Gadolinium is a chemical element; it has symbol Gd and atomic number 64. It is a silvery-white metal when oxidation is removed. Gadolinium is a ductile and malleable rare-earth element. It reacts with atmospheric oxygen or moisture slowly to form a black oxide coating. Gadolinium below its Curie point of is ferromagnetic, with an attraction to a magnetic field higher than that of nickel. Above this temperature it is the most paramagnetic element. It is found in nature only in an oxidized form. When separated, it usually has impurities of the other rare earths because of their similar chemical properties.
Gadolinium was discovered in 1880 by Jean Charles de Marignac, who detected its oxide by using spectroscopy. It is named after the mineral gadolinite, one of the minerals in which gadolinium is found, itself named for the Finnish chemist Johan Gadolin. Pure gadolinium was first isolated by the chemist Félix Trombe in 1935.
Gadolinium possesses unusual metallurgical properties, to the extent that as little as 1% of gadolinium can significantly improve the workability and resistance to oxidation at high temperatures of iron, chromium, and related metals. Gadolinium as a metal or a salt absorbs neutrons and is, therefore, used sometimes for shielding in neutron radiography and in nuclear reactors.
Like most of the rare earths, gadolinium forms trivalent ions with fluorescent properties, and salts of gadolinium(III) are used as phosphors in various applications.
Gadolinium(III) ions in water-soluble salts are highly toxic to mammals. However, chelated gadolinium(III) compounds prevent the gadolinium(III) from being exposed to the organism, and the majority is excreted by healthy kidneys before it can deposit in tissues. Because of its paramagnetic properties, solutions of chelated organic gadolinium complexes are used as intravenously administered gadolinium-based MRI contrast agents in medical magnetic resonance imaging.
The main uses of gadolinium, in addition to use as a contrast agent for MRI scans, are in nuclear reactors, in alloys, as a phosphor in medical imaging, as a gamma ray emitter, in electronic devices, in optical devices, and in superconductors.
Characteristics
Physical properties Gadolinium is the eighth member of the lanthanide series. In the periodic table, it appears between the elements europium to its left and terbium to its right, and above the actinide curium. It is a silvery-white, malleable, ductile rare-earth element. Its 64 electrons are arranged in the configuration of [Xe]4f75d16s2, of which the ten 4f, 5d, and 6s electrons are valence.
Like most other metals in the lanthanide series, three electrons are usually available as valence electrons. The remaining 4f electrons are too strongly bound: this is because the 4f orbitals penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this increases with higher ionic charge. Gadolinium crystallizes in the hexagonal close-packed α-form at room temperature. At temperatures above , it forms or transforms into its β-form, which has a body-centered cubic structure.
The isotope gadolinium-157 has the highest thermal-neutron capture cross-section among any stable nuclide: about 259,000 barns. Only xenon-135 has a higher capture cross-section, about 2.0 million barns, but is radioactive.
Gadolinium is believed to be ferromagnetic at temperatures below and is strongly paramagnetic above this temperature. In fact, at body temperature, gadolinium exhibits the greatest paramagnetic effect of any element. There is evidence that gadolinium is a helical antiferromagnetic, rather than a ferromagnetic, below . Gadolinium demonstrates a magnetocaloric effect whereby its temperature increases when it enters a magnetic field and decreases when it leaves the magnetic field. A significant magnetocaloric effect is observed at higher temperatures, up to about , in the compound .
Individual gadolinium atoms can be isolated by encapsulating them into fullerene molecules, where they can be visualized with a transmission electron microscope. Individual Gd atoms and small Gd clusters can be incorporated into carbon nanotubes.
Chemical properties
Gadolinium combines with most elements to form Gd(III) derivatives. It also combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, and arsenic at elevated temperatures, forming binary compounds.
Unlike the other rare-earth elements, metallic gadolinium is relatively stable in dry air. However, it tarnishes quickly in moist air, forming a loosely-adhering gadolinium(III) oxide (): , which spalls off, exposing more surface to oxidation.
Gadolinium is a strong reducing agent, which reduces oxides of several metals into their elements. Gadolinium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form gadolinium(III) hydroxide (): .
Gadolinium metal is attacked readily by dilute sulfuric acid to form solutions containing the colorless Gd(III) ions, which exist as complexes: .
Chemical compounds In the great majority of its compounds, like many rare-earth metals, gadolinium adopts the oxidation state +3. However, gadolinium can be found on rare occasions in the 0, +1 and +2 oxidation states. All four trihalides are known. All are white, except for the iodide, which is yellow. Most commonly encountered of the halides is gadolinium(III) chloride (). The oxide dissolves in acids to give the salts, such as gadolinium(III) nitrate and gadolinium(III) sulfate.
Gadolinium(III), like most lanthanide ions, forms complexes with high coordination numbers. This tendency is illustrated by the use of the chelating agent DOTA, an octadentate ligand. Salts of are useful in magnetic resonance imaging. A variety of related chelate complexes have been developed, including gadodiamide.
Reduced gadolinium compounds are known, especially in the solid state. Gadolinium(II) halides are obtained by heating Gd(III) halides in presence of metallic Gd in tantalum containers. Gadolinium also forms the sesquichloride , which can be further reduced to GdCl by annealing at . This gadolinium(I) chloride forms platelets with layered graphite-like structure.
