Gadolinium

• 1. Introduction

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  Gadolinium hides a paradox inside a hospital scanner. At body temperature, this silvery-white metal exhibits the greatest paramagnetic effect of any element. That single property lets doctors inject solutions of it into a patient's veins to brighten the images of a magnetic resonance scan. Yet the same metal, as a free ion in water, is highly toxic to mammals, capable of interfering with calcium-ion channels that keep cells alive.

  It carries the symbol Gd and the atomic number 64. It was named for a mineral, which was named for a man, and it was first glimpsed not by sight but by the colored lines of a spectroscope. How does an element this dangerous become safe enough to put inside most people who walk into an imaging suite? And why does a metal with no known biological role end up shielding nuclear reactors, chilling refrigerators, and listening for exploding stars? The answers live in the strange physics of its electrons and the chemistry that tames its worst tendencies.

• 2. A Mineral, A Name

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  Johan Gadolin, a Finnish chemist, first chemically analyzed a particular mineral in 1794. The mineral itself did not yet carry his name. That came in 1802, when the German chemist Martin Klaproth christened it gadolinite. The element waited even longer for its own identity.

  In 1880 the Swiss chemist Jean Charles Galissard de Marignac observed spectroscopic lines from gadolinium in samples of gadolinite. The irony is sharp. Gadolinite actually contains only traces of the element, though enough to throw a spectrum. Marignac found a richer source in a separate mineral called cerite, which held far more of whatever was casting that new spectral line.

  Marignac eventually separated a mineral oxide from cerite and recognized it as the oxide of a new element. He gave it a placeholder symbol, Yα, a chemist's note-to-self rather than a true name. The naming honor fell to the French chemist Paul-Émile Lecoq de Boisbaudran, who called the element gadolinium in 1886. For half a century after that, no one held the pure metal in hand. That milestone belonged to Félix Trombe, who first isolated the pure element in 1935.

• 3. The Electrons That Decide

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  Xe4f75d16s2 is the arrangement of gadolinium's 64 electrons, and that string of symbols explains nearly everything the element does. Of these, the ten electrons in the 4f, 5d, and 6s positions count as valence. Yet usually only three are actually available for bonding, like most metals in the lanthanide series. The remaining 4f electrons stay locked away, bound too strongly to participate, because the 4f orbitals burrow most deeply through the inert xenon core toward the nucleus.

  Gadolinium is the eighth member of the lanthanide series. On the periodic table it sits between europium on its left and terbium on its right, and directly above the actinide curium. At room temperature it crystallizes in a hexagonal close-packed form called the alpha-form. Heat it past 1235 degrees Celsius and it transforms into a body-centered cubic structure, the beta-form.

  Magnetism is where gadolinium becomes unusual. Below 20 degrees Celsius, its Curie point, it behaves as a ferromagnet, with an attraction to a magnetic field stronger than that of nickel. Some evidence suggests it is actually a helical antiferromagnet below that temperature rather than a simple ferromagnet. Above 20 degrees Celsius it becomes the most paramagnetic element. It also shows a magnetocaloric effect, warming when it enters a magnetic field and cooling when it leaves one.

• 4. The Neutron Sponge

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  About 259,000 barns is the thermal-neutron capture cross-section of the isotope gadolinium-157, the highest of any stable nuclide. Only xenon-135 captures neutrons more greedily, at about 2.0 million barns, but xenon-135 is radioactive. That makes gadolinium-157 the practical champion among stable atoms at swallowing neutrons.

  Naturally occurring gadolinium is a mixture of six stable isotopes and one radioisotope, the alpha-emitting 152Gd. The most abundant of them is 158Gd, at 24.8% natural abundance. Among the radioactive forms, 152Gd is remarkably patient, with a half-life of 1.08 times 10 to the 14th years. The predicted double beta decay of 160Gd has never been observed, with experiments only able to set a lower limit on its half-life of more than 1.3 times 10 to the 21st years.

  This hunger for neutrons gives gadolinium a job in nuclear engineering. It serves as a secondary, emergency shut-down measure in some reactors, particularly the CANDU type. In nuclear marine propulsion it acts as a burnable poison. Its appetite for neutrons also makes it useful in neutron radiography and in reactor shielding. Researchers have even investigated gadolinium-containing compounds for neutron capture therapy aimed at tumors, with promising early results.

• 5. Taming A Toxin

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  0.34 millimoles per kilogram is roughly the 50% lethal dose of free gadolinium in a mouse given the metal intravenously, expressed another way as 100 to 200 milligrams per kilogram. The danger comes from the free ion interfering with calcium-ion channel dependent processes inside the body. To use such a metal in medicine, chemists had to cage it.

  Chelation is the solution. Wrapping the gadolinium(III) ion in a chelating compound prevents it from being exposed to the organism, and healthy kidneys excrete most of it before it can settle into tissues. The chelating agent DOTA, an octadentate ligand, illustrates the lanthanide tendency to form complexes with high coordination numbers. Salts of Gd(DOTA) minus are useful in magnetic resonance imaging, and related complexes such as gadodiamide followed. Chelation cuts gadolinium's toxicity dramatically. In rodents, it lowers the toxicity relative to the free ion by a factor of 31.

