Europium

Europium is a chemical element; it has symbol Eu and atomic number 63. It is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. Europium is the most chemically reactive, least dense, and softest of the lanthanides. It is soft enough to be cut with a knife. Europium was discovered in 1896, provisionally designated as Σ; in 1901, it was named after the continent of Europe. Europium usually assumes the oxidation state +3, like other members of the lanthanide series, but compounds having oxidation state +2 are also common. All europium compounds with oxidation state +2 are slightly reducing. Europium has no significant biological role but is relatively non-toxic compared to other heavy metals. Most applications of europium exploit the phosphorescence of europium compounds. Europium is one of the rarest of the rare-earth elements on Earth.

Etymology

Its discoverer, Eugène-Anatole Demarçay, named the element after the continent of Europe.

Physical properties

Europium is a ductile metal with a hardness similar to that of lead. It crystallizes in a body-centered cubic lattice. Among the lanthanoids Europium together with ytterbium have the largest volume per mole of metal. Magnetic measurements suggest this is a consequence of these metals being effectively divalent while other lanthanoids are trivalent metals.

Chemical properties

The chemistry of europium is broadly lanthanoid chemistry, but
Europium is the most reactive lanthanoid. It rapidly oxidizes in air, so that bulk oxidation of a centimeter-sized sample occurs within several days. Its reactivity with water is comparable to that of calcium, and the reaction is
Because of the high reactivity, samples of solid europium rarely have the shiny appearance of the fresh metal, even when coated with a protective layer of mineral oil. Europium ignites in air at 150 to 180 °C to form europium oxide:
Europium dissolves readily in dilute sulfuric acid to form pale pink solutions of :

Eu(II) vs. Eu(III)

Although usually trivalent, europium readily forms divalent compounds. This behavior is unusual for most lanthanides, which almost exclusively form compounds with an oxidation state of +3. The +2 state has an electron configuration 4f⁷ because the half-filled f-shell provides more stability. In terms of size and coordination number, europium and barium are similar. The sulfates of both barium and europium are also highly insoluble in water. Divalent europium is a mild reducing agent, oxidizing in air to form Eu compounds. In anaerobic, and particularly geothermal conditions, the divalent form is sufficiently stable that it tends to be incorporated into minerals of calcium and the other alkaline earths. This ion-exchange process is the basis of the "negative europium anomaly", the low europium content in many lanthanide minerals such as monazite, relative to the chondritic abundance. Bastnäsite tends to show less of a negative europium anomaly than does monazite, and hence is the major source of europium today. The development of easy methods to separate divalent europium from the other lanthanides made europium accessible even when present in low concentration, as it usually is.

Compounds

Europium compounds tend to exist in a trivalent oxidation state under most conditions. Commonly these compounds feature Eu bound by 6–9 oxygenic ligands. The Eu sulfates, nitrates and chlorides are soluble in water or polar organic solvents. Lipophilic europium complexes often feature acetylacetonate-like ligands, such as EuFOD.

Halides

Europium metal reacts with all the halogens:
This route gives white europium fluoride, yellow europium chloride, gray europium bromide, and colorless europium iodide. Europium also forms the corresponding dihalides: yellow-green europium fluoride, colorless europium chloride , colorless europium bromide, and green europium iodide.

Chalcogenides and pnictides

Europium forms stable compounds with all of the chalcogens, but the heavier chalcogens stabilize the lower oxidation state. Three oxides are known: europium oxide, europium oxide, and the mixed-valence oxide Eu₃O₄, consisting of both Eu and Eu. Otherwise, the main chalcogenides are europium sulfide, europium selenide and europium telluride : all three of these are black solids. Europium sulfide is prepared by sulfiding the oxide at temperatures sufficiently high to decompose the Eu₂O₃:
The main nitride of europium is europium nitride.

Isotopes

Naturally occurring europium is composed of two isotopes, ¹⁵¹Eu and ¹⁵³Eu, which occur in almost equal proportions; ¹⁵³Eu is slightly more abundant. While ¹⁵³Eu is stable, ¹⁵¹Eu was found to be unstable to alpha decay with a half-life of, giving about one alpha decay per two minutes in every kilogram of natural europium. Besides the natural radioisotope ¹⁵¹Eu, 39 artificial radioisotopes have been characterized from ¹³⁰Eu to ¹⁷⁰Eu, the most stable being ¹⁵⁰Eu with a half-life of 36.9 years, ¹⁵²Eu with a half-life of 13.516 years, ¹⁵⁴Eu with a half-life of 8.592 years, and ¹⁵⁵Eu with a half-life of 4.742 years. All the others have half-lives shorter than 100 days, with the majority shorter than 3 minutes.
This element also has 27 meta states, with the most stable being ^150mEu, ^152m1Eu and ^152m5Eu. The primary decay mode for isotopes lighter than ¹⁵³Eu is electron capture to samarium isotopes, and the primary mode for heavier isotopes is beta minus decay to gadolinium isotopes.

