bonds of carbon dioxide (116 pm) is also close to the triple bond length of carbon monoxide (112.8 pm); in light of this the observed bond length of SiO may be consistent with at least some triple-bond character in the diatomic molecule. The SiO double bond structure is, notably, an exception to Lewis' octet rule for molecules composed of the light main group elements, whereas the SiO triple bond satisfies this rule. That anomaly not withstanding, the observation that monomeric SiO is short-lived and that (SiO)'n' oligomers with 'n' = 2,3,4,5 are known,[17] all having closed ring structures in which the silicon atoms are connected through bridging oxygen atoms (i.e. each oxygen atom is singly bonded to two silicon atoms; no Si-Si bonds), suggests the Si=O double bond structure, with a hypovalent silicon atom, is likely for the monomer.[3]

Condensing molecular SiO in argon matrix together with fluorine, chlorine or carbonyl sulfide (COS), followed by irradiation with light, produces the planar molecules OSiF
2 (with Si-O distance 148 pm) and OSiCl
2 (Si-O 149 pm), and the linear molecule OSiS (Si-O 149 pm, Si-S 190 pm).[3]

Matrix-isolated molecular SiO reacts with oxygen atoms generated by microwave discharge to produce molecular SiO
2 which has a linear structure.

When metal atoms (such as Na, Al, Pd, Ag, and Au) are co-deposited with SiO, triatomic molecules are produced with linear (AlSiO and PdSiO), non-linear (AgSiO and AuSiO), and ring (NaSiO) structures.[3]

Solid (polymeric) form

[edit]

Potter reported SiO solid as yellowish-brown in color and as being an electrical and thermal insulator. The solid burns in oxygen and decomposes water with the liberation of hydrogen. It dissolves in warm alkali hydroxides and in hydrofluoric acid. Even though Potter reported the heat of combustion of SiO to be 200 to 800 calories higher than that of an equilibrium mixture of Si and SiO2 (which could, arguably, be used as evidence that SiO is a unique chemical compound),[18] some studies characterized commercially available solid silicon monoxide materials as an inhomogeneous mixture of amorphous SiO2 and amorphous Si with some chemical bonding at the interface of the Si and SiO2 phases.[19][20] Recent spectroscopic studies in a correlation with Potter's report suggest that commercially available solid silicon monoxide materials can not be considered as an inhomogeneous mixture of amorphous SiO2 and amorphous Si.[21]

Interstellar occurrence

[edit]

Interstellar SiO was first reported in 1971 after detection in the giant molecular cloud Sgr B2.[22] SiO is used as a molecular tracer of shocked gas in protostellar outflows.[23]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Silicon monoxide

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from Grokipedia
Silicon monoxide, with the chemical formula SiO, is an inorganic compound in which silicon adopts the uncommon +2 oxidation state, forming a diatomic molecule in the vapor phase but typically existing as an amorphous solid that undergoes nanoscale disproportionation into silicon-rich and silica-like (SiO₂) regions. The solid form appears as a black-brown to loess-colored amorphous powder with a density of approximately 2.15 g/cm³, high melting and boiling points, and insolubility in water, though it dissolves in hydrofluoric acid, or mixtures thereof with nitric acid, to release silicon tetrafluoride. Chemically stable at room temperature and generally unreactive with water, dilute acids, or alkalis under ambient conditions, though it reacts with hydrofluoric acid and hot concentrated alkali hydroxides,[] SiO oxidizes to white silica (SiO₂) upon heating in air and can disproportionate further into elemental silicon and SiO₂ above 600°C in inert atmospheres. In its amorphous state, SiO features a heterogeneous structure with suboxide tetrahedra at interfaces between amorphous Si clusters and an SiO₂ matrix, as revealed by advanced techniques like angstrom-beam and high-energy ; this resolves long-standing debates on its composition and distinguishes it from pure Si or SiO₂. Theoretical studies indicate that while crystalline ground-state structures exist with Si–Si bonds and Si–O–Si bridges, making it semiconducting at 1 atm, the material's practical forms are predominantly amorphous without large-scale segregation, and (2SiO → Si + SiO₂) is exothermic, limiting thermodynamic stability. A two-dimensional variant, orthorhombic SiO (Orth-SiO), has been predicted to be stable with a direct of 1.52 eV, high hole mobility (up to 8.5 × 10³ cm²·V⁻¹·s⁻¹), and potential for optical and electronic applications, though it remains largely theoretical. SiO is prepared industrially by high-temperature reduction of SiO₂ with or carbon under , followed by rapid , yielding material volatile above 1100°C suitable for into thin . These exhibit strong UV absorption extending into the visible, a ranging from ~1.4 (oxidized) to ~2, high , good adhesion to substrates like , and resistance to abrasion and hygroscopicity, though they oxidize slowly in air. Notable applications include optical coatings, insulating layers in , capacitor dielectrics, protective overcoats for mirrors, anode materials in lithium-ion batteries, and precursors for high-purity silica or in ceramics and semiconductors.

