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Gadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors. Because of its paramagnetic properties, solutions of organic gadolinium complexes and gadolinium compounds are the most popular intravenous MRI contrast agents in medical magnetic resonance imaging.
Gadolinium is a silvery-white malleable and ductile rare-earth metal. It crystallizes in hexagonal, close-packed α- form at room temperature, but, when heated to temperatures above 1235 °C, it transforms into its β- form, which has a body-centered cubic structure.
Gadolinium-157 has the highest thermal neutron capture cross-section among any stable nuclides at 259,000 barns. Only xenon-135 has a higher cross section, 2 million barns, but that isotope is unstable.
Gadolinium is strongly paramagnetic at room temperature, and exhibits ferromagnetic properties below room temperature. Gadolinium demonstrates a magnetocaloric effect whereby its temperature increases when it enters a magnetic field and decreases when it leaves the magnetic field. The effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2).
Individual gadolinium atoms have been isolated by encapsulating them into fullerene molecules and visualized with transmission electron microscope. Individual Gd atoms and small Gd clusters have also been incorporated into carbon nanotubes.
Unlike other rare earth elements, gadolinium is relatively stable in dry air. However, it tarnishes quickly in moist air, forming a loosely-adhering oxide which spalls off, exposing more surface to oxidation.
4 Gd + 3 O2 → 2 Gd2O3
Gadolinium is a strong reducing agent, which reduces oxides of several metals, such as Fe, Cr, Sn, Pb, Mn and Zr, into their elements. Gadolinium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form gadolinium hydroxide:
2 Gd (s) + 6 H2O (l) → 2 Gd(OH)3 (aq) + 3 H2 (g)
Gadolinium metal reacts with all the halogens at temperature about 200 °C:
2 Gd (s) + 3 F2 (g) → 2 GdF3 (s) [white]
Gadolinium dissolves readily in dilute sulfuric acid to form solutions containing the colorless Gd(III) ions, which exist as a [Gd(OH2)9]3+ complexes:
2 Gd (s) + 3 H2SO4 (aq) → 2 Gd3+ (aq) + 3 SO2−4 (aq) + 3 H2 (g)
Gadolinium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming binary compounds. In those compounds, Gd mostly exhibit oxidation state +3. Gadolinium(II) halides are obtained by annealing Gd(III) halides in presence of metallic Gd in tantalum containers. Gadolinium also form sesquichloride Gd2Cl3, which can be further reduced to GdCl by annealing at 800 °C. This gadolinium(I) chloride forms platelets with layered graphite-like structure.
Compounds of gadolinium include
* Fluorides: GdF3
Naturally occurring gadolinium is composed of 6 stable isotopes, 154Gd, 155Gd, 156Gd, 157Gd, 158Gd and 160Gd, and 1 radioisotope, 152Gd, with 158Gd being the most abundant (24.84% natural abundance). The predicted double beta decay of 160Gd has never been observed (only lower limit on its half-life of more than 1.3×1021 years has been set experimentally ).
Twenty-nine radioisotopes have been characterized, with the most stable being alpha-decaying 152Gd (naturally occurring) with a half-life of 1.08×1014 years, and 150Gd with a half-life of 1.79×106 years. All of the remaining radioactive isotopes have half-lives less than 74.7 years. The majority of these have half-lives less than 24.6 seconds. Gadolinium isotopes have 4 metastable isomers, with the most stable being 143mGd (T½=110 seconds), 145mGd (T½=85 seconds) and 141mGd (T½=24.5 seconds).
The primary decay mode at atomic masses lower than the most abundant stable isotope, 158Gd, is electron capture, and the primary mode at higher atomic masses is beta decay. The primary decay products for isotopes of weights lower than 158Gd are the element Eu (europium) isotopes and the primary products at higher weights are the element Tb (terbium) isotopes.
In 1880, Swiss chemist Jean Charles Galissard de Marignac observed spectroscopic lines due to gadolinium in samples of didymium and gadolinite; French chemist Paul Émile Lecoq de Boisbaudran separated gadolinia, the oxide of gadolinium, from Mosander's yttria in 1886. The element itself was isolated only recently. Gadolinium, like the mineral gadolinite, is named after Finnish chemist and geologist Johan Gadolin.
Gadolinium is never found in nature as the free element, but is contained in many rare minerals such as monazite and bastnäsite. It occurs only in trace amounts in the mineral gadolinite, which was also named after Johan Gadolin. The abundance in the earth crust is about 6.2 mg/kg. The main mining areas are China, USA, 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.
Gadolinium is produced both from monazite and bastnäsite.
1. Crushed minerals are attacked by hydrochloric or sulfuric acid that transforms insoluble rare-earth oxides into soluble chlorides or sulfates.
