Neodymium

Neodymium is a chemical element with the symbol Nd and atomic number 60. It sits fourth in the lanthanide series and belongs to the rare-earth metals. This hard, slightly malleable metal has a silvery appearance but quickly develops a tarnish when exposed to air and moisture. When it oxidizes, it produces compounds in pink, purple-blue, and yellow, almost always in the +3 oxidation state, though very rare and reactive +2 and +4 forms exist. Its spectrum is considered one of the most complex among the elements.

Discovered in 1885 by Austrian chemist Carl Auer von Welsbach (who also found praseodymium), neodymium is not found naturally as a pure metal or unmixed with other lanthanides. It occurs in significant amounts in the minerals monazite and bastnäsite, and is about as common in the Earth’s crust as cobalt, nickel, or copper. Most commercial neodymium is mined in China, like many rare-earth metals.

Neodymium compounds first saw commercial use as glass dyes in 1927 and remain popular. The color, typically reddish-purple, comes from the Nd³⁺ ion and shifts under different lighting due to interactions between neodymium’s sharp absorption bands and ambient light enriched with emission bands from mercury, europium, or terbium. Neodymium-doped glasses are used in lasers emitting infrared light between 1046 and 1062 nanometers, employed in high-power applications like inertial confinement fusion. It also appears in other crystal substrates, such as yttrium aluminium garnet in Nd:YAG lasers.

Neodymium alloys produce high-strength permanent magnets, widely used in microphones, professional loudspeakers, in-ear headphones, high-performance hobby DC motors, and computer hard drives where low magnet mass or strong fields are needed. Larger neodymium magnets power electric motors with high power-to-weight ratios, such as in hybrid cars, and generators in aircraft and wind turbines.

**Physical properties:** Metallic neodymium has a bright, silvery luster. It commonly exists in two allotropic forms, shifting from double hexagonal to body-centered cubic at about 863 °C. Like most lanthanides, it is paramagnetic at room temperature, becoming antiferromagnetic below 20 K (−253.2 °C). Below this, it shows complex magnetic phases with long spin relaxation times and spin glass behavior. Neodymium made up about 18% of classical mischmetal. For magnets, it is alloyed with iron, a ferromagnet.

**Electron configuration:** As the fourth lanthanide, neodymium sits between praseodymium and radioactive promethium, above uranium. Its 60 electrons are arranged as [Xe]4f⁴6s², with six valence electrons from the 4f and 6s orbitals. Like most lanthanides, it typically uses only three valence electrons because the remaining 4f electrons are tightly bound—the 4f orbitals penetrate the xenon core most deeply, followed by 5d and 6s, with binding increasing at higher ionic charge. Neodymium can lose a fourth electron because it appears early in the lanthanides, where nuclear charge is low enough and the 4f subshell energy high enough.

**Chemical properties:** Neodymium melts at 1,024 °C and boils at 3,074 °C. Its usual oxidation state is +3, but +2 and +4 forms exist, and very rarely +0. The metal oxidizes quickly at room temperature, forming a flaking oxide layer like iron rust; a centimeter-sized sample corrodes completely in about a year. Nd³⁺ is generally water-soluble. Like praseodymium, it burns readily at about 150 °C to form neodymium(III) oxide, which peels off and exposes more metal: 4 Nd + 3 O₂ → 2 Nd₂O₃. It is electropositive, reacting slowly with cold water and quickly with hot water to form neodymium(III) hydroxide: 2 Nd(s) + 6 H₂O(l) → 2 Nd(OH)₃(aq) + 3 H₂(g). It reacts vigorously with all stable halogens: with fluorine to give violet NdF₃, chlorine to mauve NdCl₃, bromine to violet NdBr₃, and iodine to green NdI₃. It dissolves readily in dilute sulfuric acid, producing lilac Nd(III) ions as [Nd(H₂O)₉]³⁺ complexes: 2 Nd(s) + 3 H₂SO₄(aq) → 2 Nd³⁺(aq) + 3 SO₄²⁻(aq) + 3 H₂(g).

**Compounds:** Important neodymium compounds include halides (NdF₃, NdCl₂, NdCl₃, NdBr₃, NdI₂, NdI₃), oxide Nd₂O₃, hydroxide Nd(OH)₃, carbonate Nd₂(CO₃)₃, sulfate Nd₂(SO₄)₃, acetate Nd(CH₃COO)₃, and neodymium magnets (Nd₂Fe₁₄B). Some compounds change color under different lighting.

**Organoneodymium compounds:** These contain a neodymium–carbon bond and resemble other lanthanide organometallics, characterized by an inability to undergo certain reactions.