Physical and chemical properties

General characteristics

Silicon monoxide is an with the SiO, in which silicon exhibits a +2 . Its is 44.08 g/mol. In its solid form, silicon monoxide appears as a brown-black or yellowish-brown glassy solid with a of 2.13 g/cm³. It is insoluble in . In the gas phase, SiO exists as a featuring a Si–O ranging from 148.9 to 151 pm. Silicon monoxide is sensitive to air and moisture, readily undergoing surface oxidation to form a protective layer of .

Thermal and optical properties

Silicon monoxide (SiO) in its solid polymeric form exhibits notable thermal characteristics. The material sublimes appreciably at temperatures above 1,000 °C and becomes volatile above 1,100 °C, facilitating its use in processes. Thermal stability is maintained up to approximately 800 °C, beyond which into silicon and begins around 850 °C and proceeds significantly up to 1,050 °C. Vapor pressure variations with temperature are critical for evaporation applications, where SiO's equilibrium vapor pressure over a mixture of silicon and silica follows the logarithmic relation: log⁡10(pPa)=13.613−1.785×104T\log_{10} \left( \frac{p}{\mathrm{Pa}} \right) = 13.613 - \frac{1.785 \times 10^4}{T} with TT in kelvin; this equation describes pressures in the range relevant to high-temperature evaporation rates. The evaporation coefficient is approximately 0.007 over such mixtures, indicating moderately efficient sublimation under vacuum. These properties underscore SiO's heat resistance, enabling its application in thermally demanding environments like protective coatings that withstand elevated temperatures without rapid degradation. Optically, evaporated thin films of SiO display strong absorption in the ultraviolet region (below 0.4 μm), which tails into the , rendering the material opaque or brownish in thicker deposits. In the , laboratory measurements reveal a prominent absorption maximum at around 10 μm, attributed to Si-O vibrational modes, with transparency in broader mid- to far-IR ranges outside this band. The of SiO films is approximately 1.97 at visible wavelengths (e.g., 0.6 μm), decreasing slightly toward the IR, which supports its utility in optical multilayer designs. As an insulator, solid SiO possesses a dielectric constant of about 6 for pure forms, with high that resists breakdown under electric fields, making it suitable for capacitive and insulating layers in thin-film devices. This value positions SiO between elemental (dielectric constant ~11.7) and (~3.9), reflecting its intermediate oxidation state and amorphous structure.

