Gadolinium

Physical properties

Gadolinium is a silvery-white malleable and ductile rare earth metal. It crystallizes in hexagonal, in the 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.

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 this isotope is radioactive.

Gadolinium is believed to be ferromagnetic at temperatures below 20 °C (68 °F) and it is strongly paramagnetic above this temperature. There is evidence that gadolinium is a helical antiferromagnetic, rather than a ferromagnetic, below 20 °C (68 °F). Gadolinium demonstrates a magnetocaloric effect whereby its temperature increases when it enters a magnetic field, and decreases when it leaves the magnetic field. The temperature is lowered to 5 °C (41 °F) for the gadolinium alloy Gd₈₅Er₁₅, and this effect is considerably stronger for the alloy Gd₅(Si₂Ge₂), but at a much lower temperature (<85 K (−188.2 °C; −306.7 °F)). A significant magnetocaloric effect is observed at higher temperatures, up to about 300 kelvin, in the compounds Gd₅(Si_xGe_1−x)₄.

Individual gadolinium atoms can be isolated by encapsulating them into fullerene molecules, where they can be visualized with 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 (Gd₂O₃), 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 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 + 6 H₂O → 2 Gd(OH)₃ + 3 H₂

Gadolinium metal is attacked readily by dilute sulfuric acid to form solutions containing the colorless Gd(III) ions, which exist as [Gd(H₂O)₉]³⁺ complexes:

2 Gd + 3 H₂SO₄ + 18 H₂O → 2 [Gd(H₂O)₉]³⁺ + 3 SO2−
4 + 3 H₂

Gadolinium metal reacts with the halogens (X₂) at temperature about 200 °C:

2 Gd + 3 X₂ → 2 GdX₃

Chemical compounds

In the great majority of its compounds, gadolinium adopts the oxidation state +3. 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 (GdCl₃). The oxide dissolves in acids to give the salts, such as gadolinium(III) nitrate.

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 [Gd(DOTA)]⁻ 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 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.

Isotopes

Naturally occurring gadolinium is composed of six stable isotopes, ¹⁵⁴Gd, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd, ¹⁵⁸Gd and ¹⁶⁰Gd, and one radioisotope, ¹⁵²Gd, with the isotope ¹⁵⁸Gd being the most abundant (24.84% natural abundance). The predicted double beta decay of ¹⁶⁰Gd has never been observed (the only lower limit on its half-life of more than 1.3×10²¹ years has been set experimentally).

Twenty-nine radioisotopes of gadolinium have been observed, with the most stable being ¹⁵²Gd (naturally occurring) with a half-life of about 1.08×10¹⁴ years, and ¹⁵⁰Gd with a half-life of 1.79×10⁶ years. All of the remaining radioactive isotopes have half-lives of less than 75 years. The majority of these have half-lives of less than 25 seconds. Gadolinium isotopes have four metastable isomers, with the most stable being ^143mGd (t_1/2=110 seconds), ^145mGd (t_1/2=85 seconds) and ^141mGd (t_1/2=24.5 seconds).

The isotopes with atomic masses lower than the most abundant stable isotope, ¹⁵⁸Gd, primarily decay via electron capture to isotopes of europium. At higher atomic masses, the primary decay mode is beta decay, and the primary products are isotopes of terbium.

History

Gadolinium is named from the mineral gadolinite, in turn named for Finnish chemist and geologist Johan Gadolin. In 1880, the Swiss chemist Jean Charles Galissard de Marignac observed the spectroscopic lines from gadolinium in samples of gadolinite (which actually contains relatively little gadolinium, but enough to show a spectrum), and in the separate mineral cerite. 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 named the oxide "gadolinia." Because he realized that "gadolinia" was the oxide of a new element, he is credited with discovery of gadolinium. The French chemist Paul Émile Lecoq de Boisbaudran carried out the separation of gadolinium metal from gadolinia, in 1886.

Occurrence

Gadolinium is a constituent in many minerals such as monazite and bastnäsite, which are oxides. The metal is too reactive to exist naturally. Ironically, 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 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. The only known mineral with essential gadolinium, lepersonnite-(Gd), is very rare.

Production

Gadolinium is produced both from monazite and bastnäsite.

