Niobium

Niobium
Atomic number	41
Symbol	Nb
Group	5 (vanadium group)
Boiling point	4744 °C
Electron configuration	[Kr] 4d4 5s1
Density	8.57 g/cm^3
Discovery	Charles Hatchett (1801)
Melting point	2477 °C
Block	d
Phase STP	Solid
Oxidation states	+5, +4, +3
Wikidata	Q1046

Niobium (symbol Nb, atomic number 41) is a silvery-gray transition metal in group 5 of the periodic table. It is a moderately hard, ductile metal with a Mohs hardness comparable to titanium and iron. Niobium is stable in air at room temperature but forms a protective oxide film and is highly corrosion-resistant. In elemental form it is solid at standard conditions. In compounds niobium most commonly attains a +5 oxidation state, but +3 and other states occur. The name “niobium” comes from Niobe (daughter of Tantalus) in Greek mythology, reflecting its chemical similarity to tantalum. (It was once called “columbium” in older literature.) Niobium’s unique combination of high melting point, low thermal neutron absorption, and superconducting behavior at low temperatures makes it important for advanced steels, aerospace alloys, superconducting magnets, and high-tech applications.

Overview and Key Facts

Niobium (Nb) is in period 5, group 5 (the vanadium group) of the d-block. It has atomic weight about 92.91 u. At STP it is a lustrous light-grey metal (molten above about 2750 K). The element’s density is about 8.57×10^3 kg/m^3 (so ~8.57 g/cm^3). Its most stable isotope is ^93Nb (100% natural abundance). Common oxidation states are +5 and +3; it also forms compounds in oxidation states +4, +2 and even −1 in some cluster or complex compounds, but Nb(V) chemistry dominates. Niobium metal crystallizes in body-centered cubic (bcc) form (with some subtle anisotropy of thermal expansion).

Key physical constants include a melting point around 2750 K and boiling point near 5017 K (in the 2700–2800 K and 4900–5000 K ranges, respectively). Its electrical resistivity at 20 °C is about 152 nΩ·m (moderately low), and it becomes a superconductor below a critical temperature of 9.2 K – the highest Tc of any elemental superconductor. In the normal state niobium is a paramagnetic metal (unfilled d-band), and it has the largest magnetic penetration depth (weak magnetic responses) of any metal.

Niobium’s electron configuration is [Kr] 4d^4 5s^1, giving it five valence electrons. On the periodic table its neighbors are zirconium (Z=40) and molybdenum (42); it is analogous to tantalum (Ta) below it. In periodic trends, Nb’s atomic radius (~146 pm covalent) is larger than vanadium’s (about 134 pm) but similar to tantalum’s (because of lanthanide contraction). Its first ionization energy (~658 kJ/mol) and electronegativity (Pauling ~1.6) are typical for group-5 transition metals: slightly lower EN than vanadium (1.63) and higher than tantalum (1.5). Hence niobium is only weakly electronegative and its metal behaves in a fairly metallic, reducing manner (preferring to give up electrons rather than attract).

In air niobium does not burn at room temperature due to its passive oxide, though it will oxidize (to Nb2O5) if heated above ~200 °C. It resists attack by most acids (even aqua regia), but is dissolved by hydrofluoric acid or fused alkali. These chemical traits result in it being used for corrosion-resistant applications and medical implants (its inertness makes it hypoallergenic). Physically, pure niobium is relatively soft and highly ductile; alloying or impurities (e.g. oxygen, nitrogen, carbon) will harden it.

Atomic Structure and Electron Configuration

Niobium has 41 electrons arranged with krypton core plus 4d^4 5s^1 in its ground state. Its valence shell effectively has five electrons (group number 5). The presence of five valence electrons (two in 5s, three in 4d orbitals) is typical for group-5 elements, but Nb’s configuration leads to some anomalies. For example, unlike lighter group-5 vanadium which has [Ar]3d^3 4s^2, niobium’s 5s electron partially mixes into 4d, giving [Kr]4d^4 5s^1. This makes niobium somewhat unusual but not unique among middle-transition elements (ruthenium and rhodium show similar exceptions).

The atomic or covalent radius of niobium is about 146 pm. Compared to vanadium below (atomic radius ≈134 pm) and tantalum above (≈146 pm), niobium’s radius reflects the filling of the 4d shell and the so-called lanthanide contraction (which keeps Ta’s radius similar). As a result, niobium’s size, density and metallic bonding are intermediate to vanadium and tantalum.

