1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰f⁷ 5s²p⁶d¹ 6s²

History

In 1880, Swiss chemist Jean Charles Galissard de Marignac observed spectroscopic lines on the mixture then known as didymia and gadolinite, (Ce, La, Nd, Y)₂FeBe₂Si₂O₁₀; French chemist Paul Emile Lecoq de Boisbaudran separated gadolinia, the oxide of Gadolinium from Monsander's "yttria" in 1886.  Today, gadolinium is primarily obtained from the minerals monazite, (Ce, La, Th, Nd, Y)PO₄ and bastnasite, (Ce,La,Y)CO₃F.

Gadolinium (from the mineral gadolinite, is named after Finnish chemist and geologist Johan Gadolin.

In older literature the natural form of the element is often called an "earth", meaning that element came from the Earth. Accordingly - Gadolinium is the element that comes from the earth, gadolinia. Earths are compounds of the element and one or more other elements. Two common combining elements are oxygen and sulfur. For example, gadolinia contains gadolinium oxide (Gd₂O₃).

Characteristics

Gadolinium is a soft, silvery white, malleable and ductile rare earth metal with a metallic luster.  It crystallizes in hexagonal, close-packed alpha form at room temperature; when heated to 1508^oK, it transforms into its beta form, which has a body-centered cubic structure.  The metal can be prepared by the reduction of the anhydrous fluoride with metallic calcium.

Unlike other rare earth elements, gadolinium is relatively stable in dry air; however, it tarnishes quickly in moist air and forms a loosely adhering oxide that spalls off and exposes more surface to oxidation. Gadolinium reacts slowly with water and is soluble in dilute acid.

Gadolinium has the highest thermal neutron capture cross-section of any (known) element, 49,000 barns, but it also has a fast burn-out rate, limiting its usefulness as a nuclear control rod material.

Gadolinium becomes superconductive below a critical temperature of 1.083^oK.   It is strongly magnetic at room temperature, and exhibits ferromagnetic properties below room temperature.

Gadolinium demonstates 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).

The metal has a very large capacity for absorbing thermal neutrons, making it an excellent material for control rods in fission power plants

Occurrence

Gadolinium is never found in nature as the free element, but is contained in many rare minerals such as monazite and bastnasite.  It occurs only in trace amounts in the mineral gadolinite which was also named after Johan Gadolin.  Today, it is prepared by ion exchange and solvent extraction techniques, or by the reduction of its anhydrous fluoride with metallic calcium.

Applications

Gadolinium is used for making gadolinium yttrium garnets, which have microwave applications, and gadolinium compounds are used for making phosphors for colour TV tubes. Gadolinium is also used for manufacturing compact discs and computer memory.

Gadolinium is used in nuclear marine propulsion systems as a burnable poison.  The gadolinium slows the initial reaction rate, but as it decays other neutron poisons accumulate, allowing for long-running cores. Gadolinium is also used as a secondary, emergency shut-down measure in some nuclear reactors, particularly of the CANDU type.

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.

Because of their paramagnetic properties, solutions of organic gadolinium complexes gadolinium compounds are used as intravenous radiocontrast agents to enhance images in medical magnetic resanance imaging.  Magnevist is the most widespread example.

Besides MRI, gadolinium (Gd) is also used in other imaging. In X-ray, gadolinium is containing 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. Gd can emit spectrum at 540nm (Green light spectrum = 520 – 570nm), which is very useful for enhancing the imaging quality of the X-ray that are exposed to the photographic film. Beside Gd’s spectrum range, the compound also has a K-edge at 50 kiloelectron volt (keV), which means its absorption of X-ray through photoelectric interactions is great. 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 (GSO) is a single crystal that is used as a scintillator in medical imaging equipment like as Positron Emission Tomography (PET).  Another new scintillator for detecting neutron is gadolinium orthosilicate (GSO - Gd₂SiO_5: Ce).

Gadolinium gallium garnet (Gd₃Ga₅O₁₂) is a material with good optical properties, and is used in fabrication of various optical components and as substrate material for magneto–optical films.

The metal has unusual superconductive properties.  As little as 1% gadolinium has been found to improve the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation.  Gadolinium ethyl sulfate has extremely low noise characteristics and may find use in duplicating the performance of amplifiers, such as the maser.  The metal is ferromagnetic. Gadolinium is unique for its high magnetic moment and for its special Curie temperature (above which ferromagnetism vanishes) lying just at room temperature.  This suggests uses as a magnetic component that senses hot and cold.

Due the extremely high neutron cross-section of gadolinium, this element is very effective for use with neutron radiography.

Value

In 1994, the cost of gadolinium was abou US $ 0.12 per gram, and it has only increased in value by about US$ 0.01 per gram since then.

Compounds

Isotopes

Gadolinium has the greatest ability to capture thermal neutrons of all known elements and can be used as control rods for nuclear reactors.  Unfortunately, the two isotopes best suited for neutron capture, ¹⁵⁵Gd and ¹⁵⁷Gd, are present in gadolinium in small amounts.  As a result, gadolinium control rods quickly lose their effectiveness.

Naturally occurring gadolinium is composed of 6 stable isotopes,  ¹⁵⁴Gd, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd and ¹⁵⁸Gd, and 2 radioisotopes,  ¹⁵²Gd and ¹⁶⁰Gd, with ¹⁵⁸Gd being the most abundant (24.84% natural abundance). 30 radioisotopes have been characterized with the most stable being ¹⁶⁰Gd with a half-life of more than 1.3×10²¹ years (the decay is not observed, only the lower limit on the half-life is known), alpha-decaying ¹⁵²Gd 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-lifes that are less than 74.7 years, and the majority of these have half lifes that are less than 24.6 seconds. This element also has 4 meta states 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 before the most abundant stable isotope, ¹⁵⁸Gd, is electron capture and the primary mode after is beta minus decay.  The primary decay products before ¹⁵⁸Gd are element Eu (Europium) isotopes and the primary products after are element Tb (Terbium) isotopes.

Biological Role

Gadolinium has no known biological role. 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.

Precautions

In the free ionic state 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.  The US Food and Drug Administration approved Gd chelated contrast agents include: Omniscan, Multihance, Magnevist, ProHance, Vasovist and OptMARK.

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 Demopathy, 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.