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Osmium
Osmium (Os, atomic number 76) is a Group 8 HCP platinum-group metal (PGM) with the highest density of any naturally occurring element (22.59 g/cm³), the highest Young's modulus of any pure metal (~550 GPa), a melting point of 3,033 °C, and extreme hardness (~400 HB) — properties that make it the hardest and stiffest of the PGMs and one of the most physically extreme of all elements. Osmium has an HCP crystal structure stable to its melting point with no allotropic transformation, and is one of the least abundant elements in Earth's crust (~0.001 ppb, comparable to Ir and Ru), extracted exclusively as a byproduct of Ni-Cu mining from PGM-rich sulfide ores. Its most significant chemical hazard is ready oxidation to osmium tetroxide (OsO₄, mp 40 °C, bp 130 °C, vapor pressure ~7 mmHg at 20 °C) — a highly toxic, volatile oxidant (OEL ~0.0002 ppm) that causes severe corneal damage and pulmonary edema on exposure; bulk Os metal oxidizes slowly in air at room temperature but Os powder and shavings oxidize more readily. Global annual production is only ~1 tonne, making Os among the rarest commercially available metals.
The Re-Os geochronometer (¹⁸⁷Re → ¹⁸⁷Os, t½ = 41.6 Gyr, β⁻) is one of the most powerful tools in Earth sciences, uniquely dating sulfide minerals, organic-rich black shales, crude oils, and iron meteorites that cannot be dated by Rb-Sr or U-Pb methods. Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os) in marine sediments record the balance between mantle input (low ¹⁸⁷Os/¹⁸⁸Os ~0.127) and continental weathering input (high ~1.4) — the seawater Os isotope curve is a high-resolution proxy for major geological events including large igneous province emplacement, bolide impacts, and glaciation episodes. The K-Pg boundary layer is dramatically enriched in Os (and Ir) — Os isotope ratios in this layer confirm an extraterrestrial (chondritic) source, providing independent support for the Alvarez impact hypothesis. Os is produced by the s-process and r-process in stars; Os isotope patterns in presolar grains from meteorites constrain stellar nucleosynthesis models.
Osmium tetroxide (OsO₄), despite its extreme toxicity, is an indispensable reagent in both biological electron microscopy and synthetic organic chemistry — it is the standard fixative and staining agent for lipid membranes in TEM sample preparation, and a uniquely selective cis-dihydroxylation reagent for alkene functionalization in total synthesis. OsO₄-catalyzed asymmetric dihydroxylation (Sharpless AD reaction, using AD-mix-α/β with (DHQ)₂PHAL or (DHQD)₂PHAL ligands) provides some of the highest enantioselectivities achievable in homogeneous asymmetric catalysis — a reaction for which K. Barry Sharpless received the Nobel Prize in Chemistry in 2001. In TEM specimen preparation, OsO₄ vapor or solution reacts with C=C double bonds in phospholipid fatty acid chains, cross-linking membranes and providing electron-dense Os atoms that give high contrast to lipid bilayers and membranous organelles.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 76 | Group 8, Period 6; 4f¹⁴5d⁶6s²; oxidation states range from −2 to +8; most common are +4 (OsO₂, OsCl₄) and +8 (OsO₄). Os⁸⁺ in OsO₄ is the highest stable oxidation state of any element under ambient conditions. Os is one of only four elements (with Ru, Xe, and possibly Pt) with a confirmed stable +8 oxide. |
| Atomic Mass | 190.23 u | Seven naturally occurring isotopes: ¹⁸⁴Os (0.02%, Stable*), ¹⁸⁶Os (1.59%, Stable*), ¹⁸⁷Os (1.96%), ¹⁸⁸Os (13.24%), ¹⁸⁹Os (16.15%), ¹⁹⁰Os (26.26%), ¹⁹²Os (40.78%). ¹⁸⁷Os is the radiogenic daughter of ¹⁸⁷Re (t½ = 41.6 Gyr), making Os isotope ratios central to the Re-Os geochronometer. |
| Density (20 °C) | 22.59 g/cm³ | Densest naturally occurring element — marginally denser than iridium (22.56 g/cm³); the ranking depends on measurement precision and sample purity. Os's extreme density was historically exploited in Os-Ir alloy tips for fountain pen nibs and in precision instrument pivots requiring maximum hardness and density in minimum volume. |