Electronegativity (Pauling scale) is about 1.6 for Nb, slightly lower than vanadium (1.63) and higher than tantalum (1.5). The first ionization energy is ~6.76 eV (≈658 kJ/mol). These values mean niobium is not strongly electron-withdrawing; it behaves mainly as a metal.

Across the periodic table, niobium fits the expected trends: atomic size increases down the group, electronegativity and ionization energy generally decrease down the group, although Niobium’s first IE is marginally higher than vanadium’s due to the extra filled shell. Niobium’s metallic character (electrical conductivity, ductility, paramagnetism) is in line with being a transition metal with a partially filled d-band.

In summary, Nb has a [Kr]4d^4 5s^1 configuration, five valence electrons, an atomic radius ~146 pm, electronegativity ~1.6, and first ionization of 6.76 eV. Its electron configuration and trends are dominated by its position in group 5 of the d-block.

Isotopes and Nuclear Properties

Naturally occurring niobium is essentially 100% the isotope ^93Nb (atomic mass 92.906 u). No other stable isotopes exist for Z=41. ^93Nb is unique among elements in being monoisotopic. This isotope has nuclear spin 9/2^+, a magnetic dipole moment of +6.17 nuclear magnetons, and a slight electric quadrupole moment (~–0.3 barns). ^93Nb’s nuclear spin makes niobium moderately easy to study by nuclear magnetic resonance (Nb NMR is sometimes used in solid-state studies).

A few long-lived radioisotopes of niobium are known: ^92Nb has a half-life of ~34.7 million years, and ^94Nb about 20,400 years. These are found only in trace amounts and mainly arise from cosmic-ray interactions and nuclear analysis. No traditional dating methods use niobium isotopes, since the short half-lives and cosmogenic origins limit their utility. Niobium-92 and -94 have been detected in nature only via highly sensitive measurements of irradiated samples. Heavier radioisotopes (95, 96, etc.) have shorter half-lives and are found in nuclear reactors or produced in labs (e.g. ^95Nb t1/2 ~35 days, ^96Nb ~23 hours).

Because niobium has only one stable isotope, elemental niobium has an atomic weight based on ^93Nb alone. Niobium does not have radioisotopes useful for traditional radiometric dating of rocks or archaeology. Its stable isotope is not a major product of natural decay chains; it is not a fission product of uranium (no fertile-to-radioactive yield). The long-lived ^92Nb likely plays a minor role in very early Solar-System chronologies, but its very low abundance makes it insignificant for common dating methods.

In nuclear reactors, niobium’s low neutron-capture cross section (about 1.15 barns thermal) means it does not significantly become radioactive and is relatively transparent to neutrons (useful in nuclear hardware, see Applications). The combination of ^93Nb’s nuclear properties (spin, moments) and the absence of other stable isotopes make niobium of specialized interest (e.g. in studying cosmic ray muon interactions), but not broadly relevant to geologic dating or medicine.

Allotropes and Typical Compounds

Niobium has no allotropes in the sense of chemically distinct solid phases at ambient conditions. The elemental metal exists only in its metallic form, which is body-centered cubic. (Some research has found subtle deviations from perfect cubic symmetry at low temperatures, but in practical terms Nb is just the one metal phase.) There are no known molecular forms of niobium (unlike e.g. carbon allotropes). Under extreme conditions (very high pressure) a more complex orthorhombic phase may appear, but this is not encountered under normal conditions.

Niobium’s chemistry parallels tantalum’s. Its most important compounds reflect its high +5 oxidation state, but a variety of stable binary and complex compounds exist:

• Oxides and niobates: The common oxide is niobium(V) oxide, Nb₂O₅, a white solid and acidic oxide. Nb₂O₅ is a starting material for making most other niobium compounds. Niobium dioxide (NbO₂) exists (bronze-colored ceramic) and niobium monoxide (NbO) – a black metallic conductor with Nb–Nb metal bonding – also occurs. There is a lower niobium(III) oxide, Nb₂O₃. Because Nb₂O₅ is amphoteric, it dissolves in strong base to give soluble niobate anions (e.g. [NbO₄]³⁻) or in HF to give complex fluoro-niobates. Mixed niobates form many interesting materials: for example, LiNbO₃ and KTaO₃–LiNbO₃ are ferroelectric/perovskite electronics, and Ca₂Nb₂O₇ extends niobium into oxide clusters (important in photoelectrochemical cells). In general, Nb(5+) forms oxyanions and framework oxides that are used in catalysis and optics.

