Gadolinium is achemical element; it hassymbolGd andatomic number 64. It is a silvery-white metal when oxidation is removed. Gadolinium is a malleable andductilerare-earth element. It reacts with atmospheric oxygen or moisture slowly to form a blackoxide coating. Gadolinium below itsCurie point of 20 °C (68 °F) isferromagnetic, with an attraction to a magnetic field higher than that of nickel. Above this temperature it is the mostparamagnetic 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 byJean Charles de Marignac, who detected its oxide by using spectroscopy. It is named after the mineralgadolinite, one of the minerals in which gadolinium is found, itself named for the Finnish chemistJohan Gadolin. Pure gadolinium was first isolated by the chemistFélix Trombe in 1935.
Gadolinium possesses unusualmetallurgical properties, to the extent that as little as 1% of gadolinium can significantly improve the workability and resistance tooxidation at high temperatures of iron,chromium, and related metals. Gadolinium as a metal or a salt absorbsneutrons and is, therefore, used sometimes for shielding inneutron radiography and innuclear reactors.
Like most of the rare earths, gadolinium formstrivalent ions with fluorescent properties, and salts of gadolinium(III) are used asphosphors 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[9] kidneys before it can deposit in tissues. Because of itsparamagnetic properties, solutions of chelatedorganic gadoliniumcomplexes are used as intravenously administeredgadolinium-based MRI contrast agents in medicalmagnetic resonance imaging.
The main uses of gadolinium, in addition to use as acontrast 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.
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 thehexagonal 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 abody-centered cubic structure.[10]
Theisotope gadolinium-157 has the highestthermal-neutroncapture cross-section among any stable nuclide: about 259,000barns. Onlyxenon-135 has a higher capture cross-section, about 2.0 million barns, but this isotope isradioactive.[11]
Gadolinium is believed to beferromagnetic at temperatures below 20 °C (68 °F)[12] and is stronglyparamagnetic above this temperature. In fact, at body temperature, gadolinium exhibits the greatest paramagnetic effect of any element.[13] There is evidence that gadolinium is a helical antiferromagnetic, rather than a ferromagnetic, below 20 °C (68 °F).[14] Gadolinium demonstrates amagnetocaloric 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 300 kelvins, in the compounds Gd5(Si1−xGex)4.[15]
Gadolinium combines with most elements to form Gd(III) derivatives. It also combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, andarsenic at elevated temperatures, forming binary compounds.[18]
Unlike the other rare-earth elements, metallic gadolinium is relatively stable in dry air. However, ittarnishes quickly in moist air, forming a loosely-adheringgadolinium(III) oxide (Gd2O3):
4 Gd + 3 O2 → 2 Gd2O3,
whichspalls off, exposing more surface to oxidation.
Gadolinium is a strongreducing 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 formgadolinium(III) hydroxide (Gd(OH)3):
2 Gd + 6 H2O → 2 Gd(OH)3 + 3 H2.
Gadolinium metal is attacked readily by dilutesulfuric acid to form solutions containing the colorless Gd(III) ions, which exist as[Gd(H2O)9]3+ complexes:[19]
In the great majority of its compounds, like manyrare-earth metals, gadolinium adopts theoxidation 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 isgadolinium(III) chloride (GdCl3). The oxide dissolves in acids to give the salts, such asgadolinium(III) nitrate.
Gadolinium(III), like most lanthanide ions, formscomplexes with highcoordination numbers. This tendency is illustrated by the use of the chelating agentDOTA, an octadentate ligand. Salts of [Gd(DOTA)]− are useful inmagnetic resonance imaging. A variety of related chelate complexes have been developed, includinggadodiamide.
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 intantalum containers. Gadolinium also forms the sesquichlorideGd2Cl3, which can be further reduced to GdCl by annealing at 800 °C (1,470 °F). This gadolinium(I) chloride forms platelets with layered graphite-like structure.[20]
Naturally occurring gadolinium is composed of six stable isotopes,154Gd,155Gd,156Gd,157Gd,158Gd and160Gd, and oneradioisotope,152Gd, with the isotope158Gd being the most abundant (24.8%natural abundance).[21]
Thirty-three radioisotopes have been characterized, with the three most stable being alpha emitters:152Gd (naturally occurring) with a half-life of 1.08×1014 years,150Gd with a half-life of 1.79×106 years, and148Gd (theoretically notbeta-stable) with a half-life of 86.9 years. All of the remaining radioactive isotopes have half-lives less than a year, the majority of these having half-lives less than two minutes. There are also 10 metastableisomers, with the most stable being143mGd (t1/2 = 110 seconds),145mGd (t1/2 = 85 seconds) and141mGd (t1/2 = 24.5 seconds).[22] The predicted double beta decay of160Gd has never been observed (an experimental lower limit on itshalf-life of more than 1.3×1021 years has been measured[23]).
