Neodymium

• 1. Introduction

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  Neodymium, element number 60 on the periodic table, sits quietly inside the headphones on your ears, the hard drive in your computer, and the electric motor under the hood of a hybrid car. A few tens of grams of it can lift a thousand times its own weight. Handled carelessly, two small pieces can snap together from fifty centimetres away with enough force to take off a fingertip. And yet most people have never heard its name.

  This is the story of a metal that was hidden inside another metal for decades, misidentified by some of the finest chemists of the nineteenth century, and then went on to become one of the most sought-after industrial materials on Earth. How did a silvery element that tarnishes almost the moment it touches air end up inside every wind turbine and MRI machine? And why does China now mine most of the world's supply?

• 2. Hidden Inside Didymium

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  In 1751, the Swedish mineralogist Axel Fredrik Cronstedt pulled a heavy mineral from a mine at Bastnäs in Sweden. He called it cerite. Thirty years later, the fifteen-year-old Wilhelm Hisinger, whose family owned the mine, sent a sample to the chemist Carl Scheele. Scheele found nothing new in it.

  Hisinger came back to the problem as an adult. In 1803, working alongside Jöns Jacob Berzelius, he isolated a new oxide from cerite and named it ceria, after the dwarf planet Ceres, which had itself been discovered just two years earlier. Martin Heinrich Klaproth independently reached the same conclusion in Germany around the same time.

  For the next four decades, ceria was treated as a single substance. Then Carl Gustaf Mosander, a Swedish surgeon and chemist who lived in the same house as Berzelius, showed that ceria was actually a mixture of oxides. Between 1839 and 1843 he separated out two of them, naming them lanthana and didymia. The metal behind didymia was called didymium, from the Greek word for twin, because it so closely resembled lanthanum.

  Didymium held its reputation as a single element for more than forty years. Carl Auer von Welsbach, working in Vienna in 1885, finally broke it apart. Using spectroscopic analysis, he demonstrated that didymium was two different elements. One he named praseodymium. The other he named neodymium, from the Greek neos, meaning new, and didymos, meaning twin. The products, though, were of relatively low purity. Pure neodymium was not isolated until 1925.

• 3. A Complicated Atom

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  Neodymium's atomic number is 60, and its 60 electrons are arranged in the configuration Xe4f46s2. That notation captures something important: the 4f electrons sit deep inside the atom, shielded by a core of xenon-like electrons. This inner position means the 4f orbitals penetrate closest to the nucleus, which binds them tightly as the atomic number increases along the lanthanide series.

  At room temperature, neodymium is paramagnetic, meaning it is weakly attracted to magnetic fields. Cool it below 20 K and it becomes antiferromagnetic, and below that transition it passes through a series of complex magnetic phases with long spin relaxation times and spin glass behavior.

  Chemically, the metal is aggressive. It melts at 1024 degrees Celsius and boils at 3074. A centimetre-sized sample left in air corrodes completely in about a year. It burns readily at around 150 degrees Celsius to form neodymium(III) oxide, which then peels away and exposes fresh metal beneath. It reacts with cold water slowly and with hot water quickly. Dissolved in dilute sulfuric acid, it produces a distinctively lilac solution.

  Naturally occurring neodymium is composed of five stable isotopes and two radioisotopes with extremely long half-lives. The most abundant stable isotope is 142Nd, making up 27.2% of the natural abundance. In all, 35 radioisotopes of neodymium have been detected. One of the most scientifically useful properties of the element is the decay of samarium-147 to the stable 143Nd, which allows geologists to date ancient rocks and meteorites through samarium-neodymium dating.

• 4. Glass That Changes Color

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  The first commercial use of purified neodymium was not in magnets or lasers. It was in glass. In November 1927, Leo Moser began experiments that produced what the Moser glassworks still calls its signature "Alexandrite" color, a glass that shifts from lavender in daylight to pale blue under fluorescent light.

  Moser's original recipe used about 5% neodymium oxide in the glass melt, a high enough concentration that he described them as "rare-earth-doped" glasses. That level of neodymium also affected the physical melting properties of the glass, requiring adjustments to the lime content.

  By the early 1930s, American glasshouses were copying the technique. Heisey, Fostoria, Cambridge, and Steuben each produced their own versions, with trade names like "wisteria" and "heatherbloom." Lalique in France and glassmakers in Murano adopted it too. Tiffin's "twilight" glass stayed in production from around 1950 to 1980. The color shift happens because neodymium's absorption bands interact with the emission spectra of different light sources: reddish-purple under daylight or incandescent light, blue under white fluorescent lighting, and greenish under trichromatic lighting.

  The reason neodymium glass changes color but does not fade or shift with heat is that its coloration depends on what chemists call forbidden f-f transitions deep inside the atom. The chemical environment has almost no influence on those transitions. However, iron impurities in the silica used to make the glass do degrade the color, so the best neodymium glass requires especially pure raw materials.