• Halides: Niobium forms pentahalides in oxidation states +5 and +4. Niobium pentachloride (NbCl₅) is a yellow crystalline solid (sublimes at ~250 °C) with octahedral Nb centers (each Nb bound to five Cl and one bridging). Niobium pentafluoride (NbF₅) is a white solid (mp 79 °C) that is a strong Lewis acid; it hydrolyzes readily. Lower halides also occur, such as NbCl₄ and NbF₃, often as mixed-valence or cluster compounds. Niobium oxy-halides (NbOCl₃, etc.) are also known. The halides are generally synthesized by directly reacting niobium (or Nb₂O₅+halogen) at high temperature, and they serve as precursors to metal or catalysts. Niobium does not form a stable simple iodide at 1:5 stoichiometry (NbI₅ decomposes), but subiodides exist.

• Nitrides, carbides, and sulfides: Niobium carbide (NbC) and niobium nitride (NbN) are two very important compounds. NbC is an extremely hard (Mohs ~9), refractory ceramic (melts ~3500 °C) used for high-speed cutting tools and wear-resistant coatings. NbN (and Nb₂N) are also refractory and are notable superconductors with Tc ~ 16 K (NbN is used in infrared photon detectors). These binary compounds form with strong covalent bonding and high melting points, and they improve high-temperature strength and grain structure when included in alloys. Niobium disulfide (NbS₂) and diselenide (NbSe₂) are layered transition-metal dichalcogenides with interesting low-temperature charge-density-wave and superconducting behavior; they are studied as prototypes of two-dimensional electronic materials. An example is 2H-NbSe₂ (Tc ~ 7 K).

• Hydrides: Niobium can uptake hydrogen to form metal hydrides (e.g. NbH, NbH₂) when exposed to hydrogen gas at high pressure. These are non-stoichiometric metallic phases (like other group 5 metals) and are of interest for hydrogen storage and superconductivity. Bulk NbHx does not exist in isolation easily (it dissociates releasing H₂), but thin surface hydrides can form spontaneously. In practice niobium’s affinity for hydrogen is an issue in superconducting cavities (trapped H can degrade performance) and is managed by vacuum baking.

• Alloys: Although not a single chemical compound, niobium forms many important alloys. Chief among them are ferroniobium (FeNb, an alloy with ~60–70% Nb made by thermite reduction) and Nb-Ti (niobium-titanium, a ductile superconducting alloy used in magnet wire). Niobium-tungsten, niobium-zirconium, and niobium-molybdenum alloys are also used for high-temperature strength. In superalloys (for jet engines and gas turbines), niobium often enters as a strengthening element (e.g. γ″ phase Ni₃Nb precipitates in Inconel 718). In steel, trace Nb forms NbC and NbN inclusions to refine grain structure (see reactivity below).

• Acids and complexes: Aqueous Nb chemistry is limited because Nb(5+) is extremely easily hydrolyzed. In strongly acidic conditions, no simple Nb^5+ ions exist – instead transient [Nb(OH)y]^((5−y)+) and eventually polymeric hydrous oxide precipitates form. In strongly basic solution, soluble polyoxoniobate species (Niobates) appear (e.g. K4Nb6O17·4H2O). These complex anions are used in specialized catalysts and research. There is even one known “niobiumate” where Nb is formally –1 (in [Nb(C4H6)3]^−), a curiosity of cluster chemistry.

Allotropes: Aside from the metallic form, niobium has no allotropes. It does not form graphite-like or polymeric elemental networks. The metal is the only stable form, and it is bcc at all achievable pressures and temperatures (above the solidus).

In summary, niobium’s characteristic compounds include Nb₂O₅ (and derived niobates), NbCl₅/NbF₅ (Lewis-acidic halides), refractory carbides and nitrides (NbC, NbN), and mixed-metal alloys (FeNb, Nb-Ti, etc.). In all, Nb chemistry is dominated by the +5 state and strong metal–oxygen bonds; lower oxidation states often involve metal–metal bonding or interstitial compounds, lending niobium unique materials properties.

Physical Properties

Niobium is a lustrous, silvery-gray metal with metallic bonding. It is quite strong and dense: the density is roughly 8.57×10^3 kg/m^3 (20 °C). It is also heavy enough that in air it is clearly metallic in weight, heavier than aluminum or magnesium but lighter than most late-transition metals. At standard conditions niobium is ductile and malleable (similar to titanium): a thin foil of pure niobium is flexible. It has Mohs hardness similar to titanium (around 6), meaning moderate scratch resistance. With impurities (oxygen, carbon, etc.) niobium is often harder and used for wear-resistant alloys.