Gadolinium is named after the mineralgadolinite. Gadolinite was first chemically analyzed by the Finnish chemistJohan Gadolin in 1794.[24][25] In 1802 German chemist Martin Klaproth gave gadolinite its name.[26][10] In 1880, the SwisschemistJean Charles Galissard de Marignac observed the spectroscopic lines from gadolinium in samples ofgadolinite (which actually contains relatively little gadolinium, but enough to show a spectrum) and in the separate mineralcerite. The latter mineral proved to contain far more of the element with the new spectral line. De Marignac eventually separated a mineral oxide from cerite, which he realized was the oxide of this new element. He designated the element with the provisional symbol Yα. The French chemistPaul-Émile Lecoq de Boisbaudran named the element "gadolinium" in 1886.[27][28][29][30] The pure element was isolated in 1935 byFélix Trombe.[31]
Gadolinium is a constituent in many minerals, such asmonazite andbastnäsite. The metal is too reactive to exist naturally. Paradoxically, as noted above, the mineralgadolinite actually contains only traces of this element. The abundance in the Earth's crust is about 6.2 mg/kg.[10] 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.[32][33]
Gadolinium is produced both from monazite andbastnäsite.
Crushed minerals are extracted withhydrochloric acid orsulfuric 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 withammonium oxalate to convert rare earths into their insolubleoxalates. The oxalates are converted to oxides by heating.
The oxides are dissolved innitric acid that excludes one of the main components,cerium, whose oxide is insoluble in HNO3.
The salts are separated byion exchange chromatography.
The rare-earth ions are then selectively washed out by a suitable complexing agent.[10]
Gadolinium metal is obtained from its oxide or salts by heating it withcalcium at 1,450 °C (2,640 °F) in an argon atmosphere. Sponge gadolinium can be produced by reducing molten GdCl3 with an appropriate metal at temperatures below 1,312 °C (2,394 °F) (the melting point of Gd) at reduced pressure.[10]
Because gadolinium has a high neutron cross-section, it is effective for use withneutron radiography and in shielding ofnuclear reactors. It is used as a secondary, emergency shut-down measure in some nuclear reactors, particularly of theCANDU reactor type.[10] Gadolinium is used innuclear marine propulsion systems as aburnable poison. The use of gadolinium inneutron capture therapy to target tumors has been investigated, and gadolinium-containing compounds have proven promising.[34]
Gadolinium possesses unusualmetallurgic properties, with as little as 1% of gadolinium improving the workability of iron,chromium, and relatedalloys, and their resistance to high temperatures andoxidation.[35]
Gadolinium is used as a phosphor in medical imaging. It is contained in the phosphor layer ofX-ray detectors, suspended in a polymer matrix.Terbium-dopedgadolinium oxysulfide (Gd2O2S:Tb) at the phosphor layer converts the X-rays released from the source into light. This material emits green light at 540 nm because of the presence of Tb3+, which is very useful for enhancing the imaging quality. The energy conversion of Gd is up to 20%, which means that one fifth of the X-ray energy striking the phosphor layer can be converted into visible photons.[citation needed] Gadolinium oxyorthosilicate (Gd2SiO5, GSO; usually doped by 0.1–1.0% ofCe) is a single crystal that is used as ascintillator in medical imaging such aspositron emission tomography, and for detecting neutrons.[40]
Gadolinium compounds were also used for making greenphosphors for color TV tubes.[41]
Gadolinium-153 is produced in a nuclear reactor from elementaleuropium or enriched gadolinium targets. It has a half-life of240±10 days and emitsgamma 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.[42] It is also used as a gamma-ray source in X-ray absorption measurements and inbone density gauges forosteoporosis screening.[citation needed]
Gadolinium is used for makinggadolinium yttrium garnet (Gd:Y3Al5O12), which hasmicrowave applications and is used in fabrication of various optical components and as substrate material for magneto-optical films.[43]
Gadolinium is the standard reference material in the study ofmagnetic refrigeration near room temperature.[15]: 1528 Pure Gd itself exhibits a large magnetocaloric effect near itsCurie temperature of 20 °C (68 °F), and this has sparked interest into producing Gd alloys having a larger effect and tunable Curie temperature. In Gd5(SixGe1−x)4, Si and Ge compositions can be varied to adjust theCurie temperature.Gadolinium-based materials, such as Gd5(SixGe1−x)4, are currently the most promising materials, owing to their high Curie temperature and giant magneto-caloric effect. Magnetic refrigeration could provide significant efficiency and environmental advantages over conventional refrigeration methods.[15]