Europium as a nuclear fission product

Europium is produced by nuclear fission: ¹⁵⁵Eu has a fission yield of 0.033% for uranium-235 with thermal neutrons. The fission product yields of europium isotopes are low, as they are near the top of the mass range of fission products.
As with other lanthanides, many isotopes of europium have high cross sections for neutron capture, often high enough to be neutron poisons.

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

¹⁵¹Eu is the beta decay product of samarium-151, but since this has a long decay half-life and short mean time to neutron absorption, most ¹⁵¹Sm instead ends up as ¹⁵²Sm.
¹⁵²Eu and ¹⁵⁴Eu cannot be beta decay products because ¹⁵²Sm and ¹⁵⁴Sm are non-radioactive, 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. A larger amount of ¹⁵⁴Eu is produced by neutron activation of a significant portion of the non-radioactive ¹⁵³Eu; however, as shown by the cross-sections, much of this is further converted to ¹⁵⁵Eu and ¹⁵⁶Eu, ending up as gadolinium.

Occurrence

Europium is not found in nature as a free element. Many minerals contain europium, with the most important sources being bastnäsite, monazite, xenotime and loparite-.
Depletion or enrichment of europium in minerals relative to other rare-earth elements is known as the europium anomaly. Europium is commonly included in trace element studies in geochemistry and petrology to understand the processes that form igneous rocks. The nature of the europium anomaly found helps reconstruct the relationships within a suite of igneous rocks. The median crustal abundance of europium is 2 ppm; values of the less abundant elements may vary with location by several orders of magnitude.
Divalent europium in small amounts is the activator of the bright blue fluorescence of some samples of the mineral fluorite. The reduction from Eu³⁺ to Eu²⁺ is induced by irradiation with energetic particles. The most outstanding examples of this originated around Weardale and adjacent parts of northern England; it was the fluorite found here that fluorescence was named after in 1852, although it was not until much later that europium was determined to be the cause.
In astrophysics, the signature of europium in stellar spectra can be used to classify stars and inform theories of how or where a particular star was born. For instance, astronomers used the relative levels of europium to iron within the star LAMOST J112456.61+453531.3 to propose that the accretion process for the star occurred late.

Production

Europium is associated with the other rare-earth elements and is, therefore, mined together with them. Separation of the rare-earth elements occurs during later processing. Rare-earth elements are found in the minerals bastnäsite, loparite-, xenotime, and monazite in mineable quantities. Bastnäsite is a group of related fluorocarbonates, Ln. Monazite is a group of related of orthophosphate minerals , loparite- is an oxide, and xenotime is an orthophosphate PO₄. Monazite also contains thorium and yttrium, which complicates handling because thorium and its decay products are radioactive. For the extraction from the ore and the isolation of individual lanthanides, several methods have been developed. The choice of method is based on the concentration and composition of the ore and on the distribution of the individual lanthanides in the resulting concentrate. Roasting the ore, followed by acidic and basic leaching, is used mostly to produce a concentrate of lanthanides. If cerium is the dominant lanthanide, then it is converted from cerium to cerium and then precipitated. Further separation by solvent extractions or ion exchange chromatography yields a fraction which is enriched in europium. This fraction is reduced with zinc, zinc/amalgam, electrolysis or other methods converting the europium to europium. Europium reacts in a way similar to that of alkaline earth metals and therefore it can be precipitated as a carbonate or co-precipitated with barium sulfate. Europium metal is available through the electrolysis of a mixture of molten EuCl₃ and NaCl in a graphite cell, which serves as cathode, using graphite as anode. The other product is chlorine gas.
A few large deposits produce or produced a significant amount of the world production. The Bayan Obo iron ore deposit in Inner Mongolia contains significant amounts of bastnäsite and monazite and is, with an estimated 36 million tonnes of rare-earth element oxides, the largest known deposit. The mining operations at the Bayan Obo deposit made China the largest supplier of rare-earth elements in the 1990s. Only 0.2% of the rare-earth element content is europium. The second large source for rare-earth elements between 1965 and its closure in the late 1990s was the Mountain Pass rare earth mine in California. The bastnäsite mined there is especially rich in the light rare-earth elements and contains only 0.1% of europium. Another large source for rare-earth elements is the loparite found on the Kola peninsula. It contains besides niobium, tantalum and titanium up to 30% rare-earth elements and is the largest source for these elements in Russia.