Niobium exhibits high melting and boiling points: it melts at about 2750 K (2477 °C) and boils around 5017 K (4744 °C). These values make it one of the refractory metals. Its heat of fusion is ~30 kJ/mol and heat of vaporization ~690 kJ/mol. The specific heat at room temperature is ~24.6 J/(mol·K) (≈0.26 J/(g·K)), typical of metals. It has a respectable thermal conductivity (~53.7 W/m·K), meaning it conducts heat well, though less than copper or silver.

In the solid state, niobium adopts a body-centered cubic (bcc) crystal structure from room temperature up to its melting point. High-precision measurements have indicated minor anisotropy in thermal expansion along crystallographic axes, but for practical purposes Nb is treated as cubic. (Under very high pressures an orthorhombic polymorph has been observed, but only above ~60 GPa.)

Magnetic and electrical properties: Above 9.2 K, niobium is metallic and paramagnetic (there are unpaired d electrons). It has magnetic susceptibility typical of a Pauli paramagnet (weak, positive). Below the superconducting critical temperature (9.2 K at ambient pressure), niobium becomes a superconductor of type-II, with zero electrical resistance and expulsion of magnetic flux (Meissner effect) for fields below ~200 mT. Its superconducting penetration depth and coherence length are record-setting among elements. In the normal state at 20 °C, the electrical resistivity is ~152 nanohm·m (0.152 μΩ·cm).

In optics/electron spectroscopy, metallic niobium has no visible color (it is grey-white) and a typical metal sheen. In ultraviolet/visible atomic spectra, niobium has strong spectral lines in the near-UV and violet (for instance around 294 nm, 316 nm) used in discharge lamps or plasma diagnostics, but these are specialized (not widely used reference lines).

Niobium’s chemistry yields oxide films that are often colorful. Anodizing niobium (forming an oxide layer of varying thickness) can produce an array of iridescent colors from gold to blue, via thin-film interference; this is why niobium is popular for body jewelry. These colors are not from pigments but from controlled oxide layers.

Finally, niobium is notable for its neutron capture cross-section: thermal neutrons are only rarely captured by ^93Nb (cross-section ~1.15 barns). This low absorption means that niobium is “transparent” to slow neutrons. As a result, niobium is used in nuclear engineering where non-absorbing structural metals are needed.

Chemical Reactivity and Trends

Niobium is a refractory metal that is remarkably inert under many conditions. At room temperature it is almost impervious to water or air, forming only a very thin and stable Nb₂O₅ oxide layer. Bulk niobium metal does not react with water or oxygen at ambient temperature. When heated, niobium vigorously reacts with oxygen starting around 200–400 °C, burning to give Nb₂O₅ (with a brilliant white flame at very high heat). It is also flammable in iodine vapor or when finely divided.

Most acids do not dissolve niobium due to passivation. Concentrated nitric, sulfuric, phosphoric, and even aqua regia have little effect on cold niobium metal. This corrosion resistance comes from the spontaneous formation of an oxide layer that protects the metal surface. Only hydrofluoric acid (HF) or hydrofluoric/nitric mixtures will attack niobium significantly, since HF breaks down oxides. Concentrated sulfuric acid at elevated temperature can oxidize it to Nb₂O₅. Highly oxidizing melts (e.g. fused alkali hydroxides or alkaline carbonates) can dissolve Nb by converting it to niobate salts.

Niobium also resists alkalis to an extent, but hot concentrated NaOH/KOH will react, especially at high temperature. Overall, the metal is amphoteric: it dissolves in very strong acid or very strong base, but is passive otherwise. Its strong affinity for oxygen and fluorine make it tolerant of aggressive fluorine chemistry (hence use in nuclear fuel reprocessing equipment, where HF is present).

In solid state chemistry, niobium frequently forms direct metal–metal bonds in low-oxidation compounds. Compounds like NbO (niobium monoxide) have a crystal structure where about one quarter of Nb atoms are unoxidized and bond directly to each other (Nb–Nb distance < 300 pm). Similarly, cluster compounds of Nb exist (e.g. Nb₆Cl₁₈ where six Nb atoms form an octahedral cluster bridged by chlorine). This tendency for Nb–Nb bonding is characteristic of its nonaqueous chemistry in oxidation states <+5; above that, Nb behaves as an isolated cation.