Reactivity and stability

Silicon monoxide (SiO) exhibits distinct reactivity depending on its form and environmental conditions, primarily driven by its +2 , which renders it prone to further oxidation. In the gaseous molecular form, SiO reacts with oxygen to form (SiO₂), as described by the reaction 2SiO + O₂ → 2SiO₂, though this process is endothermic with high activation barriers, making it kinetically slow at (upper limit rate constant of 4.5 × 10⁻¹⁵ cm³ s⁻¹ at 293 K). Upon exposure to air, solid SiO undergoes gradual oxidation, forming a protective passivation layer of SiO₂ that limits further degradation, similar to the native on surfaces. This passivation enhances the durability of SiO films, which remain abrasion-resistant and non-hygroscopic under ambient conditions below 200°C. In the gas phase, SiO interacts with H₂O to form stable intermediates like Si(OH)₂ rather than directly decomposing to SiO₂ and H₂, as the latter pathway is endothermic (ΔH = 56.6 kJ/mol) and features high activation energies, rendering it inefficient at typical temperatures. SiO films deposited under certain conditions (e.g., grazing incidence angles) show increased instability in atmospheres, buckling due to hydrolytic stress. Stability of SiO varies markedly between inert and oxidizing environments. In inert atmospheres such as vacuum or dry , solid SiO maintains at temperatures up to 200°C, with films exhibiting long-term durability after (e.g., 250°C annealing). Conversely, in oxidizing environments, reactivity accelerates, with SiO burning in pure oxygen to yield SiO₂, driven by the thermodynamic favorability at elevated temperatures (e.g., above 1000 K in combustion-like conditions). The gaseous form is particularly reactive in such settings, facilitating rapid oxidation pathways involving radicals like OH, which are exothermic and barrier-free (ΔH ≈ -5.0 kJ/mol for SiO + OH → H + SiO₂). Differences in reactivity between the gaseous and solid forms stem from their structural disparities. Gaseous SiO exists as a with high volatility above 1100°C, enabling efficient participation in gas-phase oxidation and reactions in astrophysical or high-temperature settings. In contrast, the solid polymeric form is an amorphous network, often viewed as a disproportionated of Si and SiO₂, which reduces its reactivity due to polymerization and the encapsulating SiO₂ layer, making it more stable against immediate decomposition in ambient conditions. This polymeric structure confers lower sensitivity to environmental interactions compared to the monomeric gas, though prolonged exposure still promotes slow oxidation.

Structural forms

Gaseous molecular form

Silicon monoxide in its gaseous molecular form exists as the diatomic species SiO, characterized by a strong Si–O bond with significant triple bond character due to the involvement of pπ–pπ multiple bonding interactions, analogous to but with reduced π-backbonding from silicon's larger atomic size. The bond dissociation energy D0D_0 for SiO(g) → Si(g) + O(g) is approximately 800 kJ/mol. The vibrational frequency of the ground electronic state, derived from and , is ωe≈1245\omega_e \approx 1245 cm−1^{-1}, reflecting the stiff nature of the bond. Spectroscopic investigations reveal that the SiO molecule exhibits a permanent of 3.10 in its vibrational (v=0), enabling detection via rotational and vibrational transitions in the , , and regions. These transitions, particularly the fundamental vibrational band near 8 μm and pure rotational lines in the J=1–0 transition at approximately 43 GHz for 28^{28}Si16^{16}O, are crucial for identifying the in laboratory and astrophysical contexts. The diatomic SiO monomer is stable only at elevated temperatures above 1000°C, where it predominates in the vapor phase over polymeric forms; below this threshold, it tends to disproportionate according to the equilibrium 22SiO(g) ⇌ Si(s) + SiO2_2(s), with the forward reaction favored at higher temperatures due to entropy gains from gas production. This transient nature limits its persistence under ambient conditions, as the equilibrium constant KpK_p increases with temperature, reaching values that support significant SiO partial pressures around 1200–1500 K. In laboratory settings, gaseous SiO is generated as a transient through of solid SiO or mixtures of and silica at temperatures exceeding 1000°C, producing monomeric vapor via the reaction Si(s) + SiO2_2(s) → 2SiO(g). Alternatively, of targets in controlled oxygen atmospheres yields SiO monomers, allowing spectroscopic interrogation in expansion-cooled molecular beams. These methods ensure isolation of the diatomic form for studies of its electronic (X1Σ+X^1\Sigma^+) and excited states.

