Gadolinium

europium ← gadolinium → terbium - ↑ Gd ↓ Cm 64Gd Periodic table
Appearance
silvery white
General properties
Name, symbol, number	gadolinium, Gd, 64
Pronunciation	/ˌɡædɵˈlɪniəm/ GAD-o-LIN-ee-əm
Element category	lanthanide
Group, period, block	n/a, 6, f
Standard atomic weight	157.25g·mol−1
Electron configuration	[Xe] 4f7 5d1 6s2
Electrons per shell	2, 8, 18, 25, 9, 2 (Image)
Physical properties
Phase	solid
Density (near r.t.)	7.90 g·cm−3
Liquid density at m.p.	7.4 g·cm−3
Melting point	1585 K, 1312 °C, 2394 °F
Boiling point	3546 K, 3273 °C, 5923 °F
Heat of fusion	10.05 kJ·mol−1
Heat of vaporization	301.3 kJ·mol−1
Specific heat capacity	(25 °C) 37.03 J·mol−1·K−1
Vapor pressure (calculated)
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1836 2028 2267 2573 2976 3535
Atomic properties
Oxidation states	1, 2, 3 (mildly basic oxide)
Electronegativity	1.20 (Pauling scale)
Ionization energies	1st: 593.4 kJ·mol−1
	2nd: 1170 kJ·mol−1
	3rd: 1990 kJ·mol−1
Atomic radius	180 pm
Covalent radius	196±6 pm
Miscellanea
Crystal structure	hexagonal
Magnetic ordering	ferromagnetic/paramagnetic transition at 292 K[1]
Electrical resistivity	(r.t.) (α, poly) 1.310 µΩ·m
Thermal conductivity	(300 K) 10.6 W·m−1·K−1
Thermal expansion	(100 °C) (α, poly) 9.4 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 2680 m/s
Young's modulus	(α form) 54.8 GPa
Shear modulus	(α form) 21.8 GPa
Bulk modulus	(α form) 37.9 GPa
Poisson ratio	(α form) 0.259
Vickers hardness	570 MPa
CAS registry number	7440-54-2
Most stable isotopes
Main article: Isotopes of gadolinium
iso NA half-life DM DE (MeV) DP 152Gd 0.20% 1.08×1014 y α 2.205 148Sm 154Gd 2.18% 154Gd is stable with 90 neutrons 155Gd 14.80% 155Gd is stable with 91 neutrons 156Gd 20.47% 156Gd is stable with 92 neutrons 157Gd 15.65% 157Gd is stable with 93 neutrons 158Gd 24.84% 158Gd is stable with 94 neutrons 160Gd 21.86% >1.3×1021y β−β− 1.7 160Dy

Gadolinium (pronounced /ˌɡædɵˈlɪniəm/ GAD-o-LIN-ee-əm) 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.

Characteristics

A sample of gadolinium

Physical

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.^[2]

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.^[3]

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).^[4]

Individual gadolinium atoms have been isolated by encapsulating them into fullerene molecules and visualized with transmission electron microscope.^[5] Individual Gd atoms and small Gd clusters have also been incorporated into carbon nanotubes.^[6]

Chemical

Unlike other rare earth elements, metallic 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.^[2] 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 dissolves readily in dilute sulfuric acid to form solutions containing the colorless Gd(III) ions, which exist as a [Gd(OH₂)₉]³⁺ complexes:^[7]

2 Gd (s) + 3 H₂SO₄ (aq) → 2 Gd³⁺ (aq) + 3 SO2−4 (aq) + 3 H₂ (g)

Gadolinium metal reacts with all of the halogens at temperature about 200 °C:

2 Gd (s) + 3 F₂ (g) → 2 GdF₃ (s) [white]

2 Gd (s) + 3 Cl₂ (g) → 2 GdCl₃ (s) [white]

2 Gd (s) + 3 Br₂ (g) → 2 GdBr₃ (s) [white]

2 Gd (s) + 3 I₂ (g) → 2 GdI₃ (s) [yellow]

Gadolinium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming binary compounds.^[2]

Lower oxidation state derivatives

In the great majority of its compounds, Gd has oxidation state +3, but other oxidation states are known especially in the solid state. Gadolinium(II) halides are obtained by heat 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.^[8]

Compounds

Compounds of gadolinium include

• Fluorides: GdF₃
• Chlorides: GdCl₃
• Bromides: GdBr₃
• Nitrates: Gd(NO₃)₃
• Iodides: GdI₃
• Oxides: Gd₂O₃
• Sulfides: Gd₂S₃
• Nitrides: GdN
• Organics: gadodiamide

Isotopes

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 ^[9]).

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 (T_½=110 seconds), ^145mGd (T_½=85 seconds) and ^141mGd (T_½=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.

History

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.^[2]

Occurrence

Gadolinite

Gadolinium is a constituent in many minerals such as monazite and bastnäsite, which are oxides. The metal is too reactive to exist naturally. Ironically, the mineral gadolinite contains only traces of Gd. The abundance in the earth crust is about 6.2 mg/kg.^[2] 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.

Production

Gadolinium is produced both from monazite and bastnäsite.

1. Crushed minerals are extracted with hydrochloric or sulfuric acids, 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 removed.
3. The remaining solution is treated with ammonium oxalate to convert rare earths in to 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 suitable complexing agent.^[2]

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 at temperatures below 1312 °C (melting point of Gd) in a reduced pressure.^[2]

Applications

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

With 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.^[2] Gadolinium is also used in nuclear marine propulsion systems as a burnable poison.

Gadolinium also possesses unusual metallurgic properties, and 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.^[10] 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.^[11][12] Nanotubes packed with gadolinium, dubbed "gadonanotubes," are 40 times more effective than this traditional gadolinium contrast agent.^[13] 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.

Gadolinium-153 helps calibrate positron emission tomography (PET) systems that are used in nuclear medicine for functional imaging. This PET image of the human brain shows the difference between a normal brain and the clinically depressed patient. The blue color indicates less glucose metabolism in a normal brain. The green, yellow, and red colors indicate areas of higher glucose metabolism characteristic of a depressed patient.^[14]

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.^[15]

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.^[14] 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.^[16]

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 and compact disc.

Gadolinium Gallium Garnet (GGG, Gd₃Ga₅O₁₂) was used as diamond imitation and for computer bubble memory.^[17]

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, as well as to stretch activated ion channels.^[18]

Safety

Some gadolinium are toxic, but MRI contrast agents, being chelated compound, are considered safe. The toxicity depends on the strength of the chelating agent. ^[19]. US Food and Drug Administration approved Gd chelated contrast agents include: Omniscan, Multihance, Magnevist, ProHance, Vasovist, Eovist and OptiMARK.^[20]

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%.^[21]

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^[22] and nephrogenic fibrosing dermopathy^[23], 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.^[24]

References

External links

• WebElements.com – Gadolinium
• Nephrogenic Systemic Fibrosis – Complication of Gadolinium MR Contrast (series of images at MedPix website)
• It's Elemental – Gadolinium
• refrigerator uses gadolinium metal that heats up when exposed to magnetic field
• FDA Advisory on Gadolinium-Based Contrast
• Abdominal MR imaging: important considerations for evaluation of gadolinium enhancement Rafael O.P. de Campos, Vasco Herédia, Ersan Altun, Richard C. Semelka, Department of Radiology University of North Carolina Hospitals Chapel Hill