Niobium’s industrial chemistry exploits these reactivities. For example, in steel production Nb atoms in molten iron will tie up carbon and nitrogen by forming finely dispersed NbC and NbN particles (with strong covalent bonds). These precipitates greatly refine the steel’s microstructure. Because NbC/NbN formation removes free carbon/nitrogen from the matrix, steel containing ~0.05–0.1% Nb can be very strong and tough. This is why Nb is called a microalloying element in steels, similar to vanadium and titanium (which form VC and TiC). The Nb precipitates slow grain growth during hot working and inhibit recrystallization, boosting strength and weldability.

Another chemical trend: in aqueous solution niobium has essentially only the +5 state. Dissolved Nb always hydrolyzes to form hydrous oxide Nb₂O₅·nH₂O, precipitating unless very acidic. In strong base, it can form complex oxoanions (polyoxoniobates). Niobium(III) and (IV) exist only in solid compounds or organometallics; in water they would be immediately oxidized to +5.

As a reductant, metallic Nb can reduce many nonmetals at high temperature. It reacts with halogens (F₂, Cl₂, Br₂, I₂) to form NbX₅ (and sometimes lower halides). It burns in nitrogen at ~400 °C to form NbN, and in chlorine at ~200 °C to form NbCl₅. In hydrogen it forms hydrides (as noted above). Niobium does not react with dilute acids or alkalis to release hydrogen gas (unlike e.g. aluminum, which is rapidly oxidized by halogens only in halogen-free acid).

In terms of the metal reactivity series, niobium is quite noble. It is far less reactive than alkali/alkaline earth metals, and even less so than mid-transition metals. It is only slowly attacked by aqua regia, so it is more inert than lead or tin, for example. It behaves more like platinum-group elements in its corrosion resistance.

In summary, niobium’s chemistry is dominated by its high positive oxidation state: it is a good Lewis acid in +5, highly stable oxide former, and very corrosion-resistant. When forced into lower states, it shows strong metal–metal bonding and cluster formation. Its reactivity hierarchy is: it does not corrode easily, will oxidize at high heat, and forms strong covalent bonds in its compounds (especially with C, N, O).

Occurrence and Production

Niobium is the 33rd most abundant element in Earth’s crust (~20 parts per million). This is modestly common for a heavy metal, though its high density means much of it likely sank into Earth’s core during formation. Niobium is not found uncombined; it occurs in a variety of rare minerals, usually together with tantalum.

The chief niobium minerals are pyrochlore (a complex calcium–niobium oxide-fluoride) and columbite-tantalite (Nb,Ta-bearing iron-manganese niobate). Columbite Fe,Mn)Nb₂O₆] and tantalite Fe,Mn)Ta₂O₆] form a continuous series (“coltan”) and are commonly mined for tantalum; they typically contain ~30–70% Nb₂O₅ with the rest Ta₂O₅. Large pyrochlore deposits are found in carbonatite and alkaline intrusive rocks. Examples of niobate minerals include euxenite ((Y,Ca,U,Nb,Ta) oxide) and fergusonite (Y(Ta,Nb)O₄), though these are rarer. Niobium is often associated with rare-earth element (REE) minerals and uranium–thorium minerals in such ores.

Commercially, niobium is produced mostly from pyrochlore-rich ores. The largest known vein-type deposits are in Brazil (Minas Gerais region) and Canada. Brazil accounts for roughly 70–90% of world niobium production. The two biggest Brazilian mines are the Araxá and Catalão deposits (both in carbonatite intrusions), operated by Companhia Brasileira de Metalurgia e Mineração (CBMM) and China Molybdenum, respectively. These produce ferro-niobium at a massive scale. In total, Brazil’s output is roughly 80–90% of global supply. Canada’s Niobec mine (Ontario/Quebec) is the next largest, providing most of the rest (~7–10%). A few other countries (e.g. China, Russia) have smaller production, but currently Brazil and Canada dominate. Historically, niobium (as columbium) was also mined in Nigeria and Zimbabwe, and tantalum mining in the Democratic Republic of Congo yields some niobium as a by-product, but these supplies are comparatively minor.

Extraction of niobium metal begins with concentration of ore to Nb₂O₅ (often together with Ta₂O₅). A typical process is as follows: the ore is treated with hydrofluoric acid to form soluble fluoro-niobate complexes. In the classic Marignac process, potassium fluoride and organic solvents are used to separate niobium from tantalum by fractional precipitation of K₂NbOF₅·H₂O (niobium) vs. K₂TaF₇ (tantalum). Modern methods often use solvent extraction or ion exchange to achieve the separation. Once purified Nb₂O₅ is obtained, it is converted to niobium metal by reduction.