Solid polymeric form

The solid polymeric form of silicon monoxide, denoted as (SiO)_n, adopts an amorphous structure featuring interconnected Si-O-Si chains and Si-Si bonds, with silicon maintaining an average of +2. This network forms a glass-like morphology, often manifesting as nanoscale clusters embedded within an SiO₂-like matrix, connected by suboxide tetrahedra at the interfaces. Upon condensation from the vapor phase, typically via or sublimation, the material solidifies into a brown-black, brittle solid with a of approximately 2.15 g/cm³. Deviations from ideal 1:1 are common, resulting in Si-rich regions that arise from partial during formation, enhancing its heterogeneous character. Electrically, amorphous SiO functions as a wide-bandgap insulator or , with an optical bandgap of about 4 eV for compositions near x=1 in SiO_x, though defect-induced states can enable hopping conduction in Si-rich variants. Paramagnetism originates from unpaired spins at structural defects, such as silicon dangling bonds, as observed via in evaporated films. In contrast to SiO₂, which exhibits a higher (~2.2 g/cm³), a larger bandgap (~9 eV), and greater chemical inertness, the suboxide nature of SiO imparts enhanced reactivity, including facile oxidation to SiO₂ and decomposition into elemental and silica at elevated temperatures.

Synthesis and production

Historical methods

Silicon monoxide (SiO) was first synthesized and reported in 1887 by American chemist Charles F. Mabery, who obtained it as a greenish-yellow, vitreous, amorphous substance through the partial reduction of silica-containing ores, such as or clay, with in an electric furnace like the Cowles process. Mabery's analysis confirmed the composition, with samples yielding 96.45% to 99.88% SiO upon conversion to , and a specific gravity of 2.893. An improved synthesis method was developed in 1905 by Henry Noel Potter, an engineer at Westinghouse, who heated a mixture of granular and silica in a electric furnace to approximately 1,700 °C, producing SiO vapor that condensed into a solid upon cooling. Potter's approach used a slight excess of to minimize residual silica in the product and avoided the formation of or by maintaining low pressure, marking a significant advancement in isolating purer SiO. By the early , SiO was recognized as a silicon suboxide, with the key reaction understood as the equilibrium: SiO2+Si⇌2SiO (g)\text{SiO}_2 + \text{Si} \rightleftharpoons 2\text{SiO (g)} This gaseous intermediate forms at high temperatures and condenses to the solid form, as detailed in Potter's work and subsequent studies. Isolation of stable SiO proved challenging due to its tendency to disproportionate into and , even at moderate temperatures, complicating efforts to obtain pure samples without rapid or conditions.

Modern laboratory and industrial production

In modern laboratory settings, silicon monoxide (SiO) is commonly produced in gaseous form through vacuum thermal evaporation of a mixture of silicon (Si) and silicon dioxide (SiO₂). The mixture is heated to temperatures between 1,800°C and 2,000°C under high vacuum conditions, facilitating the reaction Si + SiO₂ → 2SiO, which generates SiO vapor for subsequent deposition as amorphous thin films. This method allows for controlled deposition rates and is widely used for optical and protective coatings due to the material's volatility above 1,100°C. Chemical vapor deposition (CVD) variants, including plasma-assisted techniques, enable the synthesis of SiO thin films with enhanced uniformity and adhesion. In plasma-enhanced CVD (PECVD), precursors such as (SiH₄) or tetraethoxysilane () are activated in a low-pressure plasma environment to deposit SiO-rich films at substrate temperatures below 400°C, offering scalability for applications. Recent advancements post-2000 incorporate for gas-phase SiO studies, where pulsed lasers (e.g., 266 nm Nd:YAG) ablate targets in oxygen-containing atmospheres to generate SiO molecules for spectroscopic analysis. On an industrial scale, SiO is generated as a during the carbothermic reduction of SiO₂ for silicon metal production, following the reaction SiO₂ + 2C → Si + 2CO, where intermediate SiO forms at temperatures exceeding 1,800°C in submerged arc furnaces. The gaseous SiO can be captured and purified through to separate it from contaminants like and unreacted silica, yielding material suitable for further processing. Laboratory production of SiO achieves yields up to 99% purity through optimized and , though challenges persist with contamination from residual oxygen or carbon impurities, necessitating inert atmospheres and high-vacuum systems. A 2022 study demonstrated gas-phase SiO formation via non-adiabatic reactions between atomic and molecular oxygen in single-collision events, providing insights for astrophysical simulations while highlighting potential for high-purity, controlled synthesis.