  Risk does not vanish entirely. In patients with kidney failure, gadolinium-based contrast agents can trigger a rare but serious illness called nephrogenic systemic fibrosis, which resembles scleromyxedema and to some extent scleroderma. The illness can appear months after injection, and its link to gadolinium rather than the carrier molecule is confirmed because it occurs with very different carrier molecules. Anaphylactoid reactions are rare, occurring in roughly 0.03 to 0.1% of cases. The Canadian Association of Radiologists advises that dialysis patients receive these agents only where essential and undergo dialysis after the exam.

• 6. Brighter Images

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  Paramagnetic ions speed up nuclear spin relaxation rates, and that single physical fact is why gadolinium brightens an MRI scan. Solutions of organic gadolinium complexes are administered intravenously to enhance images in medical scans and in magnetic resonance angiography procedures. Magnevist is the most widespread example of such an agent.

  Gadonanotubes push the technique further. These are nanotubes packed with gadolinium, and they are reported to be 40 times more effective than the usual gadolinium contrast agent. Traditional agents are un-targeted, spreading throughout the body after injection, but they will not readily cross the intact blood-brain barrier. Brain tumors and other disorders that degrade that barrier let the agents slip into the brain, where contrast-enhanced MRI can detect them.

  Gadolinium also works inside imaging hardware as a phosphor. Terbium-doped gadolinium oxysulfide sits in the phosphor layer of X-ray detectors, suspended in a polymer matrix, converting X-rays into light. It emits green light at 540 nanometers thanks to the Tb3+ ions, with an energy conversion of up to 20%. A related material, gadolinium oxyorthosilicate doped with cerium, serves as a scintillator in positron emission tomography and for detecting neutrons. Gadolinium compounds were once used to make green phosphors for color TV tubes as well.

• 7. From Refrigerators To Reactors

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  Gadolinium is the standard reference material in the study of magnetic refrigeration near room temperature. Its large magnetocaloric effect near its Curie temperature of 20 degrees Celsius is the reason. That has driven interest in alloys with a larger effect and an adjustable Curie temperature. In the compounds Gd5(SixGe1 minus x)4, the silicon and germanium proportions can be varied to tune that temperature, and a significant magnetocaloric effect appears at higher temperatures, up to about 300 kelvins.

  Superconductors bring gadolinium into power generation. Gadolinium barium copper oxide, or GdBCO, is a superconductor with uses in motors or generators such as those in wind turbines. It shares an analogous chemical composition with the well-studied yttrium barium copper oxide. In 2014 it set a world record for the highest trapped magnetic field in a bulk high temperature superconductor, with 17.6 tesla trapped within two GdBCO bulks.

  Alloys and electronics fill out the list. As little as 1% of gadolinium can significantly improve the workability of iron, chromium, and related metals, along with their resistance to oxidation at high temperatures. Gadolinium yttrium garnet finds microwave applications and serves as a substrate for magneto-optical films. As a dopant in cerium oxide, gadolinium-doped ceria becomes an electrolyte for solid oxide fuel cells with high ionic conductivity. Gadolinium-153, produced in a reactor and carrying a half-life of 240 days, emits gamma radiation used in bone density gauges for osteoporosis screening.

• 8. Rare In The Ground

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  About 6.2 milligrams per kilogram is gadolinium's abundance in the Earth's crust, and the metal is too reactive to exist there in pure form. It is found in nature only in an oxidized state, locked into minerals such as monazite and bastnäsite. The only known mineral with essential gadolinium, lepersonnite-(Gd), is very rare. World production of pure gadolinium runs about 400 tonnes per year, drawn mainly from mining areas in China, the US, Brazil, Sri Lanka, India, and Australia, with reserves expected to exceed one million tonnes.

  Pulling the metal out is a patient chemical relay. Crushed minerals are extracted with hydrochloric or sulfuric acid to convert insoluble oxides into soluble chlorides or sulfates. The filtrates are partially neutralized with caustic soda so that thorium precipitates out as its hydroxide. Ammonium oxalate then turns the rare earths into insoluble oxalates, which are heated into oxides. Nitric acid separates out cerium, whose oxide will not dissolve in it, and magnesium nitrate crystallizes a mixture of double salts of gadolinium, samarium, and europium, finally split apart by ion exchange chromatography. The metal itself emerges by heating its oxide or salts with calcium at 1450 degrees Celsius in an argon atmosphere.

  The element's reach now extends beyond Earth's troubles to the cosmos. In the Japanese Super-Kamiokande detector, gadolinium helps sense supernova explosions by detecting antineutrinos. Low-energy neutrons from antineutrino absorption are captured by gadolinium nuclei in the detector's ultrapure water, which then emit gamma rays that mark the antineutrino's passage.