Principal metal production routes include:

• Aluminothermic reduction: Niobium oxide mixed with iron oxide and aluminum powder will undergo an exothermic thermite reaction. The result is alumina (Al₂O₃) plus an alloy called ferroniobium (Fe–Nb, ~60–75% Nb). Ferroniobium is the main commercial form of niobium, used directly in steelmaking.
• Electrolysis: Molten salts such as K₂NbF₇ dissolved in molten alkali chlorides can be electrolyzed, yielding niobium metal at the cathode. This route yields high-purity metal, but is energy-intensive.
• Chemical reduction: Niobium fluoride (e.g. K₂NbF₇) or oxide can be reduced by sodium or magnesium in a high-temperature reaction, analogous to the Kroll process for titanium. This also gives high-purity powder.
• Hydrogen or carbon reduction: Direct reduction of pure Nb₂O₅ with hydrogen (forming suboxides or metal) or carbon (carbothermic reduction) is not usually used industrially for the highest grades, but can be used for intermediate products.

Today, most usable niobium metal is consumed in alloy form. Thus, many factories produce ferroniobium (an alloy) rather than pure Nb. Only a small fraction is further refined. Leading producers (CBMM in Brazil; Magris Resources at Niobec in Canada; possibly China Molybdenum) make ferroniobium for the steel industry. Pure niobium metal or cathodes (99.9%+) are made for specialized uses (e.g. superconductors, aerospace). Some is also cast into Niobium-Titanium and Niobium-Tin alloys for superconducting wires.

In summary, niobium’s abundance in Earth’s crust (~20 ppm) is moderate. It is mined mainly from highly concentrated rare ores in a few deposits (Brazil and Canada dominate). Most of it enters commerce as Nb₂O₅ concentrate, which is then chemically converted (often via fluoride complexes) and reduced to metal or alloy. Brazil (via CBMM and similar) is the world leader, controlling the majority of supply.

Applications and Technology

Niobium’s unique properties make it critical in several high-technology sectors. The vast majority of niobium mined (roughly 90% or more) is used in alloy form to strengthen steel. Even a small addition of niobium (0.01–0.1%) dramatically increases steel’s yield strength, toughness, and weldability. Niobium-containing microalloyed steels are used for automotive frames, pipelines (oil and gas transmission), bridges, and construction. By forming stable NbC and NbN precipitates, niobium allows steel producers to use less weight to achieve the same strength, improving fuel efficiency in vehicles and reducing material usage. Examples include high-strength low-alloy (HSLA) steels and modern pipeline grades (X60–X80), where Nb content is optimized.

About 5–10% of niobium goes into superalloys (nickel, cobalt or iron-base) for jet engines, gas turbines, and rocket engines. Here niobium, often as the γ″-phase Ni₃Nb, provides high-temperature strength. A famous example is Inconel 718 (a Ni-Cr-Fe superalloy) which contains ~3–5% Nb; this alloy operates at ~700–900 °C and is used in supersonic jet parts and rocket nozzles. Some aerospace alloys (e.g. certain titanium-aluminum-niobium alloys) use niobium to stabilize the microstructure and prevent creep. In the space industry, niobium and its alloys (such as Nb–1%Zr) are used for liquid rocket engine components and pressure vessels, owing to its strength at high temperature and weldability.

Another key domain is superconductivity. Niobium is a premier superconductor metal. Niobium–titanium (NbTi) and niobium–tin (Nb₃Sn) alloys are the workhorse materials for superconducting magnets. Tens of thousands of tons of NbTi wire are used in MRI machines and high-energy physics detectors (like the Large Hadron Collider). These alloys carry immense currents with zero loss at cryogenic temperature. Pure niobium (and niobium sheets) is also used for radio-frequency superconducting accelerator cavities (for particle accelerators). Niobium’s high critical temperature (9.2 K) and critical magnetic field allow more operational margin than most other elements (only vanadium, Tc=5.3 K, and Tc=7.2 K beside it). Niobium films (e.g. NbN, Tc~16 K) are used in superconducting sensors and photon detectors.

In the electronics and capacitor industry, niobium pentoxide is an important dielectric. Niobium oxide-based electrolytic capacitors (sometimes called niobium solid capacitors) serve as alternatives to tantalum capacitors. These devices can achieve high capacitance in a small volume. Niobate crystals (especially lithium niobate, LiNbO₃) are widely used in optical modulators, surface acoustic wave devices, and nonlinear optics, thanks to their strong piezoelectric and electro-optic properties. Niobium-doped piezoelectric ceramics (like K0.5Na0.5NbO3) are lead-free ferroelectrics under development for actuators.

