Samarium (Sm)

Samarium is named after a man who had nothing to do with chemistry. Colonel Vasili Samarsky-Bykhovets was chief of staff of the Russian Corps of Mining Engineers, and his contribution was administrative: he gave Heinrich Rose access to mineral specimens from the Ilmen Mountains. Rose named the mineral samarskite after him in 1847, and when an element was eventually pulled out of that mineral it inherited the name at one remove.

This is why samarium is usually described as the first element named after a person. The claim survives on date order alone: Boisbaudran named it in 1879 and named gadolinium — the only other nineteenth-century element that honours someone, and by exactly the same indirect route through a mineral — seven years afterwards. Neither honorand was ever the intended recipient of an element. Samarsky-Bykhovets, who never published a line of chemistry, ended up on the periodic table for lending out mineral specimens, and he was still alive when Rose put his name on the rock, which is more than can be said for most of the people commemorated up there.

Pulling it out of the twin

The raw material was didymium, a substance Carl Gustaf Mosander had separated in 1841 and named from the Greek for twin, because it accompanied lanthanum everywhere. Didymium behaved consistently enough to be treated as an element for forty years. It was not one.

Paul-Émile Lecoq de Boisbaudran attacked didymium from samarskite in 1879 and found a sharp absorption line that belonged to none of the known rare earths. He had samarium. Marc Delafontaine had reported something in the same territory the previous year under the name decipium, and part of what he had was almost certainly samarium — the credit is not entirely clean, and Delafontaine's material was a mixture rather than an element.

Boisbaudran was working with the best analytical eye of his generation; he had found gallium four years earlier by the same spectroscopic method and would find dysprosium seven years later.

The magnet that keeps working when neodymium quits

Samarium's most important product came out of an air force laboratory in the late 1960s, when Karl Strnat and Alden Ray demonstrated that samarium and cobalt combine into a permanent magnet far stronger than anything then available. Samarium-cobalt was the first rare-earth magnet, and from its arrival in the late 1960s until neodymium-iron-boron appeared in 1983 nothing beat it.

Then neodymium-iron-boron arrived in the early 1980s with a higher energy product and a much lower price, and samarium-cobalt was written off in most of the market. It did not disappear, because it holds two advantages that no amount of neodymium engineering has closed.

The first is temperature. Samarium-cobalt's Curie temperature is around 700 to 800 °C against neodymium magnets' 310 to 400 °C, and it keeps a usable field at operating temperatures of 250 °C and above where a neodymium magnet has demagnetised. Its output also drifts far less with temperature — a coefficient roughly a third of neodymium's — which matters enormously when a magnet is part of a measuring instrument rather than a motor.

The second is corrosion. Neodymium magnets rust visibly and must be nickel- or epoxy-coated to survive. Samarium-cobalt is chemically stable bare.

Those two properties define where the material still lives: satellite travelling-wave tubes, aerospace actuators, military motors and generators, magnetic bearings, downhole tools for oil and gas, and precision sensors whose calibration must not wander when the equipment warms up. It is the magnet you specify when the environment is hostile and you cannot afford to be surprised.

Why reactor operators watch samarium-149

Nuclear fission produces samarium-149, and samarium-149 has a thermal neutron absorption cross-section of around 40,000 barns. That makes it one of the strongest neutron absorbers known and a serious nuisance inside a reactor core.

What distinguishes it from the other notorious fission poison, xenon-135, is that samarium-149 is stable. Xenon-135 decays with a nine-hour half-life, so xenon poisoning builds and then clears; a reactor can wait it out. Samarium-149 does not go away. It accumulates until it reaches an equilibrium set by the rate at which neutrons destroy it, and after a shutdown it gets worse, because its precursor promethium-149 keeps decaying into it with a 53-hour half-life while no neutrons are being produced to burn it off.

Reactor physics therefore treats the two poisons as different problems: a transient one that constrains restart timing, and a permanent one that has to be built into the fuel's reactivity budget from the start.

A clock for the oldest rocks

About 15% of natural samarium is samarium-147, which alpha-decays to neodymium-143 with a half-life of 106 billion years. The pair makes an unusually durable geological chronometer.

Most dating systems pair elements with very different chemical behaviour, so heating or fluid flow can reset them by moving one and not the other. Samarium and neodymium are adjacent lanthanides with nearly identical chemistry, so they tend to stay together through metamorphism, and the clock survives events that would erase a rubidium-strontium or potassium-argon age. That resilience is why samarium-neodymium dating is the method of choice for the oldest terrestrial rocks, for lunar samples and for meteorites, and why the neodymium isotope ratio doubles as a tracer distinguishing mantle-derived material from continental crust.

A second, extinct isotope does the finer work. Samarium-146 was present when the solar system formed and has long since decayed, so its former presence has to be inferred from excess neodymium-142 in ancient rocks — which dates events in the first few hundred million years of planetary history. In 2012 a redetermination of samarium-146's half-life revised it downward from about 103 million years to about 68, which compressed the inferred timeline for the early differentiation of the Moon and Mars considerably. The revision has been challenged and the question of which value to use is not fully closed, which means a portion of early planetary chronology currently rests on a contested nuclear measurement.

A reagent chemists reach for by name

Samarium is one of a small group of lanthanides that will accept a second oxidation state, and one compound has made a career out of it. Samarium diiodide — introduced by Henri Kagan in 1980 and universally known as Kagan's reagent — is a deep blue-green solution that delivers single electrons to organic molecules under mild conditions.

Single-electron chemistry lets synthetic chemists form carbon-carbon bonds in ways that two-electron reagents cannot, and samarium diiodide is tolerant, selective and predictable enough that it appears throughout the literature of natural-product synthesis. It is one of the few places where a rare earth is a routine bench reagent rather than a component of a device.

Samarium also has a modest medical role: samarium-153 attached to a bone-seeking phosphonate was licensed in 1997 for relieving the pain of cancer that has spread to bone, delivering a short-range beta dose where the skeleton takes it up.