Gadolinium metal is obtained from its oxide or salts by heating with calcium at 1450 °C under argon atmosphere. Sponge gadolinium can be produced by reducing molten GdCl3 with an appropriate metal oxide at temperatures below 1312 °C (melting point of Gd) in a reduced pressure.
Gadolinium has the highest neutron cross-section among any stable nuclides, 61,000 barns for 155Gd and 259,000 barns for 157Gd (compare with Gd ionic radius ~100,000,000 barns). 157Gd has been used to target tumors in neutron therapy. This element is very 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 type. Gadolinium is also used in nuclear marine propulsion systems as a burnable poison.
Gadolinium is an efficient catalyst used for decarboxylation of oxaloacetic acid, conversion of ortho-hydrogen to para-hydrogen and polymerization of ethylene.
Gadolinium also possesses unusual metallurgic properties, with as little as 1% of gadolinium improving the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation. Magnesium alloy Elektron21 produced by Magnesium Elektron is a high strength magnesium alloy that uses gadolinium and neodymium as strengthening additions.
Gadolinium is paramagnetic at room temperature, with a ferromagnetic Curie point of 17 °C.Paramagnetic ions, such as gadolinium, move differently within a magnetic field. This trait makes gadolinium useful for magnetic resonance imaging (MRI). Solutions of organic gadolinium complexes and gadolinium compounds are used as intravenous MRI contrast agent to enhance images in medical magnetic resonance imaging and magnetic resonance angiography (MRA) procedures. Magnevist is the most widespread example.Nanotubes packed with gadolinium, dubbed "gadonanotubes," are 40 times more effective than this traditional gadolinium contrast agent.  Once injected, gadolinium-based contrast agents accumulate in abnormal tissues of the brain and body. This accumulation provides a greater contrast between normal and abnormal tissues, allowing doctors to better locate uncommon cell growths and tumors.
Beside MRI, gadolinium is also used in other imaging. In X-ray, gadolinium is contained in the phosphor layer, suspending in a polymer matrix at the detector. Terbium-doped gadolinium oxysulfide (Gd2O2S: Tb) at the phosphor layer is to convert the X-rays releasing from the source into light. This material emits green light at 540 nm due to the presence of Tb3+, which is very useful for enhancing the imaging quality of the X-ray that is exposed to the photographic film. The energy conversion of Gd is up to 20%, which means, one-fifth of the X-ray striking on the phosphor layer can be converted into light photons. Gadolinium oxyorthosilicate (Gd2SiO5, GSO; usually doped by 0.1-1% of Ce) is a single crystal that is used as a scintillator in medical imaging such as positron emission tomography or for detecting neutrons.
Gadolinium-153 is produced in a nuclear reactor from elemental europium or enriched gadolinium targets. It has a half-life of 240±10 days and emits gamma radiation with strong peaks at 41 keV and 102 keV. It is used in many quality assurance applications, such as line sources and calibration phantoms, to ensure that nuclear medicine imaging systems operate correctly and produce useful images of radioisotope distribution inside the patient. It is also used as a gamma ray source in X-ray absorption measurements or in bone density gauges for osteoporosis screening, as well as in the Lixiscope portable X-ray imaging system.
Gadolinium is used for making gadolinium yttrium garnet (Gd:Y3Al5O12); it has microwave applications and is used in fabrication of various optical components and as substrate material for magneto–optical films.
Gadolinium compounds are also used for making green phosphors for colour TV tubes, compact disc
Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) was used as diamond imitation and for computer bubble memory.
Gadolinium has no known native biological role, but in research on biological systems it has a few roles. It is used as a component of MRI contrast agents, as, in the 3+ oxidation state, the metal has 7 unpaired f electrons. This causes water around the contrast agent to relax quickly, enhancing the quality of the MRI scan. Second, as a member of the lanthanides, it is used in various ion channel electrophysiology experiments, where it is used to block sodium leak channels, as well as to stretch activated ion channels.
As a free ion, gadolinium is highly toxic but is generally regarded as safe when administered as a chelated compound. The compounds can be classified by whether they are macro-cyclic or linear geometry and whether they are ionic or not. Cyclical ionic Gd compounds being considered the least likely to release the Gd ion and hence the most safe. US Food and Drug Administration approved Gd chelated contrast agents include: Omniscan, Multihance, Magnevist, ProHance, Vasovist, Eovist and OptiMARK.
Gadolinium MRI contrast agents have proved safer than the iodinated contrast agents used in X-ray radiography or computed tomography. Anaphylactoid reactions are rare, occurring in approx. 0.03-0.1%.
Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illnesses, such as nephrogenic systemic fibrosis and nephrogenic fibrosing dermopathy, that may be linked to the use of certain gadolinium-containing agents. Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan is complete, in order to remove the agent from the body promptly.
1. ^ Charles Kittel (1996). Introduction to Solid State Physics. New York: Wiley. p. 449.