Other applications include:

• Electronics: Niobium and its alloys are used in vacuum tubes and electron guns (in early TV tubes, Nb electrodes were stable at high temperature). Niobium oxide films serve as high-k dielectric layers in some specialized semiconductor devices.
• Optics: Niobium coatings enhance the corrosion resistance of glass (e.g. Nb₂O₅ layers for durable optics). Niobium oxide is used in electrochromic windows (it changes color when charged).
• Jewelry/Miscellaneous: Because niobium is hypoallergenic and can be anodized to brilliant colors, it is popular for body jewelry (rings, earrings). It’s also used decoratively (e.g. in anodized watches). Niobium magnets and magnets blanks (Nd-Fe-B magnet glue) are niche uses.
• Chemical: Niobium pentachloride is used as a catalyst and reagent in organic synthesis (for example in transforming certain chlorides to carbonyls). Niobium oxide has been explored as a catalyst for selective oxidation (e.g. propene to acrylic acid catalysts contain Nb^5+). Some high-performance lubricants contain Nb to form protective tribofilms.

A new and emerging application is in battery and energy storage. Brazilian producers are exploring niobium compounds (such as niobium oxide or niobate anodes) for use in advanced lithium-ion batteries. Preliminary research suggests Nb2O5 anodes could allow faster charging and longer cycle life with reduced cobalt use. Niobium-containing supercapacitors are also under investigation.

Finally, nuclear technology takes advantage of niobium’s low neutron absorption: it is used in superconducting magnets for fusion reactors, in shielding and support structures of test reactors, and as spacers in reactor fuel assemblies when chemical compatibility with fuel is needed.

Biology, Environment, and Safety

Niobium has no known biological role. Because it is biologically inert, niobium is generally considered non-toxic. The metal and many of its compounds are poorly soluble in water and are not absorbed into biological tissues. Elemental niobium is hypoallergenic; that is why it is widely used in jewelry and implants (surgical devices, hypodermic needle hubs) without causing allergic reactions.

Niobium dust or powder is a mild irritant: it can irritate eyes, skin, and lungs if inhaled. In animal studies, very high doses of niobium compounds are required before observing acute toxic effects. For example, rats given single injections of soluble niobium salts (niobium pentachloride or niobates) had median lethal doses (LD50) between 10 and 100 mg per kg of body weight. Oral toxicity is much lower; one study reported a 7-day LD50 of ~940 mg/kg for niobium compounds. By comparison, these figures indicate that niobium is far less toxic than heavy metals like lead or cadmium.

As a precaution, workers should avoid breathing niobium dust and avoid ingestion. Niobium metal itself is safe to handle. Soluble niobium chemicals (e.g. NbCl₅, niobates) should be treated as hazardous because they can strongly irritate tissues (especially the lungs and digestive tract). As with all fine metal powders, niobium powder is a potential fire hazard if dispersed in air (it can form flammable dust clouds). Niobium metal readily passivates, however, so routine oxidation is not rapid.

Environmental fate: Niobium compounds are stable and tend to remain in soil or tailings. They do not bioaccumulate significantly in plants or animals. There are no widely recognized environmental standards for niobium, simply because it does not pose the kind of toxicity issues that heavy metals do. Niobium mining has environmental impacts mainly from habitat disturbance and waste water/acid from processing (as with any mining), but the element itself is not particularly mobile or harmful in soil and water.

Because niobium is present in rocks and river sediments where ore is mined, trace amounts can appear in lakes and streams near mining areas; however, niobium concentration in ocean water is extremely low (~0.1 ppb). Typical soil levels (due to weathered rocks) might be tens of ppm, posing no danger. Niobium is not listed as a carcinogen by agencies such as IARC or OSHA, and no occupational exposure limits are specifically assigned for niobium (other than general dust limits).

In summary, niobium is regarded as a low-hazard metal. It is largely inert in the body, and only concentrated soluble compounds or metal dust present any risk. Jewelry-grade niobium and niobium-containing medical implants are used safely. As always, appropriate industrial hygiene (ventilation, dust control) is advised when machining or grinding niobium metal or handling niobium compounds.