  During the First World War, didymium mirrors were reportedly used to transmit Morse code across battlefields, exploiting the sharp absorption bands that make neodymium so distinctive. Those same bands make neodymium glass useful in astronomical work, where astronomers use it to calibrate spectral lines and to filter out light pollution from sodium and mercury vapor lamps.

• 5. Lasers Powerful Enough to Fuse Atoms

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  The Nd:CaWO4 laser was developed in 1961, making it historically the third laser ever put into operation, after the ruby laser and the U3+:CaF laser. In 1964, a researcher named Geusic and colleagues demonstrated the operation of neodymium ions in an yttrium aluminium garnet matrix, producing what became one of the most widely used laser configurations in the world.

  The reason neodymium works so well as a laser gain medium lies in the structure of its energy levels. As a four-level laser, the Nd:YAG system has a lower threshold than many alternatives and excellent mechanical and temperature stability. Light can pump it either with non-coherent flashlamp radiation or with a coherent diode beam.

  Neodymium-doped crystals, typically Nd:YVO4, generate high-powered infrared beams that are then converted to the green laser light found in commercial handheld laser pointers.

  At the far end of the power scale, the UK Atomic Weapons Establishment operates a laser called HELEN, short for High Energy Laser Embodying Neodymium. HELEN is a one-terawatt neodymium-glass laser used to reach the midpoints of pressure and temperature conditions inside warheads, generating plasmas of around one million degrees Kelvin. Multiple-beam neodymium-glass systems at the terawatt scale and megajoule energy level are used in inertial confinement fusion research. In these devices, Nd:glass lasers are usually frequency tripled to the third harmonic at 351 nanometres.

• 6. The Magnet That Changed Industry

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  Neodymium magnets, made from the alloy Nd2Fe14B, are the strongest permanent magnets known. A magnet weighing a few tens of grams can lift a thousand times its own weight and snap together with enough force to break bones. They are cheaper, lighter, and stronger than the samarium-cobalt magnets they largely replaced, though they do lose magnetism more readily at elevated temperatures and are more prone to corrosion.

  To improve performance in heated conditions, manufacturers often add the heavy rare-earth elements dysprosium and terbium as substituents, because the magnets lose performance rapidly above room temperature.

  Each Toyota Prius requires one kilogram of neodymium per vehicle for its drive electric motors. Wind turbines that use permanent magnet generators each require a significant neodymium alloy component. The demand from hybrid vehicles, plug-in hybrids, electric vehicles, fuel cell vehicles, and wind turbines has been climbing sharply. Toward meeting the goals of the Paris Agreement, demand for NdFeB magnets is expected to increase significantly in coming years.

  In consumer electronics, neodymium magnets appear in microphones, professional loudspeakers, headphones, guitar and bass pick-ups, and computer hard disk drives. Medical devices, including MRI machines, also depend on them.

  The safety risks of these magnets are not trivial. There is at least one documented case of a person losing a fingertip when two magnets snapped together from 50 centimetres away. If multiple small magnets are swallowed, they can pinch soft tissue in the gastrointestinal tract. This danger led to an estimated 1,700 emergency room visits in the United States and prompted the recall of the Buckyballs line of toys, which were construction sets built from small neodymium magnets.

• 7. Where the Metal Comes From

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  Neodymium is not rare in the geological sense. Its abundance in the Earth's crust is about 41 milligrams per kilogram, comparable to lanthanum and roughly in the same league as cobalt, nickel, or copper. What makes it difficult to obtain is that it never appears as a free metal; it is always mixed with other lanthanides in minerals like monazite and bastnäsite.

  The main mining regions are China, the United States, Brazil, India, Sri Lanka, and Australia. As of 2015, China holds the bulk of both production capacity and reserves of rare-earth elements, and that position extends to neodymium. The world's production of neodymium was about 7,000 tons in 2004.

  Processing the ore is an involved task. After crushing and grinding, the ore is treated with hot concentrated sulfuric acid, which releases carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The residue is dried and then leached with water to bring the early lanthanide ions, including neodymium, into solution. The final step is electrolysis of halide salts to produce the metal.

  Purification methods have evolved considerably. Double nitrate crystallization was the commercial standard until the 1950s. Lindsay Chemical Division was the first to commercialize ion-exchange purification at scale. From the 1950s onward, ion exchange became the primary route to high-purity neodymium, above 99%. Today, solvent extraction is the standard for most commercial production, while ion exchange is reserved for the highest purities, typically above 99.99%.

  The uncertainty over pricing and availability has pushed some companies, particularly Japanese ones, to develop permanent magnets and electric motors that use fewer rare-earth metals, attempting to reduce dependence on a single dominant supplier.