Niobium compounds are compounds containing the element niobium (Nb). In many ways, niobium is similar to tantalum and zirconium. It reacts with most nonmetals at high temperatures; with fluorine at room temperature; with chlorine at 150 °C and hydrogen at 200 °C; and with nitrogen at 400 °C, with products that are frequently interstitial and nonstoichiometric. The metal begins to oxidize in air at 200 °C. It resists corrosion by fused alkalis and by acids, including aqua regia, hydrochloric, sulfuric, nitric and phosphoric acids. Niobium is attacked by hydrofluoric acid and hydrofluoric/nitric acid mixtures.
Although niobium exhibits all of the formal oxidation states from +5 to −1, the most common compounds have niobium in the +5 state. Characteristically, compounds in oxidation states lower than +5 frequently display Nb–Nb bonding. In aqueous solution, niobium exhibits essentially only the +5 oxidation state. It is readily prone to hydrolysis and is barely soluble in dilute solutions of hydrochloric, sulfuric, nitric and phosphoric acids because of the precipitation of hydrous niobium oxide. Nb(V) is somewhat soluble in alkaline media owing to the formation of soluble polyoxoniobate species.
Properties of selected niobium compounds Formula appearance symmetry space group No. a (nm) b (nm) c (nm) density,g/cm3 NbO2 dark solid tetragonal I41/a 88 1.37 0.599 5.68 LiNbO3 colourless crystals trigonal R3c 161 0.51516 1.3869 2H-NbS2 dark grey solid hexagonal P63/mmc 194 0.3418 1.1860 2H-NbSe2 dark metallic crystals hexagonal P63/mmc 194 0.34425 1.2547 NbN dark solid cubic Fmm 225 0.4394 NbC grey to black solid cubic Fmm 225 0.4469 NbB2 dark grey solid hexagonal P6/mmm 191 0.3116 0.3264 NbP grey metallic crystals tetragonal I41md 109 0.3335 1.1379 Nb3Sn metallic solid cubic Pmn 223 0.5289
Halides
Niobium forms halides in the oxidation states +5 and +4 as well as diverse substoichiometric compounds. The pentahalides () feature octahedral Nb centres. Niobium pentafluoride () is a white solid with a melting point of 79.0 °C, whereas niobium pentachloride () is yellow and melts at 203.4 °C. Both are hydrolysed to give oxides and oxyhalides such as .
The pentahalides can be prepared by direct reaction of niobium with the corresponding halogen. Niobium pentachloride, for example, is obtained by chlorination of the metal:
2 Nb + 5 Cl2 → 2 NbCl5
The pentachloride is a versatile reagent for the preparation of other niobium compounds, including organometallic derivatives such as niobocene dichloride (). The tetrahalides () are dark-coloured polymers with Nb–Nb bonds; examples include black hygroscopic niobium tetrafluoride () and brown niobium tetrachloride (). Lower halides can be obtained by reduction of the corresponding pentahalides.
Anionic halide compounds of niobium are well known, owing in part to the Lewis acidity of the pentahalides. An important example is [NbF7]2−, an intermediate in the separation of niobium and tantalum from their ores. This heptafluoride tends to form the oxopentafluoride more readily than does the corresponding tantalum compound. Other halide complexes include octahedral []:
+ 2 Cl → 2 []
A variety of reduced niobium halide cluster ions is also known, a prominent example being [].
Oxides and oxyanions
Niobium forms three principal binary oxides: niobium monoxide (), niobium dioxide (), and niobium pentoxide (), corresponding formally to the +2, +4 and +5 oxidation states, respectively. The niobium–oxygen system is complex, and several nonstoichiometric and intermediate oxide phases are also known. The pentoxide is the most stable and technologically important oxide and is a precursor to many other niobium compounds and alloys.
Heating niobium in oxygen gives niobium pentoxide:
4 Nb + 5 O2 → 2 Nb2O5
Niobium pentoxide occurs in several polymorphs whose stability depends on temperature and preparation conditions. Reduction of gives lower oxides including and several intermediate phases, while stronger reduction produces .
Niobates can be prepared by reaction of with basic oxides, carbonates or hydroxides. They comprise a structurally diverse group of ternary and polynary oxides containing Nb(V). Examples include lithium niobate (), potassium niobate () and lanthanum niobate (). Lithium niobate has a trigonally distorted structure and is widely used for its nonlinear optical and piezoelectric properties, while niobates more generally adopt structures including perovskite-related, tungsten-bronze and complex block frameworks.
In strongly alkaline aqueous solution, Nb(V) can form soluble polyoxoniobate ions. Among the best known is the hexaniobate ion [Nb6O19]8−, whose chemistry forms the basis of a large family of condensed niobium oxoanions.
Thin films of niobium(V) oxide can be deposited by chemical vapor deposition or atomic layer deposition, including by thermal decomposition of volatile niobium alkoxides such as niobium(V) ethoxide.
Chalcogenides
Niobium forms binary compounds with sulfur, selenium and tellurium. The most prominent are the layered dichalcogenides , and , in which niobium–chalcogen sheets are separated by van der Waals gaps. Their structures and physical properties differ substantially with the chalcogen and with crystal polytype.
Niobium disulfide can be prepared by solid-state reaction of elemental niobium and sulfur at elevated temperature. The overall reaction may be represented as:
Nb + 2 S → NbS2
The polytype formed depends strongly on reaction temperature and sulfur content; both 2H-NbS2 and 3R-NbS2 can be obtained, and mixed-polytype products readily form under intermediate conditions.
