Samarium

• 1. Introduction

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  Samarium, element 62 on the periodic table, sits at the center of some of the most consequential technologies of the modern age. A single F-35 fighter jet contains samarium magnets. A drug built around the isotope samarium-153 targets cancer cells in the lung, the prostate, the breast, and bone. And in the control rooms of nuclear reactors, samarium-149 plays a quiet but critical role in keeping fission from spiraling out of control.

  Yet almost nobody has heard of it. Samarium is one of the lanthanide metals, a family of elements sometimes called rare earths, and it was first isolated not quite 150 years ago from a mineral found in the Ural Mountains. The man who identified it did so by watching how light passed through it. The mineral itself was named after a Russian bureaucrat who simply let two German scientists look at some rock samples. That bureaucrat became, indirectly, the first person in history to have a chemical element named after him.

  How did a silvery metal that spontaneously ignites at 150 degrees Celsius become indispensable to military aviation, cancer medicine, and nuclear engineering? And why does China now control all of the world's usable supply?

• 2. Discovery and Naming

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  In 1879, French chemist Paul-Emile Lecoq de Boisbaudran was working in Paris with a dark, complex mineral called samarskite. He detected a new element inside it by observing sharp optical absorption lines, the spectroscopic signature of an atom no one had catalogued before. He called his discovery samarium, after the mineral itself.

  The mineral samarskite had its own story. It was named after Vassili Samarsky-Bykhovets, who lived from 1803 to 1870 and served as Chief of Staff of the Russian Corps of Mining Engineers. Samarsky-Bykhovets had done something modest: he granted two German mineralogists, the brothers Gustav and Heinrich Rose, permission to examine mineral samples from the Urals. That act of bureaucratic hospitality made him the first person ever to have a chemical element named after him, even if the honor arrived indirectly, through the mineral, through the element.

  The discovery itself was messier than textbook accounts suggest. Swiss chemist Marc Delafontaine had announced a new element he called decipium, meaning "deceptive" or "misleading," in 1878. By 1880-1881 he showed that decipium was actually a mixture of several elements, one of them identical to Boisbaudran's samarium. The samarium Boisbaudran had isolated was itself impure; it contained a comparable amount of europium. Pure samarium(III) oxide was not produced until 1901, by Eugene-Anatole Demarçay, and the pure metal was not isolated until 1903, by Wilhelm Muthmann.

  For decades after, chemists debated even the symbol. Sm was the suggested abbreviation, but the alternative Sa was used widely until the 1920s.

• 3. A Metal of Unusual Character

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  Samarium has a hardness and density similar to zinc, and when freshly prepared it carries a bright silvery lustre. That lustre fades quickly: the metal oxidizes slowly at room temperature and will spontaneously ignite at just 150 degrees Celsius. Even sealed under mineral oil, it gradually develops a grayish-yellow powder of oxide-hydroxide on its surface. Keeping a pure sample intact requires sealing it under argon or another inert gas.

  With a boiling point of 1,794 degrees Celsius, samarium is the third most volatile lanthanide, ranking behind only ytterbium and europium. That volatility is practically useful: when samarium oxide is reduced with lanthanum to produce the pure metal, the product is distilled to separate samarium from lanthanum, whose boiling point is 3,464 degrees Celsius. The gap between those two numbers does the separation work.

  Samarium's atomic radius of 238 picometres is one of the largest among all elements; only potassium, praseodymium, barium, rubidium, and caesium are larger. At room temperature the metal takes a rhombohedral crystal structure. Heat it to 731 degrees Celsius and the structure shifts to hexagonal close-packed. Push it to 922 degrees Celsius and it becomes body-centered cubic. Apply both heat and pressure of around 40 kilobars and a double-hexagonal structure appears. At pressures in the hundreds to thousands of kilobars, still more transformations unfold, including a tetragonal phase appearing at around 900 kilobars.

  Samarium doped into iron-based superconductors raises their transition temperature up to 56 K, the highest value achieved so far in that series of materials.

• 4. The Magnet That Outlasts Heat

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  Samarium-cobalt magnets carry a permanent magnetization roughly 10,000 times that of iron. Only neodymium magnets exceed them in magnetic strength, but samarium magnets hold an advantage that neodymium cannot match: they remain stable above 700 degrees Celsius, compared to 300-400 degrees Celsius for neodymium. Where heat is a constraint, samarium wins.

  That heat resistance explains why samarium magnets turn up in military hardware, modern aircraft, and missiles. The F-35 fighter jet is one documented example. But the same magnets also appear in small motors, high-end headphones, and guitar and bass pickups, including a line called Samarium Cobalt Noiseless. They power the motors of the Solar Challenger, a solar-powered electric aircraft.

