gadolinium

gadolinium (Gd), chemical element, a rare-earth metal of the lanthanide series of the periodic table.

Gadolinium is a moderately ductile, moderately hard, silvery white metal that is fairly stable in air, although with time it tarnishes in air, forming a thin film of Gd₂O₃ on the surface. Gadolinium reacts slowly with water and rapidly with diluted acids—except hydrofluoric acid (HF), in which a stable protective layer of GdF₃ forms and prevents the metal from further reaction. Gadolinium is the only lanthanide that is ferromagnetic near room temperature; its Curie point (ferromagnetic ordering) is 293 K (20 °C, or 68 °F). Above this temperature the metal is a very strong paramagnet.

Gadolinium was discovered by Jean-Charles Galissard de Marignac and Paul-Émile Lecoq de Boisbaudran. Marignac separated (1880) a new rare earth (metallic oxide) from the mineral samarskite, and Lecoq de Boisbaudran obtained (1886) a fairly pure sample of the same earth, which with Marignac’s assent he named gadolinia, after a mineral in which it occurs that in turn had been named for the Finnish chemist Johan Gadolin. Gadolinium occurs in many minerals along with the other rare earths but is obtained primarily from bastnasite. It is also found in products of nuclear fission. In Earth’s crust gadolinium is as abundant as nickel and arsenic.

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In nature the element occurs as a mixture of six stable isotopes—gadolinium-158 (24.84 percent), gadolinium-160 (21.86 percent), gadolinium-156 (20.47 percent), gadolinium-157 (15.65 percent), gadolinium-155 (14.8 percent), and gadolinium-154 (2.18 percent)—and one radioactive isotope, gadolinium-152 (0.20 percent). Odd-numbered isotopes have extremely high nuclear absorption cross sections, with that of gadolinium-157 reaching 259,000 barns. As a result, the naturally occurring mixture of gadolinium isotopes also has a very high nuclear absorption cross section on the order of 49,000 barns. Excluding nuclear isomers, a total of 32 radioactive isotopes of gadolinium ranging in mass from 133 to 169 and having half-lives from 1.1 seconds (gadolinium-135) to 1.08 × 10¹⁴ years (gadolinium-152) have been characterized.

Commercial separation of the metal is done using solvent-solvent extraction or ion-exchange techniques. The metal has been produced by metallothermic reduction of the anhydrous chloride or fluoride by calcium. Gadolinium exists in two allotropic forms. The α-phase is close-packed hexagonal with a = 3.6336 Å and c = 5.7810 Å at room temperature. The β-phase is body-centred cubic with a = 4.06 Å at 1,265 °C (2,309 °F).

The major uses of gadolinium compounds include hosts for phosphors for fluorescent lamps, X-ray intensifying screens, and scintillators for X-ray tomography, and as a magnetic resonance imaging (MRI) contrast agent (in the form of water-soluble chelates). Other uses are in shields and control rods of nuclear reactors (due to its very high nuclear absorption cross section) and as a component of yttrium gadolinium garnet, which is employed in communications.

Gadolinium sulfate, Gd₂(SO₄)₃―7H₂O, was used by American chemist William F. Giauque and his graduate student D.P. MacDougal in 1933 to reach temperatures below 1 K (−272 °C, or −458 °F) by adiabatic demagnetization. Gadolinium metal was employed by Gerald V. Brown as an active element of a near-room-temperature magnetic refrigerator prototype, which in 1976–78 reached a temperature span of nearly 80 °C (176 °F) using a magnetic field of 7 teslas and a water-based heat-exchange fluid. Since then the metal became the magnetic refrigerant material of choice for numerous continuously operating laboratory magnetic refrigeration devices. In 1997 American materials scientists Vitalij Pecharsky and Karl Gschneidner, Jr., discovered the giant magnetocaloric effect in Gd₅(Si_{1 − x}Ge_x)₄ compounds; this discovery gave a strong impetus toward the development and commercialization of magnetic refrigeration technology.

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Gadolinium displays the oxidation state +3 in all its compounds; it behaves as a typical rare earth. Its salts are white, and its solutions are colourless.