1. Crushed minerals are extracted with hydrochloric acid or sulfuric acid, which converts the insoluble oxides into soluble chlorides or sulfates.
2. The acidic filtrates are partially neutralized with caustic soda to pH 3–4. Thorium precipitates as its hydroxide, and is then removed.
3. The remaining solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by heating.
4. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO₃.
5. The solution is treated with magnesium nitrate to produce a crystallized mixture of double salts of gadolinium, samarium and europium.
6. The salts are separated by ion exchange chromatography.
7. 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 1450 °C in an argon atmosphere. Sponge gadolinium can be produced by reducing molten GdCl₃ with an appropriate metal at temperatures below 1312 °C (the melting point of Gd) at a reduced pressure.

Applications

Gadolinium has no large-scale applications, but it has a variety of specialized uses.

Because ¹⁵⁷Gd has a high neutron cross-section, it is used to target tumors in neutron therapy. This element 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 also used in nuclear marine propulsion systems as a burnable poison.

Gadolinium possesses unusual metallurgic properties, with as little as one percent of gadolinium improving the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation.

Gadolinium is paramagnetic at room temperature, with a ferromagnetic Curie point of 20 °C. Paramagnetic ions, such as gadolinium, enhance nuclear relaxation rates, making 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 the usual gadolinium contrast agent. Once injected, gadolinium-based contrast agents accumulate in abnormal tissues of the brain and body, which provides a greater image contrast between normal and abnormal tissues, facilitating location of abnormal cell growths and tumors.

Gadolinium as a phosphor is also used in other imaging. In X-ray systems gadolinium is contained in the phosphor layer, suspended in a polymer matrix at the detector. Terbium-doped gadolinium 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 due to 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-rays striking 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 compounds are also used for making green phosphors for color TV tubes.

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 gallium garnet (GGG, Gd₃Ga₅O₁₂) was used for imitation diamonds and for computer bubble memory.

Gadolinium can also serve as an electrolyte in solid oxide fuel cells (SOFCs). Using gadolinium as a dopant for materials like cerium oxide (in the form of gadolinium doped ceria) creates an electrolyte with both high ionic conductivity and low operating temperatures that are optimal for cost-effective production of fuel cells.

Research is being conducted on magnetic refrigeration near room temperature, which could provide significant efficiency and environmental advantages over conventional refrigeration methods. Gadolinium-based materials, such as Gd₅(Si_xGe_1−x)₄, are currently the most promising materials owing to their high Curie temperature and giant magnetocaloric effect. Pure Gd itself exhibits a large magnetocaloric effect near its Curie temperature of 20 °C, and this has sparked great interest into producing Gd alloys with a larger effect and tunable Curie temperature. In Gd₅(Si_xGe_1−x)₄, Si and Ge compositions can be varied to adjust the Curie temperature. This technology is still very early in development and significant material improvements still need to be made before it is commercially viable.

Biological role

Gadolinium has no known native biological role, but its compounds are used as research tools in biomedicine. Gd³⁺ compounds are components of MRI contrast agents. It is used in various ion channel electrophysiology experiments to block sodium leak channels and stretch activated ion channels.

Safety

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 100–200 mg/kg. Toxicities have not been reported following low dose exposure to gadolinium ions. Toxicity studies in rodents, however show that chelation of gadolinium (which also improves its solubility) decreases its toxicity with regard to the free ion by at least a factor of 100 (i.e., the lethal dose for the Gd-chelate increases by 100 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.

GBCAs have proved safer than the iodinated contrast agents used in X-ray radiography or computed tomography. Anaphylactoid reactions are rare, occurring in approximately 0.03–0.1%.

Although gadolinium agents are useful for patients with renal impairment, in patients with severe renal failure requiring dialysis, there is a risk of a rare but serious illness called nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy, that is linked to the use of MRI contrast agents containing gadolinium. The disease resembles scleromyxedema and to some extent scleroderma. 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. Similar but not identical symptoms to NSF may occur in subjects with normal or near normal renal function within hours to 2 months following the administration of GBCAs; the name "gadolinium deposition disease" (GDD) has been proposed for this condition, which occurs in the absence of pre-existent disease or subsequently developed disease of an alternate known process. The causal relationship was being investigated as of May 2016; preliminary investigation suggested a true disease process.

Current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential and to consider performing an iodinated contrast-enhanced CT when feasible. If a contrast-enhanced MRI must be performed on a dialysis patient, it is recommended that certain high-risk contrast agents be avoided and that a lower dose be considered. 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.