Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a silvery-white metal belonging to the lanthanide series and is one of the rare-earth elements. The metal tarnishes in air and reacts with water. Its compounds predominantly contain neodymium in the +3 oxidation state, although +2 and +4 compounds are also known. Natural neodymium contains seven isotopes, two of which are radioactive with extremely long half-lives.
The Austrian chemist Carl Auer von Welsbach discovered neodymium in 1885 when he separated didymium, previously regarded as an element, into neodymium and praseodymium. Its name derives from the Greek for 'new twin'. Neodymium occurs with other rare-earth elements in minerals such as bastnäsite and monazite, from which it is separated during refining. China is the largest producer and refiner of the rare-earth elements used in magnets, including neodymium.
Neodymium is used in neodymium–iron–boron magnets, which produce strong magnetic fields from small, lightweight components. These permanent magnets are used in audio equipment, computer hard disk drives, electric vehicle motors and the generators of some wind turbines. Neodymium compounds are also used as catalysts in the manufacture of synthetic rubber.
Neodymium imparts color to glass and is used in decorative glassware and optical filters. Its selective absorption of light can make the glass appear different colors under different illumination. Neodymium ions also provide the active component in solid-state lasers, including Nd:YAG lasers and neodymium-glass lasers, used in manufacturing, medicine and inertial confinement fusion research. Neodymium isotope ratios are used in dating rocks and tracing geological processes and past ocean circulation. Neodymium can serve as an enzyme cofactor in certain bacteria, while finely divided metal and powerful neodymium magnets present chemical and physical hazards.
Physical properties
Neodymium is a silvery-white metal with a density of at room temperature. The freshly exposed metal has a bright luster but soon tarnishes in air. It melts at approximately and boils at .
High-purity polycrystalline neodymium has a measured electrical resistivity of approximately at . Ultrasonic measurements have yielded ambient-pressure bulk and shear moduli of approximately 33 and 14 GPa, respectively, describing the metal's resistance to compression and shear deformation.
At room temperature and atmospheric pressure, neodymium has a double hexagonal close-packed (dhcp) structure. Its close-packed atomic layers repeat in an ABAC sequence, creating two distinct environments for the atoms: those in the A layers have approximately cubic local symmetry, whereas those in the B and C layers have hexagonal local symmetry. On heating at atmospheric pressure, this structure transforms into a body-centered cubic phase at about .
Compression produces a different sequence of transformations. At room temperature, the metal passes through face-centered cubic and distorted face-centered cubic phases before adopting a monoclinic structure at pressures above approximately 75 GPa.
Neodymium is paramagnetic at room temperature and develops antiferromagnetic order below approximately 19.9 K. Further cooling produces several magnetic phases involving the two types of atomic site. Their arrangement depends on temperature and on the strength and direction of an applied magnetic field.
Experiments on crystalline neodymium films have identified a self-induced spin-glass state, in which the magnetic arrangement remains disordered and evolves slowly despite the ordered crystal lattice. On warming, these films can become more magnetically ordered, with a glassy state observed near 5 K and extended ordered domains at 11 K.
Chemical properties
Neodymium lies between praseodymium and promethium in the lanthanide series of the periodic table. Its electron configuration is [Xe]4f46s2: four 4f electrons and two 6s electrons lie outside a closed-shell xenon core. Its most common oxidation state is +3. Formation of the ion removes the two 6s electrons and one 4f electron, leaving the configuration [Xe]4f3. The remaining 4f electrons are more tightly bound, favoring the trivalent state. Compounds containing neodymium in the +2 and +4 oxidation states are also known.
The metal tarnishes in air. Its surface oxide can flake away, exposing fresh metal and allowing oxidation to continue. On burning in oxygen, it forms neodymium(III) oxide:
Neodymium is electropositive and reacts with water to produce neodymium(III) hydroxide and hydrogen. The reaction is slow in cold water and faster in hot water:
It also dissolves in dilute sulfuric acid, releasing hydrogen and forming a solution containing hydrated neodymium(III) ions.
In aqueous solution, water molecules bind to neodymium(III) ions. Spectroscopic measurements are consistent with a hydrated ion containing nine directly coordinated water molecules, .
Compounds
Neodymium most commonly forms compounds in the +3 oxidation state. Its chloride and nitrate are soluble in water, whereas its oxide and hydroxide are poorly soluble. Dissolved neodymium salts can precipitate as the hydroxide at neutral or alkaline pH.
Neodymium salts can appear different colors under different types of illumination.
Oxides
Neodymium(III) oxide, , also called neodymia, occurs in hexagonal and cubic crystal forms. Both react with atmospheric moisture and carbon dioxide, undergoing hydration and carbonation; these changes occur more rapidly in the cubic samples studied. The oxide can also be prepared by heating neodymium(III) oxalate, which decomposes through stages that depend on the surrounding atmosphere.
Neodymium monoxide, , has also been synthesized at high pressure and temperature. It adopts the rock-salt structure and is metastable. One preparation used a pressure of 5 GPa and a temperature of . Thin NdO films 45 nm thick have been reported to become ferromagnetic below about 19 K.
