Europium

Notable characteristics

Europium is the most reactive of the ductile.

Applications

There are few commercial applications for europium metal, although it has been used to 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 europium anomaly found is used to help reconstruct the relationships within a suite of igneous rocks.

History

Europium was first found by 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 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
• 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 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 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 gadolinium (Gd).

Europium in nuclear power

Thermal neutron capture cross sections

Isotope	151Eu	152Eu	153Eu	154Eu	155Eu
Yield	~10	low	1580	>2.5	330
Barns	5900	12800	312	1340	3950

Medium-lived fission products
t½(y)		Yield%	KeV	β
155Eu	4.76	.0330	252	γ
85Kr	10.76	.2717	687	γ
113mCd	14.1	.0003	316
90Sr	28.9	5.7518	2826	β
137Cs	30.23	6.0899	1176	γ
121mSn	43.9	.00003	390	γ
151Sm	90	.4203	77

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 poisons.

¹⁵¹Eu is the 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 neutron activation of a significant portion of the nonradioactive¹⁵³Eu; however, much of this will be further converted to ¹⁵⁵Eu.

Gadolinium-156 by the end of fuel burnup.

Overall, europium is overshadowed 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