Applications

Thin film deposition and coatings

Silicon monoxide (SiO) is commonly employed in deposition through thermal evaporation in systems, where solid SiO is heated to temperatures between 1,000 and 1,250 °C to produce vapors that condense as amorphous on substrates such as optical lenses and mirrors. This method, established since the in the industry, allows for precise control of thickness using rate monitoring to achieve uniform layers typically ranging from tens to hundreds of nanometers. During deposition, exposure to residual oxygen often results in , forming SiO_x with compositions intermediate between SiO and SiO_2, which enhances adhesion and stability. The deposited films exhibit a typically in the range of 1.45 to 1.7 in the , depending on deposition conditions like substrate temperature and oxygen , making them suitable for anti-reflective applications by minimizing light reflection at interfaces. These amorphous layers demonstrate low absorption in the near-infrared region, enabling their use in optical components requiring high transmission beyond 1 μm, while compatibility with environments facilitates integration into multi-layer stacks. In electronics, evaporated SiO films serve as passivation layers on semiconductors, such as in high-electron-mobility transistors (HEMTs), where they reduce surface leakage currents and improve breakdown voltage by mitigating trap states. Historically, since the mid-1950s, SiO-based coatings have been widely adopted in the sector for anti-reflective treatments on precision lenses and mirrors, enhancing light transmission in cameras and telescopes. SiO antireflection coatings have also been applied to InP/CdS solar cells to increase short-circuit currents.

Metallurgical and ceramic uses

In metallurgical processes, gaseous (SiO) serves as a key intermediate for transfer during ironmaking in blast furnaces. Formed by the reduction of in coke ash or at high temperatures, SiO gas reacts with carbon-saturated iron to dissolve into the hot metal, contributing to the desired content in . This mechanism is critical for controlling levels, which influence the mechanical properties of subsequent products. In production via carbothermic reduction, SiO emerges as a volatile from the reaction of silica (SiO₂) with carbon, potentially leading to yield losses of 10-20% if not recovered. efforts capture and reintroduce SiO into the process to improve and reduce material waste, often through and reintegration in the furnace. Additionally, SiO facilitates vapor-phase transport in certain techniques, aiding the deposition of layers or related compounds during high-temperature synthesis. SiO is used as a precursor in the production of ceramics, including high-purity silica and . Industrial production of SiO for metallurgical and applications occurs on a significant scale, with global market values exceeding USD 160 million annually as of , reflecting thousands of tons utilized in these sectors. However, the carbothermic synthesis process generates substantial CO emissions, prompting ongoing efforts to mitigate environmental impacts through process optimizations and alternative reduction methods. SiO has also found application as an anode material in lithium-ion batteries, where its nanostructured form accommodates volume expansion during cycling, improving battery performance and capacity.