| Melting Point | 3,033 °C (3,306 K) | Fourth highest melting point of any element after W (3,422 °C), Re (3,186 °C), and Os is above Ta (3,017 °C). Processing requires arc melting or powder metallurgy under inert atmosphere; Os cannot be easily hot-worked and is typically used as sintered powder compacts or as alloying additions to Ir and Pt. |
| Boiling Point | 5,012 °C | Very high boiling point; however the more practically important vapor-phase Os species is OsO₄ (bp 130 °C), which forms at much lower temperatures and constitutes the primary handling hazard for all Os materials including bulk metal at elevated temperatures. |
| Thermal Conductivity | 87.6 W/m·K | Moderate thermal conductivity for a PGM — lower than Ir (147 W/m·K) and Ru (117 W/m·K). Adequate for the small-volume precision components (contacts, pivot bearings) where Os is typically used; not a primary selection criterion for Os applications. |
| Electrical Resistivity | 81 nΩ·m (20 °C) | Moderate resistivity for a PGM; Os is a superconductor below Tc = 0.66 K — studied in the context of HCP elemental superconductors. OsO₂ is a metallic conductor (~60 µΩ·cm) and has been investigated as a corrosion-resistant conductive coating. |
| Crystal Structure | HCP, a = 2.734 Å, c = 4.319 Å; c/a = 1.580 (close to ideal 1.633) | HCP structure with near-ideal c/a ratio; stable from RT to melting point with no allotropic transformation. The HCP structure with strong d-electron bonding gives Os its extreme incompressibility — Os has the smallest isothermal compressibility of any element at ambient conditions, relevant to its use in ultra-hard alloys and reference materials for high-pressure studies. |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Tensile Strength | 410–500 MPa (estimated) | Source value noted as estimated — direct measurement is difficult due to Os's extreme brittleness at room temperature and limited availability of bulk samples. Os is one of the most brittle of the PGMs; it fractures intergranularly and cannot be drawn into wire or rolled into sheet without extreme difficulty. |
| Young's Modulus | 550 GPa | Highest Young's modulus of any pure metal — slightly higher than Ir (528 GPa). The extreme stiffness reflects Os's very strong d-electron bonding and small atomic volume. Used as a reference point in computational materials science for validating DFT calculations of elastic constants in transition metals. |
| Hardness | 392 HB | Hardest of the platinum-group metals and one of the hardest pure elements. Os's hardness combined with its extreme density makes Os-Ir alloys (osmiridium, iridosmine) the material of choice for the most demanding wear-resistant precision applications — pen nibs, instrument pivots, and electrical contacts. |
| Poisson's Ratio | 0.25 | Typical for HCP metals with strong d-electron character. Used in elasticity modeling of Os alloy contacts under cyclic mechanical loading and in high-pressure equation-of-state studies of Os compressibility. |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Oxidation States | +8 (OsO₄); +4 (OsO₂, OsCl₄); +3, +6; range −2 to +8 | OsO₄ (Os⁸⁺) is the highest confirmed stable oxidation state of any element. Os displays the widest oxidation state range of any PGM. Lower states (+2, +3) occur in Os carbonyl clusters and organometallic compounds used as catalysts and electron microscopy stains for specific biological targets. |
| Corrosion Resistance | Very high in bulk form; resistant to most acids; bulk Os oxidizes slowly in air at RT; fine powder and heated metal oxidize readily to volatile OsO₄ | Bulk Os is resistant to HCl, H₂SO₄, and HNO₃ at room temperature, but is attacked by oxidizing fused alkalis. The primary hazard of any Os material — including bulk metal — is OsO₄ formation on heating or grinding; OsO₄ vapor is immediately hazardous at sub-ppm concentrations and causes irreversible corneal damage. |
| Volatile Compounds | Osmium tetroxide (OsO₄): mp 40 °C, bp 130 °C, vapor pressure ~7 mmHg at 20 °C; highly toxic (OEL 0.0002 ppm TWA) | OsO₄ is simultaneously Os's greatest hazard and most important chemical application — it is a selective dihydroxylation agent for alkenes (Sharpless AD reaction, Nobel Prize 2001) and the standard fixative/stain for lipid membranes in biological TEM. All Os handling must be performed in a fume hood; OsO₄ solutions are stabilized by storage at 4 °C in sealed glass ampules. |