Gadolinium barium copper oxide (GdBCO) is a superconductor[44][45][46] with applications in superconducting motors or generators such as in wind turbines.[47] It can be manufactured in the same way as the most widely researched cuprate high temperature superconductor,yttrium barium copper oxide (YBCO) and uses an analogous chemical composition (GdBa2Cu3O7−δ ).[48] It was used in 2014 to set a new world record for the highest trapped magnetic field in a bulkhigh temperature superconductor, with a field of 17.6T being trapped within two GdBCO bulks.[49][50]
Gadolinium is being investigated as a possible treatment for preventing lung tissue scarring inasthma. A positive effect has been observed in mice.[51]
Gadolinium is used forantineutrino detection in the JapaneseSuper-Kamiokande detector in order to sensesupernova explosions. Low-energy neutrons that arise from antineutrino absorption by protons in the detector's ultrapure water are captured by gadolinium nuclei, which subsequently emitgamma rays that are detected as part of the antineutrino signature.[52]
As a free ion, gadolinium is reported often to be highly toxic, but MRI contrast agents arechelated 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. The50% lethal dose is about 0.34 mmol/kg (IV, mouse)[55] 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).[56][57][58] It is believed therefore that clinical toxicity of gadolinium-based contrast agents (GBCAs[59]) in humans will depend on the strength of the chelating agent; however this research is still not complete.[when?] About a dozen different Gd-chelated agents have been approved as MRI contrast agents around the world.[60][61][62]
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 onkidney function, structure of the chelates (linear or macrocyclic) and the dose administered.[63] In patients with kidney failure, there is a risk of a rare but serious illness callednephrogenic systemic fibrosis (NSF)[64] that is caused by the use of gadolinium-based contrast agents. The disease resemblesscleromyxedema and to some extentscleroderma. It may occur months after a contrast agent has been injected. Its association with gadolinium and not the carrier molecule is confirmed by its occurrence with various contrast materials in which gadolinium is carried by very different carrier molecules. Because of the risk of NSF, use of these agents is not recommended for any individual with end-stage kidney failure as they may require emergent dialysis.
Included in the current guidelines from the Canadian Association of Radiologists[65] are that dialysis patients should receive gadolinium agents only where essential and that they should receive dialysis after the exam. If a contrast-enhanced MRI must be performed on a dialysis patient, it is recommended that certain high-risk contrast agents be avoided but not that a lower dose be considered.[65] The American College of Radiology recommends that contrast-enhanced MRI examinations be performed as closely before dialysis as possible as a precautionary measure, although this has not been proven to reduce the likelihood of developing NSF.[66] The FDA recommends that potential for gadolinium retention be considered when choosing the type of GBCA used in patients requiring multiple lifetime doses, pregnant women, children, and patients with inflammatory conditions.[67]
Gadolinium has no known native biological role, but its compounds are used as research tools in biomedicine. Gd3+ compounds are components ofMRI contrast agents.[71] It is used in variousion channel electrophysiology experiments to block sodium leak channels and stretch activated ion channels.[72] Gadolinium has recently been used to measure the distance between two points in a protein viaelectron paramagnetic resonance, something that gadolinium is especially amenable to thanks to EPR sensitivity at w-band (95 GHz) frequencies.[73]
^The thermal expansion of a Gd crystal is highlyanisotropic and temperature-dependent: the parameters for each crystal axis at 20 °C are: αa = 9.37×10−6/K, αc = −83.0×10−6/K, and αaverage = αV/3 = −21.4×10−6/K. At 100 °C: αa = 6.6×10−6/K, αc = 20.1×10−6/K, and αaverage = 11.1×10−6/K.
^abcArblaster, John W. (2018).Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International.ISBN978-1-62708-155-9.
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