Isotopes
Naturally occurring gadolinium is composed of the element's six stable isotopes - Gd, Gd, Gd, Gd, Gd, and Gd - as well as the primordial radionuclide Gd, which has an extremely long half-life of 1.08×1014 years. Out of these isotopes, Gd has the highest natural abundance at 24.8%.
Gadolinium has 35 known radioisotopes, ranging in mass number from 133 to 173. The three most stable of these all undergo alpha decay into isotopes of samarium: Gd (t½ = 1.08×1014 years), Gd (t½ = 1.79×106 years), and Gd (t½ = 86.9 years). All remaining radioisotopes have half-lives less than a year, and the majority have half-lives less than two minutes. Radioisotopes of gadolinium with mass numbers at or below 147 primarily decay via electron capture into isotopes of europium, while those with mass numbers at or above 161 primarily decay via beta minus decay into isotopes of terbium. Gadolinium also has fourteen known metastable isomers, with the three most stable being Gd (t½ = 110 seconds), Gd (t½ = 85 seconds), and Gd (t½ = 24.5 seconds).
Gd, Gd, and Gd are observationally stable, meaning that they are predicted to decay, but their decay has never been observed. Gd and Gd are expected to alpha decay into Sm and Sm respectively, while Gd is expected to double beta decay into Dy. An experimental lower bound for the half-life of Gd has been measured to be 1.3×1021 years.
History In 1794, the Finnish chemist and mineralogist Johan Gadolin became the first to chemically analyze the mineral now known as gadolinite, which was named after him by the German chemist Martin Klaproth in 1802. Because Gadolin had found a new ore ("einer unbekannten Erdart" (an unknown type of ore)) in a mineral which had previously been called "Ytterbite" (because it had been found near the town of Ytterby in Sweden), Klaproth proposed to rename the mineral "Gadolinite". From p. 54: "Herr Gadolin hat also das Verdienst, diese neue Erde im gegenwärtigen Fossil zuerst entdeckt zu haben; weshalb ich auch, mit mehrern Naturforschern, dessen Namen Gadolinit der erstern Benennung Ytterbit vorziehe." (Mr. Gadolin thus has the merit of having first discovered this new ore in the present rock; for which reason I, with several [other] scientists, prefer the name "gadolinite" to the first name "ytterbite".) Klaproth used the name "gadolinite" as early as 1801: In 1880, the Swiss chemist Jean Charles Galissard de Marignac observed the spectroscopic lines of a then-unknown element, gadolinium, in samples of gadolinite (which actually contains relatively little gadolinium, but enough to show a spectrum) and the separate mineral cerite, which proved to contain far more of the new element. De Marignac eventually separated a mineral oxide from cerite, which he realized was the oxide of the element. He designated the element with the provisional symbol Yα. The French chemist Paul-Émile Lecoq de Boisbaudran named the element "gadolinium" after gadolinite in 1886. Pure gadolinium metal was isolated for the first time in 1935 by Félix Trombe.
Occurrence
Gadolinium is a constituent in many minerals, such as monazite and bastnäsite. The metal is too reactive to exist naturally. Paradoxically, as noted above, the mineral gadolinite actually contains only traces of this element. The abundance in the Earth's crust is about 6.2 mg/kg. The main mining areas are in China, the US, Brazil, Sri Lanka, India, and Australia with reserves expected to exceed one million tonnes. World production of pure gadolinium is about 400 tonnes per year. The only known mineral with essential gadolinium, lepersonnite-(Gd), is very rare.
Production Gadolinium is produced both from monazite and bastnäsite. Crushed minerals are extracted with hydrochloric acid or sulfuric acid, which converts the insoluble oxides into soluble chlorides or sulfates. The acidic filtrates are partially neutralized with caustic soda to pH 3–4. Thorium precipitates as its hydroxide, and is then removed. The remaining solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by heating. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in . The solution is treated with magnesium nitrate to produce a crystallized mixture of double salts of gadolinium, samarium and europium. The salts are separated by ion exchange chromatography. The rare-earth ions are then selectively washed out by a suitable complexing agent.
Gadolinium metal is obtained from its oxide or salts by heating it with calcium at in an argon atmosphere. Sponge gadolinium can be produced by reducing molten with an appropriate metal at temperatures below (the melting point of Gd) at reduced pressure.
Applications
Gadolinium has no large-scale applications, but it has a variety of specialized uses.
Neutron absorber Because gadolinium has a high neutron cross-section, it is effective for use with neutron radiography and in shielding of nuclear reactors. It is used as a secondary, emergency shut-down measure in some nuclear reactors, particularly of the CANDU reactor type. Gadolinium is used in nuclear marine propulsion systems as a burnable poison. The use of gadolinium in neutron capture therapy to target tumors has been investigated, and gadolinium-containing compounds have proven promising.
Alloys Gadolinium possesses unusual metallurgic properties, with as little as 1% of gadolinium improving the workability of iron, chromium, and related alloys, and their resistance to high temperatures and oxidation.
Magnetic contrast agent