| Identifier | Value |
|---|---|
| Symbol | Os |
| Atomic Number | 76 |
| CAS Number | 7440-04-2 |
| UN Number | UN3089 (powder) |
| EINECS Number | 231-114-0 |
| Isotope | Type | Notes |
|---|---|---|
| ¹⁸⁴Os | Stable* | 0.02% natural abundance; I = 0; Stable* — alpha decay to ¹⁸⁰W is energetically allowed; t½ > 5.6 × 10¹³ yr (experimental lower limit). The least abundant Os isotope and the least studied geochemically due to its low natural abundance and proximity to the isobar ¹⁸⁴W, which must be corrected for in MC-ICP-MS measurements. |
| ¹⁸⁶Os | Stable* | 1.59% natural abundance; I = 0; Stable* — alpha decay to ¹⁸²W measured: t½ = 2.0 × 10¹⁵ yr. Used as a reference isotope in Os isotope ratio normalization (¹⁸⁶Os/¹⁸⁸Os). Small excesses of ¹⁸⁶Os from ¹⁹⁰Pt alpha decay (t½ = 6.5 × 10¹¹ yr) in Pt-rich mantle domains constrain the history of ancient subducted oceanic crust in the deep mantle. |
| ¹⁸⁷Os | Stable | 1.96% natural abundance; I = 1/2, NMR-active. The radiogenic daughter of ¹⁸⁷Re (t½ = 41.6 Gyr, β⁻) — the basis of the Re-Os geochronometer, which uniquely dates sulfide minerals (molybdenite, pyrite), Re-rich black shales, crude oils, and Fe-Ni meteorites. ¹⁸⁷Os/¹⁸⁸Os ratios measured by N-TIMS or MC-ICP-MS to ±0.002% are the primary tool for tracing mantle vs. crustal Os contributions in magmas and sediments. |
| ¹⁸⁸Os | Stable | 13.24% natural abundance; I = 0. The primary normalization isotope for Os isotope ratio measurements — ¹⁸⁷Os/¹⁸⁸Os is the standard Re-Os geochronology ratio. ¹⁸⁸Os(n,γ)¹⁸⁹Os (σ = 4.7 barn) produces ¹⁸⁹ᵐOs (t½ = 5.8 hr), which has been investigated as a radiopharmaceutical precursor for ¹⁸⁸Re/¹⁸⁸Os generators for targeted radionuclide therapy. |
| ¹⁸⁹Os | Stable | 16.15% natural abundance; I = 3/2, NMR-active. ¹⁸⁹Os NMR (chemical shift range ~3,000 ppm; quadrupolar broadening limits routine use in low-symmetry environments) characterizes Os coordination in organoosmium compounds, Os carbonyl clusters, and OsO₄ adducts used in asymmetric synthesis research. |
| ¹⁹⁰Os | Stable | 26.26% natural abundance; I = 0. Used as a reference isotope alongside ¹⁸⁸Os in Os isotope ratio measurement by MC-ICP-MS; the ¹⁹⁰Os/¹⁸⁸Os ratio is used for instrumental mass bias correction in Re-Os geochronology when N-TIMS is not available. |
| ¹⁹²Os | Stable | 40.78% natural abundance — the most abundant Os isotope; I = 0. Dominates natural Os isotopic composition. ¹⁹²Os(n,γ)¹⁹³Os (σ = 2.0 barn) produces ¹⁹³Os (t½ = 30.1 hr, β⁻, 139 keV gamma), used as a radiotracer in Os dissolution and corrosion studies in nuclear and chemical process environments. |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Biological TEM Staining & Fixation | OsO₄ solution (1–2% aqueous or buffered); OsO₄ vapor (from crystals in sealed container) | OsO₄ reacts with unsaturated lipid fatty acid chains (C=C bonds) to cross-link and stain phospholipid bilayers — the standard secondary fixative in biological TEM that provides high contrast to membranes, myelin, and lipid droplets. OsO₄ also reacts with proteins (amino acid side chains) contributing to overall tissue contrast. All handling requires a fume hood due to acute toxicity at sub-ppm vapor concentrations. |
| Re-Os Geochronology & Isotope Geochemistry | Enriched ¹⁸⁵Re and ¹⁹⁰Os IDMS spikes; Os standard solutions (DROsS, UMCP Os standards) | Re-Os dating (¹⁸⁷Re→¹⁸⁷Os, t½ = 41.6 Gyr) dates molybdenite (MoS₂, highest Re/Os ratio of any mineral), black shales, crude oils (petroleum systems analysis), and iron meteorites — materials inaccessible to U-Pb or Rb-Sr methods. Measurement by N-TIMS (negative ionization as OsO₃⁻) or MC-ICP-MS achieves ±0.001–0.01% precision on ¹⁸⁷Os/¹⁸⁸Os ratios. |
| Asymmetric Dihydroxylation Catalysis | OsO₄ (catalytic, 0.2–5 mol%) or K₂OsO₄·2H₂O (air-stable precatalyst) with NMO or K₃Fe(CN)₆ co-oxidant | The Sharpless asymmetric dihydroxylation (AD) reaction uses OsO₄ with chiral cinchona alkaloid ligands (AD-mix-α/β) to convert prochiral alkenes to enantiopure vicinal diols in up to >99% ee. The AD reaction is a key step in total synthesis of complex natural products and pharmaceutical intermediates — among the highest-impact uses of Os despite the minute quantities involved. |