Chalcogenides
Neodymium forms compounds with sulfur, selenium, and tellurium, including the monochalcogenides , , and . These are metallic compounds with the rock-salt structure. Neutron diffraction measurements show that all three develop antiferromagnetic order at low temperatures.
Neodymium telluride is stable at high temperatures, with a reported melting point of about . Its formation has been investigated as a way of binding neodymium produced in uranium–zirconium nuclear fuel, thereby limiting its availability to react with the fuel cladding.
Halides
Neodymium reacts with fluorine, chlorine, bromine, and iodine to form the corresponding trihalides: , , , and . These reactions can be represented by the general equation below, where X denotes a halogen:
The fluoride and bromide are violet, the chloride is mauve, and the iodide is green.
Neodymium(III) chloride can also be prepared in an anhydrous form by heating neodymium(III) oxide with ammonium chloride. The resulting chloride is sensitive to moisture.
Lower halides include neodymium(II) chloride, , and neodymium(II) iodide, . Neodymium(II) iodide can be prepared by reacting neodymium metal with iodine under controlled conditions. The diiodide acts as a reducing agent: in tetrahydrofuran, it converts several alkyl and aryl halides into the corresponding hydrocarbons.
Borides
Neodymium forms borides including the tetraboride, , and hexaboride, . The tetraboride has a tetragonal structure. Competing magnetic interactions give rise to several low-temperature phases, and its response to an applied magnetic field depends strongly on the field's direction relative to the crystal axes.
Neodymium hexaboride, , has a cubic structure and becomes antiferromagnetic below a Néel temperature of approximately 8.6 K. Large single crystals of both borides can be grown by the floating-zone method, in which a molten region is moved along a rod of the starting material.
Hydroxide and salts
Neodymium(III) hydroxide, , is sparingly soluble in water but dissolves in hydrochloric acid.
Several neodymium salts form hydrates, whose crystals incorporate water molecules. These include the octahydrates of neodymium(III) carbonate, , and neodymium(III) sulfate, .
Neodymium(III) acetate, , also forms a monohydrate. Heating removes its water of crystallization; further heating decomposes the acetate through intermediate compounds to an oxycarbonate.
Other inorganic compounds
Neodymium forms hydrides whose structures depend on their hydrogen content. The trihydride, , has a trigonal structure related to that of tysonite, whereas hydrogen-deficient compositions can have a cubic metal sublattice.
Neodymium nitride, , has been studied as a magnetic semiconductor. Thin films grown by molecular beam epitaxy have shown an optical band gap of about 0.9 eV and ferromagnetic ordering below about 43 K.
Neodymium also forms binary compounds with other pnictogens, including the phosphide , arsenide , and antimonide . These have the rock-salt structure and show antiferromagnetic ordering at low temperatures. Its carbides include the tetragonal dicarbide, , and cubic sesquicarbide, .
Organometallic compounds
Organoneodymium compounds contain bonds between neodymium and carbon. They include cyclopentadienyl complexes, in which neodymium binds to carbon rings, and compounds with σ-bonded alkyl or aryl groups. Studies of substituted cyclopentadienyl complexes have found that their neodymium–ligand bonding is predominantly electrostatic, with little covalent character.
Isotopes
Natural neodymium contains seven isotopes: Nd, Nd, Nd, Nd, Nd, Nd, and Nd. The most abundant is Nd, which accounts for approximately 27.2% of natural neodymium. Five of these isotopes have no observed radioactive decay. Of those five, Nd, Nd, Nd, and Nd are predicted to undergo extremely slow alpha decay and are therefore described as observationally stable.
The other two natural isotopes are primordial radionuclides, whose long half-lives have allowed them to survive since before the formation of Earth. Neodymium-144 undergoes alpha decay to Ce with a half-life of years. Neodymium-150 undergoes double beta decay to Sm; the NEMO-3 experiment measured a half-life of approximately years for decay to the ground state of samarium-150.
The other known radioisotopes have much shorter half-lives. The longest-lived of these is Nd, with a half-life of 10.98 days, followed by Nd at 3.37 days. Neutron-deficient isotopes generally decay by electron capture or positron emission to praseodymium, whereas neutron-rich isotopes generally undergo beta-minus decay to promethium. Neodymium also has numerous nuclear isomers, in which the nucleus occupies a relatively long-lived excited state. The longest-lived is Nd, with a half-life of 5.5 hours; other examples include Nd at 5.5 minutes and Nd at approximately 70 seconds.
The alpha decay of Sm to Nd, with a half-life of approximately years, forms the basis of samarium–neodymium dating.
Neodymium isotopes are also used as targets for producing other radionuclides. Neutron capture by Nd produces Nd, which undergoes beta-minus decay to Pm. Irradiation of enriched Nd targets has been investigated as a source of Pm for betavoltaic batteries, which convert energy from beta radiation into electricity. Bombardment of natural neodymium with deuterons has also been used to produce promethium isotopes. Enriched Nd targets have been used with fluorine and neon ion beams to produce short-lived thulium and ytterbium isotopes.
Several neodymium isotopes occur among the products of nuclear fission. Measurements of Nd in spent nuclear fuel are used to determine burnup, the extent to which the fuel has undergone fission. Measurements of other neodymium isotopes can provide additional estimates and reduce uncertainty.