History and Etymology

Niobium’s discovery history is intertwined with that of tantalum (Ta, Z=73). In 1801 the British chemist Charles Hatchett analyzed a mineral from Connecticut (then called “New England stone”) and reported a new element. He named it columbium (Cb), after Columbia (a poetic name for the United States). However, in 1802 the Swedish chemist Ekeberg identified “tantaline” (Ta) and published it. Over the next decade, chemists Wollaston and Rose argued whether “tantalum” and “columbium” were the same element or different. Wollaston (1809) concluded they were identical, and for a while the name “tantalum” dominated. In 1846–1847 the German chemist Heinrich Rose reexamined columbite and tantalum minerals and showed they contained two distinct elements. He named the new element niobium, after Niobe (daughter of Tantalus), linking it to tantalum by myth.

For about a century, both names “columbium” and “niobium” were used. Eventually, in 1949 the name niobium was officially chosen by IUPAC for the element. (As a note of historical trivia: the chemical symbol Nb was suggested by Rose’s son, and this symbol was kept.) In the United States the name columbium remained in use for many years, especially in metallurgy standards (ASTM, SAE), but today “niobium” is universal in scientific contexts.

Isolation of pure niobium metal came later. Early chemists could only isolate it as oxides or compounds. In 1864 several teams (Blomstrand, de Marignac, Hermann) succeeded in producing minute amounts of metal. However, niobium was scarce and expensive, so it saw little practical use in the 19th century. It was not until the early 20th century that methods for producing larger quantities of niobium became available. In 1907, attempts to use niobium in incandescent light bulbs showed that it could withstand high temperatures (but tungsten proved superior).

The first major commercial uses began in the 1930s. In 1930–33, niobium’s effect on corrosion in stainless steel was discovered: small additions (0.1–0.2%) of niobium (as columbium) prevented chromium carbide precipitation in 18-8 stainless, improving its performance. This led to niobium’s steady adoption in steels. Later, during and after World War II, niobium alloys were used in rocket engines and jet turbines. In 1950s, when superconductivity was discovered, niobium’s 9.2 K transition made it a prime candidate: WWII-era research into cryogenic engineering accelerated Nb-Ti and Nb3Sn alloy development.

In the late 20th century, Brazil became a niobium powerhouse. Large carbonatite deposits were developed, and mining of niobium exploded from the 1960s onward. CBMM in Brazil (founded 1955) developed ferroniobium production for the steel industry, eventually supplying the global market (with ~75–90% of world demand). In parallel, niobium capacitors and optical applications extended the market. Modern research continues to find new uses (e.g. battery materials, high-speed electronics).

Etymology: The names “columbium” (Cb) and “niobium” (Nb) reflect this history. “Niobium” is derived from Niobe, a figure in Greek myth (daughter of Tantalus, thus symbolizing the element’s close relation to tantalum). “Columbium” honored Christopher Columbus/COLUMBIA (America), the source of the mineral niobium was discovered in. Both names appear in historical patents and older literature, though only Nb/niobium is official now.

In summary, niobium was discovered at the dawn of modern chemistry (1801), confused with tantalum for decades, and finally isolated and industrialized in the 20th century. Its name reflects mythology and early American heritage, and its uses grew with the development of high-strength alloys and superconductor technologies.

Data Table

Property | Niobium (Nb) --- | --- Atomic number | 41 Symbol | Nb Standard atomic weight | 92.90638 Electron config. | [Kr] 4d^4 5s^1 Electrons per shell | 2, 8, 18, 12, 1 Group / Period / Block | 5 (V group) / 5 / d-block (transition metal) Category | Transition metal Oxidation states | +5 (most common), +4, +3, +2, –1 and metals Nb⁰ Phase at STP | Solid metal (body-centered cubic) Density (295 K) | 8.57×10^3 kg/m³ Melting point | 2750 K (2477 °C) Boiling point | 5017 K (4744 °C) Heat of fusion | ≈30 kJ/mol Heat of vaporization | ≈690 kJ/mol Specific heat (298 K) | 24.6 J/(mol·K) Thermal conductivity (300 K) | 53.7 W/(m·K) Electrical resistivity (20 °C) | 152 nΩ·m (0.152 μΩ·cm) Magnetic ordering | Paramagnetic (superconducts <9.2 K) First ionization energy | 658 kJ/mol (≈6.76 eV) Electronegativity (Pauling) | 1.6 Atomic (covalent) radius | ~146 pm Thermal neutron capture cross-section | 1.15 barns (for ^93Nb) Most abundant isotope | ^93Nb (stable, spin 9/2⁺) – 100% natur. abundance Isotopic composition | ^93Nb (stable); ^92Nb (t1/2≈3.47×10^7 y), ^94Nb (20.3 k y) trace CAS registry number | 7440-03-1