| Thin-Film Deposition & Hard Coatings Research | Os sputtering targets (99.9%); Os powder for thermal spray | Os thin films and Os-containing hard coatings are investigated for ultra-high hardness applications — OsB₂ (synthesized under pressure) has been reported with hardness rivaling diamond in certain orientations. Os sputtering targets produce conductive Os coatings for SEM specimen preparation as an alternative to Au or Pt, providing finer grain size and higher resolution imaging capability. |
| Isotope Geochemistry — Mantle & Meteorite Studies | Os standard solutions; Os metal dissolved for spike calibration | Os is a strongly siderophile (iron-loving) element concentrated in Earth's core and in mantle peridotites relative to the crust — the ¹⁸⁷Os/¹⁸⁸Os ratio distinguishes pristine mantle Os (~0.127) from crustal Os (~1.4), enabling identification of mantle plume components in basalts and of meteoritic material in impact layers. Chondritic ¹⁸⁷Os/¹⁸⁸Os (~0.1268) serves as the reference for "primitive" solar system Os. |
Industrial & Commercial Applications
| Sector | Form / Grade Used | Description |
|---|---|---|
| Wear-Resistant Alloys (Osmiridium / Iridosmine) | Os-Ir natural alloys or sintered Os-Ir PM compacts (typically 30–80% Os) | Natural Os-Ir alloys (osmiridium ~35% Os, iridosmine ~50% Os) are among the hardest naturally occurring materials. Sintered Os-Ir alloy tips have been used for fountain pen nibs, instrument pivot bearings, and record player styli requiring extreme hardness and wear resistance in minimum volume. The combination of Os density, hardness, and Ir toughness outperforms either pure metal. |
| Electrical Contacts | Os-Ir, Os-Ru alloy contacts (99.9%+ Os content) | Os alloy contacts resist arc erosion and mechanical wear in precision relays, scientific instruments, and aerospace switching systems requiring millions of operating cycles without dimensional change. Os's extreme hardness minimizes contact deformation under mechanical load and arc erosion under electrical switching. |
| OsO₄ Organic Synthesis | OsO₄ crystals or solution (from Os metal oxidation); K₂OsO₄·2H₂O (stable solid) | OsO₄ is used stoichiometrically or catalytically (with co-oxidant regeneration) for cis-selective dihydroxylation of alkenes in pharmaceutical and fine chemical synthesis. K₂OsO₄·2H₂O is the preferred laboratory reagent as an air- and moisture-stable OsO₄ equivalent that regenerates OsO₄ in situ under reaction conditions. |
| Catalysis | OsO₄, Os carbonyl clusters, Os/C heterogeneous catalysts | Os-based catalysts have been studied for ammonia synthesis (the original Haber-Bosch catalysts tested Os before settling on Fe), Fischer-Tropsch synthesis, and hydrogenation reactions. Os carbonyl clusters are model catalysts for studying metal-metal bonding and catalytic mechanisms in organometallic chemistry research. |
| Purity | Description |
|---|---|
| 99.9% (3N) | High-quality osmium suitable for research, catalyst preparation, and powder metallurgy applications. |
| Synonym / Alternative Name | Context |
|---|---|
| Osmium metal | Commercial designation for elemental Os in powder, pellet, or target form; used in PGM commodity market documentation, Johnson Matthey PGM market reports, and procurement specifications for Os-Ir alloy and OsO₄ production. |
| Osmium precious metal | Trade designation used in PGM commodity markets to distinguish Os from base metals; used in London Platinum and Palladium Market (LPPM) and precious metals refinery documentation, though Os is traded in far smaller volumes than Pt, Pd, or Rh. |
| Elemental Osmium | Scientific designation distinguishing the pure element from OsO₄, K₂OsO₄, Os carbonyl clusters, and other Os compounds; used in geochemistry and materials science literature specifying the metallic form in Re-Os geochronology and thin-film deposition studies. |
| Element 76 | Periodic table designation; used in XRF/ICP-MS analytical software, nuclear data libraries (ENDF/B), and geochemical databases (PetDB, GEOROC) where atomic number is the primary identifier for PGM geochemistry and cosmochemistry studies. |