Astrophysical occurrence

Detection in interstellar medium

Silicon monoxide (SiO) was first detected in the in 1971 through observations of its rotational J=3–2 transition near 130.2 GHz toward the dense Sagittarius B2 (Sgr B2) using the 11 m telescope at . This millimeter-wave revealed emission at a radial velocity consistent with other molecules in the region, confirming SiO as a gaseous in the interstellar environment with an estimated column of approximately 4 × 10^{13} cm^{-2}. Subsequent surveys have identified SiO abundances in circumstellar envelopes surrounding late-type stars, where it often manifests as emission in vibrational states (v=1 or v=2) and rotational transitions such as J=1–0 at 43.4 GHz or J=2–1 at 86.8 GHz, particularly in bipolar outflows from evolved stars like . SiO is also prevalent in supernova remnants, such as , where it traces shocked molecular gas through thermal emission in the ground vibrational state. These detections highlight SiO's role as a shock tracer, with enhanced abundances (up to 10^{-7} relative to H_2) in regions of high-velocity gas interactions. Recent observations using the Atacama Large Millimeter/submillimeter Array (ALMA) have mapped SiO emission in shocked regions of massive star-forming clouds and protostellar outflows, revealing detailed spatial distributions with resolutions down to ~0.1 pc. These studies, spanning 2015 to 2025, report column densities ranging from ~10^{12} to 10^{16} cm^{-2} in environments like infrared dark clouds and high-mass protostellar objects, often associated with broad-line profiles indicative of velocities exceeding 20 km s^{-1}. Isotopic variants, including ^{29}SiO and ^{30}SiO, have been observed in dense molecular clouds such as Orion and Sgr B2, enabling determinations of silicon isotopic ratios (e.g., ^{28}Si/^{29}Si ≈ 19–20 and ^{28}Si/^{30}Si ≈ 90–100) that reflect Galactic chemical evolution. In these clouds, SiO abundances correlate with those of H_2O, both elevated in shocked layers where sputtering releases silicon and oxygen from dust grains.

Role in cosmic chemistry

Silicon monoxide (SiO) serves as a critical precursor in the formation of within cosmic environments, particularly through gas-phase oxidation reactions that facilitate the growth of nanoparticles. In these processes, SiO reacts with hydroxyl radicals (OH), such as in the pathway SiO + OH → H + SiO₂, which is nearly thermoneutral with a low barrier, efficient oxidation even at low temperatures typical of interstellar regions. This reaction proceeds via an HOSiO intermediate and supports the of silica (SiO₂) and higher-order s, ultimately contributing to the aggregation of grains essential for . Recent studies highlight non-adiabatic pathways for SiO formation itself, where ground-state atomic (Si) collides with molecular oxygen (O₂) in a barrierless, exoergic reaction involving intersystem crossings, producing highly rovibrationally excited SiO without forming SiO₂. This mechanism underscores SiO's role as a fundamental building block for interstellar nanoparticles in cold molecular clouds. In astrophysical shocks, SiO is released into the gas phase via of grains, where high-velocity impacts by neutral particles erode grain cores, injecting SiO and tracing regions of turbulent, high-velocity gas flows. This dominates in C-type shocks driven by , with SiO abundances enhanced by factors of up to 10⁶ relative to quiescent interstellar values. Such releases are particularly evident in protostellar jets, where SiO abundance spikes—reaching at least 10% of total elemental in dusty jets like HH212—signal recent grain disruption and provide kinematic probes of outflow dynamics. Within dark clouds, SiO contributes to silicon chemistry primarily through neutral-neutral reactions, which dominate over ion-molecule pathways due to the prevalence of atomic silicon in excited states at temperatures above 30 K. Key reactions include Si + OH → SiO + H and Si + O₂ → SiO + O, with the latter favored by the higher abundance of O₂, enabling SiO production under dense conditions (n(H₂) ≥ 10⁶ cm⁻³). These processes link SiO to broader networks influencing by facilitating dust grain growth and H₂ formation on surfaces, while also informing the chemical heritage of solar system materials through presolar inclusions. Post-2019 laboratory simulations have advanced understanding of SiO's role in dust formation by modeling the clustering of SiO with SiO₂ to form proto-silicate via , revealing aggregation pathways that mimic interstellar at low temperatures. These studies demonstrate bottom-up assembly of SiO aggregates into larger silicates, supporting efficient production in circumstellar outflows. Furthermore, SiO detections in atmospheres, such as the ultrahot WASP-121b where Si/H ratios exceed stellar values by over 2.5 times, imply ongoing rocky accretion and thermal inversions driven by SiO opacity, with implications for volatile enrichment beyond H₂O ice lines (as of June 2025).

References

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