Lanthanide
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 20 °C (68 °F) 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. …
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. 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.
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 1,235 °C (2,255 °F), 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 20 °C (68 °F) 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 20 °C (68 °F). …
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.
Gadolinium has no large-scale applications, but it has a variety of specialized uses.
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.
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.
Paramagnetic ions, such as gadolinium, increase nuclear spin relaxation rates, making gadolinium useful as a contrast agent for magnetic resonance imaging (MRI). Solutions of organic gadolinium complexes and gadolinium compounds are used as intravenous contrast agents to enhance images in medical and magnetic resonance angiography (MRA) procedures. Magnevist is the most widespread example. Nanotubes packed with gadolinium, called "gadonanotubes", are 40 times more effective than the usual gadolinium contrast agent. Traditional gadolinium-based contrast agents are un-targeted, generally distributing throughout the body after injection, but will not readily cross the intact blood–brain barrier. …
Naturally occurring gadolinium is composed of the element's six stable isotopes—154Gd, 155Gd, 156Gd, 157Gd, 158Gd, and 160Gd—as well as the primordial radionuclide 152Gd, which has an extremely long half-life of 1.08×1014 years. Out of these isotopes, 158Gd 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: 152Gd (t½ = 1.08×1014 years), 150Gd (t½ = 1.79×106 years), and 148Gd (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 143mGd (t½ = 110 seconds), 145mGd (t½ = 85 seconds), and 141mGd (t½ = 24.5 seconds).
154Gd, 155Gd, and 160Gd are observationally stable, meaning that they are predicted to decay, but their decay has never been observed. 154Gd and 155Gd are expected to alpha decay into 150Sm and 151Sm respectively, while 160Gd is expected to double beta decay into 160Dy. An experimental lower bound for the half-life of 160Gd has been measured to be 1.3×1021 years.
As a free ion, gadolinium is reported often to be highly toxic, but MRI contrast agents are chelated compounds and are considered safe enough to be used in most persons. The toxicity of free gadolinium ions in animals is due to interference with a number of calcium-ion channel dependent processes. The 50% lethal dose is about 0.34 mmol/kg (IV, mouse) or 100–200 mg/kg. Toxicity studies in rodents show that chelation of gadolinium (which also improves its solubility) decreases its toxicity with regard to the free ion by a factor of 31 (i.e., the lethal dose for the Gd-chelate increases by 31 times). It is believed therefore that clinical toxicity of gadolinium-based contrast agents (GBCAs) in humans will depend on the strength of the chelating agent; however this research is still not complete. About a dozen different Gd-chelated agents have been approved as MRI contrast agents around the world.
Use of gadolinium-based contrast agents results in deposition of gadolinium in tissues of the brain, bone, skin, and other tissues in amounts that depend on kidney function, structure of the chelates (linear or macrocyclic) and the dose administered. In patients with kidney failure, there is a risk of a rare but serious illness called nephrogenic systemic fibrosis (NSF) that is caused by the use of gadolinium-based contrast agents. The disease resembles scleromyxedema and to some extent scleroderma. It may occur months after a contrast agent has been injected. …