Europium

Europium (pronounced /jʊˈroʊpiəm/) is a chemical element with the symbol Eu and atomic number 63. It was named after the continent Europe.

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Notable characteristics

Europium is the most reactive of the rare earth elements; it rapidly oxidizes in air, and resembles calcium in its reaction with water; deliveries of the metal element in solid form, even when coated with a protective layer of mineral oil, are rarely shiny. Europium ignites in air at about 150 °C to 180 °C. It is about as hard as lead and quite ductile.

Applications

There are few commercial applications for europium metal, although it has been used to dope some types of glass to make lasers, as well as for screening for Down syndrome and some other genetic diseases. Due to its ability to absorb neutrons, it is also being studied for use in nuclear reactors. Europium oxide (Eu₂O₃) is widely used as a red phosphor in television sets and fluorescent lamps, and as an activator for yttrium-based phosphors. Whereas trivalent europium gives red phosphors, divalent europium gives blue phosphors. The two europium phosphor classes, combined with the yellow/green terbium phosphors, give the "trichromatic" lights that are becoming so important to provide economical lighting. It is also being used as an agent for the manufacture of fluorescent glass. Europium fluorescence is used to interrogate biomolecular interactions in drug-discovery screens. It is also used in the anti-counterfeiting phosphors in Euro banknotes. ^[1]

Europium is commonly included in trace element studies in geochemistry and petrology to understand the processes that form igneous rocks (rocks that cooled from magma or lava). The nature of the europium anomaly found is used to help reconstruct the relationships within a suite of igneous rocks.

History

Europium was first found by Paul Émile Lecoq de Boisbaudran in 1890, who obtained basic fraction from samarium-gadolinium concentrates which had spectral lines not accounted for by samarium or gadolinium; however, the discovery of europium is generally credited to French chemist Eugène-Anatole Demarçay, who suspected samples of the recently discovered element samarium were contaminated with an unknown element in 1896 and who was able to isolate europium in 1901. When the europium-doped yttrium orthovanadate red phosphor was discovered in the early 1960s, and understood to be about to cause a revolution in the color television industry, there was a mad scramble for the limited supply of europium on hand among the monazite processors. (Typical europium content in monazite was about 0.05%.) Luckily, Molycorp, with its bastnäsite deposit at Mountain Pass California, whose lanthanides had an unusually "rich" europium content of 0.1%, was about to come on-line and provide sufficient europium to sustain the industry. Prior to europium, the color-TV red phosphor was very weak, and the other phosphor colors had to be muted, to maintain color balance. With the brilliant red europium phosphor, it was no longer necessary to mute the other colors, and a much brighter color TV picture was the result. Europium has continued in use in the TV industry ever since, and, of course, also in computer monitors. California bastnäsite now faces stiff competition from Bayan Obo, China, with an even "richer" europium content of 0.2%. Frank Spedding, celebrated for his development of the ion-exchannge technology that revolutionized the rare earth industry in the mid-1950's once related the story of how, in the 1930's, he was lecturing on the rare earths when an elderly gentleman approached him with an offer of a gift of several pounds of europium oxide. This was an unheard-of quantity at the time, and Spedding did not take the man seriously. However, a package duly arrived in the mail, containing several pounds of genuine europium oxide. The elderly gentleman had turned out to be the Dr. McCoy who had developed a famous method of europium purification involving redox chemistry.

Occurrence

Europium is never found in nature as a free element; however, there are many minerals containing europium, with the most important sources being bastnäsite and monazite. Europium has also been identified in the spectra of the sun and certain stars. Depletion or enrichment of europium in minerals relative to other rare earth elements is known as the europium anomaly.

Divalent europium in small amounts happens to be the activator of the bright blue fluorescence of some samples of the mineral fluorite (calcium difluoride). The most outstanding examples of this originated around Weardale, and adjacent parts of northern England, and indeed it was this fluorite that gave its name to the phenomenon of fluorescence, although it was not until much later that europium was discovered or determined to be the cause.

Compounds

Europium compounds include:

• Fluorides: EuF₂ EuF₃
• Chlorides: EuCl₂ EuCl₃
• Bromides: EuBr₂ EuBr₃
• Iodides: EuI₂ EuI₃
• Oxides: EuO Eu₂O₃ Eu₃O₄
• Sulfides: EuS
• Selenides: EuSe
• Tellurides: EuTe
• Nitrides: EuN

Europium(II) compounds tend to predominate, in contrast to most lanthanides: (which generally form compounds with an oxidation state of +3). Europium(II) chemistry is very similar to barium(II) chemistry, as they have similar ionic radii. Divalent europium is a mild reducing agent, such that under atmospheric conditions, it is the trivalent form that predominates. Under anaerobic, and particularly under geothermal conditions, the divalent form is sufficiently stable such that it tends to be incorporated into minerals of calcium and the other alkaline earths. This is the cause of the "negative europium anomaly", that depletes europium from being incorporated into the most usual light lanthanide minerals such as monazite, relative to the chondritic abundance. Bastnaesite tends to show less of a negative europium anomaly than monazite does, and hence is the major source of europium today. The accessible divalency of europium has always made it one of the easiest lanthanides to extract and purify, even when present, as it usually is, in low concentration. See also europium compounds.