  For most of the 20th century, getting samarium pure enough to be commercially useful was impossible. Before ion-exchange separation technology arrived in the 1950s, samarium had no standalone commercial applications. What did exist was a by-product of purifying neodymium: a mixture of samarium and gadolinium that a company called Lindsay gave its name to, known as Lindsay Mix. That mixture found use in nuclear control rods in early reactors. The modern equivalent is samarium-europium-gadolinium concentrate, or SEG, prepared by solvent extraction from the mixed lanthanides in bastnäsite or monazite ore. SEG typically makes up only 12% of the original ore.

• 5. Medicine, Reactors, and Lasers

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  Samarium-153 is a beta emitter with a half-life of 46.285 hours. Chelated with a compound called ethylene diamine tetramethylene phosphonate and injected intravenously, it travels to cancer sites in the lung, prostate, breast, and bone. The chelation prevents free samarium from accumulating elsewhere in the body. The drug is sold under the trade name Quadramet.

  Samarium-149 works differently. Its neutron cross section for thermal neutrons is 41,000 barns, a measure of how readily it captures neutrons. That number makes it a powerful poison for nuclear chain reactions, second in importance for reactor design and operation only to xenon-135. Unlike xenon-135, samarium-149 is not radioactive and does not decay away on its own; it disappears only through neutron capture. Its equilibrium concentration in an operating reactor builds over roughly 500 hours, about three weeks. Most of its neutron-capture products are other samarium isotopes that are themselves strong absorbers, which makes the poisoning effect self-sustaining in a way that competing materials like boron and cadmium cannot match.

  In early 1961, Peter Sorokin and Mirek Stevenson at IBM research labs built one of the first solid-state lasers using samarium-doped calcium fluoride crystals. It produced pulses of red light at 708.5 nm but required liquid helium cooling, which kept it from practical use. A later samarium-based laser became the first saturated X-ray laser operating at wavelengths shorter than 10 nanometres. It gave 50-picosecond pulses at 7.3 and 6.8 nm, with a peak energy of 0.3 millijoules, opening possibilities in holography, high-resolution microscopy of biological specimens, and radiography of dense plasmas relevant to fusion research.

• 6. Geopolitics of a Critical Metal

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  Western militaries depended on a single samarium production plant in La Rochelle, France from the 1970s until that facility closed in 1994. The plant had sourced its samarium from Australia. When it shut, the supply chain for a metal essential to fighter jets, missiles, and advanced electronics collapsed into a single country.

  A United States government effort costing roughly one billion dollars to reopen a closed rare earths mine in Mountain Pass, California ended in bankruptcy. The Biden administration signed two contracts for samarium production plants inside the United States; neither materialized. China, which processes its samarium primarily in the city of Baotou, now produces all of the world's usable samarium.

  World samarium resources are estimated at two million tonnes, distributed across China, the United States, Brazil, India, Sri Lanka, and Australia. Annual production runs to about 700 tonnes. When China imposed strict export limits on samarium during the tariff disputes of the Trump administration's second term, the vulnerability of that single-country dependency became impossible to ignore. Samarium oxide sells at around US$30 per kilogram, making it one of the cheaper lanthanide oxides, a price that reflects chronic oversupply from Chinese production rather than scarcity in the earth's crust. The element ranks as the 40th most abundant in the crust, present at a median concentration of 7 parts per million.

• 7. Isotopes and Deep Time

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  Natural samarium is composed of five stable isotopes and two radioisotopes with half-lives long enough to persist since the formation of the solar system. The isotope samarium-147 has a half-life of 1.066 times ten to the eleventh years; samarium-148 has a half-life of 6.3 times ten to the fifteenth years. The most abundant stable isotope is samarium-152, which makes up 26.75% of natural samarium.

  The alpha decay of samarium-147 to neodymium-143 is the basis of samarium-neodymium dating, a technique for determining the age and origin of rocks and meteorites. Because samarium and neodymium are both lanthanides with very similar physical and chemical properties, the dating method is largely insensitive to the partitioning effects that complicate other radiometric techniques. Natural samarium has a radioactivity of 127 becquerels per gram, driven primarily by samarium-147.

  Samarium-146 is an extinct radionuclide; its half-life of 9.20 times ten to the seventh years is long enough that minute quantities might in principle persist from primordial times, and searches for it as a primordial nuclide have been conducted. The known isotopes of samarium span from samarium-129 to samarium-168. The half-lives of samarium-151 and samarium-145 are 94.6 years and 340 days respectively, placing them in a middle range between the near-eternal long-lived isotopes and the lightest unstable varieties, most of which decay in under 48 seconds.