Niobium disulfide () is a layered metallic solid. Several polytypes are known; phase-pure 2H-NbS2 is a bulk superconductor, whereas phase-pure 3R-NbS2 has not been observed to superconduct above 1.75 K. Niobium diselenide () is likewise a layered metallic compound. The 2H polytype undergoes both a charge density wave transition and a superconducting transition at low temperature. Niobium ditelluride () has a strongly distorted layered structure consisting of buckled Te–Nb–Te sheets separated by van der Waals gaps.
Pnictides
Niobium forms binary compounds with the pnictogens nitrogen, phosphorus, arsenic and antimony. Many are refractory or metallic solids, while NbP and NbAs have attracted particular interest because of their topological electronic structures.
Nitrides
Several phases occur in the niobium–nitrogen system. Niobium nitride commonly refers to compositions near NbN, although phases with other Nb:N ratios and several crystal structures are known. Cubic NbN is a hard metallic material that becomes superconducting at low temperatures. Because of its relatively high superconducting transition temperature and rapid electrical response, thin films of NbN are used in superconducting electronics and superconducting nanowire single-photon detectors.
Niobium begins to react appreciably with nitrogen at elevated temperature. Formation of stoichiometric NbN may be represented by:
2 Nb + N2 → 2 NbN
In practice, the niobium–nitrogen system contains several intermediate and nonstoichiometric nitride phases, so the product depends on temperature, nitrogen pressure and preparation conditions.
Phosphides and heavier pnictides
Among the niobium phosphides, niobium phosphide (NbP) is a metallic crystalline compound and a non-centrosymmetric Weyl semimetal. Other binary niobium phosphides, including , are also known.
Polycrystalline NbP can be prepared by direct reaction of elemental niobium and red phosphorus in an evacuated tube at elevated temperature. Single crystals can subsequently be grown by chemical vapor transport using iodine as the transport agent. The overall reaction for formation of NbP may be represented as:
4 Nb + P4 → 4 NbP
Niobium arsenide (NbAs) is another non-centrosymmetric Weyl semimetal. Polycrystalline NbAs can be prepared by heating stoichiometric niobium and arsenic under vacuum, while single crystals have been grown by iodine-mediated chemical vapour transport. Niobium also forms antimonides and other arsenide phases.
Carbides and borides
Carbides
Niobium carbide () is a very hard, refractory metallic carbide with the rock-salt structure over a broad composition range. It combines high hardness and melting temperature with good electrical and thermal conductivity. NbC-based materials are used in wear-resistant and cutting-tool applications.
NbC can be produced by direct reaction of niobium with carbon at high temperature:
Nb + C → NbC
Carbothermal reduction of niobium oxide provides another preparation route. Because the carbide has a broad homogeneity range caused by carbon vacancies, its composition may deviate substantially from ideal stoichiometric NbC.
Borides
Niobium forms several borides, including NbB, Nb3B4 and niobium diboride (). The borides are hard, refractory materials. NbB2 has a broad homogeneity range and adopts the hexagonal aluminium diboride structure, consisting of alternating planar layers of niobium atoms and graphite-like boron sheets.
NbB2 can be prepared by direct reaction of niobium and boron at high temperature:
Nb + 2 B → NbB2
It can also be produced from niobium oxide by high-temperature reduction routes involving carbon and boron-containing reagents. Superconductivity reported in NbB2-type material is strongly dependent on composition and defect concentration.
Hydrides
Niobium readily absorbs hydrogen, which occupies interstitial sites in the metal lattice. With increasing hydrogen concentration, several ordered hydride phases can form, and their stability depends on hydrogen content and temperature.
Hydrogen uptake may be represented generally as:
Nb + H2 ⇌ NbHx
Hydride phases identified experimentally include β-NbH0.89 and hydrogen-rich δ-NbH2. Hydride formation is accompanied by structural changes in the niobium lattice and can influence the material's mechanical and superconducting behaviour. Hydrogen can also be introduced electrochemically; electrochemical hydrogenation has been used to form and characterise both β and δ hydride phases.
Silicides
Several intermetallic compounds occur in the binary niobium–silicon system, principally , and . They are refractory solids with strong Nb–Si bonding and high thermal stability.
Niobium silicides can be produced by reaction of elemental niobium and silicon at high temperature. Nb5Si3, for example, has also been produced from elemental powder compacts by self-propagating high-temperature synthesis. Idealised formation reactions include:
5 Nb + 3 Si → Nb5Si3
Nb + 2 Si → NbSi2
The phase assemblage obtained depends on the overall Nb:Si ratio and thermal history.
occurs in several structural forms. The low-temperature α form has the Cr5B3-type structure, whereas the high-temperature β form adopts the W5Si3-type structure. A metastable hexagonal γ form is also known, particularly in alloyed systems.
Nb–Si alloys containing a niobium solid solution together with and related silicides have been investigated as ultra-high-temperature structural materials. In such two-phase materials, Nb5Si3 provides high-temperature strength and creep resistance while the niobium-rich solid solution improves room-temperature fracture toughness.
Intermetallic compounds
Niobium forms numerous ordered intermetallic compounds with metallic elements. Several niobium-based intermetallics having the cubic A15 structure are important superconductors.
The most technologically important is niobium–tin (Nb3Sn). Nb3Sn has the A15 structure, a superconducting transition temperature near 18 K and an upper critical magnetic field that can approach 30 T. These properties allow Nb3Sn conductors to operate in substantially higher magnetic fields than conventional Nb–Ti conductors, although Nb3Sn is comparatively brittle and strain-sensitive.