Isotopes

Template:Isotopes of europium Naturally occurring europium is composed of 2 stable isotopes, ¹⁵¹Eu and ¹⁵³Eu, with ¹⁵³Eu being the most abundant (52.2% natural abundance). 35 radioisotopes have been characterized, with the most stable being ¹⁵⁰Eu with a half-life of 36.9 years, ¹⁵²Eu with a half-life of 13.516 years, and ¹⁵⁴Eu with a half-life of 8.593 years. All of the remaining radioactive isotopes have half-lives that are less than 4.7612 years, and the majority of these have half-lives that are less than 12.2 seconds. This element also has 8 meta states, with the most stable being ^150mEu (t_½ 12.8 hours), ^152m1Eu (t_½ 9.3116 hours) and ^152m2Eu (t_½ 96 minutes).

The primary decay mode before the most abundant stable isotope, ¹⁵³Eu, is electron capture, and the primary mode after is beta minus decay. The primary decay products before ¹⁵³Eu are isotopes of samarium (Sm) and the primary products after are isotopes of gadolinium (Gd).

Europium in nuclear power

Europium is produced by nuclear fission, but as it is near the top of the mass range for fission products, the fission product yields of europium isotopes are low.

Like other lanthanides, many isotopes, especially isotopes with odd mass numbers and neutron-poor isotopes like ¹⁵²Eu, have high cross sections for neutron capture, often high enough to be neutron poisons.

¹⁵¹Eu is the beta decay product of Sm-151, but since this has a long decay halflife and short mean time to neutron absorption, most ¹⁵¹Sm instead winds up as ¹⁵²Sm.

¹⁵²Eu (halflife 13.516 years) and ¹⁵⁴Eu (halflife 8.593 years) cannot be beta decay products because ¹⁵²Sm and ¹⁵⁴Sm are nonradioactive, but ¹⁵⁴Eu is the only long-lived "shielded" nuclide, other than ¹³⁴Cs, to have a fission yield of more than 2.5 parts per million fissions.^[2] A larger amount of ¹⁵⁴Eu will be produced by neutron activation of a significant portion of the nonradioactive¹⁵³Eu; however, much of this will be further converted to ¹⁵⁵Eu.

¹⁵⁵Eu (halflife 4.7612 years) has a fission yield of 330 ppm for U-235 and thermal neutrons. Most will be transmuted to nonradioactive and nonabsorptive Gadolinium-156 by the end of fuel burnup.

Overall, europium is overshadowed by Cs-137 and Sr-90 as a radiation hazard, and by samarium and others as a neutron poison.

Precautions

The toxicity of europium compounds has not been fully investigated, but there are no clear indications that europium is highly toxic compared to other heavy metals. The metal dust presents a fire and explosion hazard. Europium has no known biological role.

Isolation of Europium

Europium metal is available commercially so it is not normally necessary to make it in the laboratory, which is just as well as it is difficult to isolate as the pure metal. This is largely because of the way it is found in nature. The lanthanoids are found in nature in a number of minerals. The most important are xenotime, monazite, and bastnaesite. The first two are orthophosphate minerals LnPO4 (Ln deonotes a mixture of all the lanthanoids except promethium which is vanishingly rare) and the third is a fluoride carbonate LnCO3F. Lanthanoids with even atomic numbers are more common. The most comon lanthanoids in these minerals are, in order, cerium, lanthanum, neodymium, and praseodymium. Monazite also contains thorium and ytrrium which makes handling difficult since thorium and its decomposition products are radioactive.

For many purposes it is not particularly necessary to separate the metals, but if separation into individual metals is required, the process is complex. Initially, the metals are extracted as salts from the ores by extraction with sulphuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modern purification techniques for these lanthanoid salt mixtures are ingenious and involve selective complexation techniques, solvent extractions, and ion exchange chromatography.

Pure europium is available through the electrolysis of a mixture of molten EuCl3 and NaCl (or CaCl2) in a graphite cell which acts as cathode using graphite as anode. The other product is chlorine gas.

References

• Los Alamos National Laboratory – Europium