Lanthanide
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 due to their tendency to get oxidised into the more stable +3 state. It 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.
Although europium is present in most of the minerals containing the other rare elements, due to the difficulties in separating the elements it was not until the late 1800s that the element was isolated. William Crookes first noted some anomalous lines in the optical spectrum of samarium-yttrium ores in 1885. In 1892, Paul Émile Lecoq de Boisbaudran obtained basic fractions from samarium-gadolinium concentrates which had spectral lines not accounted for by samarium or gadolinium. French chemist Eugène-Anatole Demarçay made detailed studies of the spectral lines and suspected these samples of the recently discovered element samarium were contaminated with an unknown element in 1896. Demarçay was able to isolate it in 1901; he then named it europium. Crookes confirmed the discovery in 1905 and observed the phosphorescent spectra of the rare elements including those eventually assigned to europium.
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.
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-(Ce).
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 (rocks that cooled from magma or lava). 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 (Eu2+) in small amounts is the activator of the bright blue fluorescence of some samples of the mineral fluorite (CaF2). 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. …
Relative to most other elements, commercial applications for europium are few and rather specialized. Almost invariably, its phosphorescence is exploited, either in the +2 or +3 oxidation state.
It is a dopant in some types of glass in lasers and other optoelectronic devices. Europium oxide (Eu2O3) is widely used as a red phosphor in television sets and fluorescent lamps, and as an activator for yttrium-based phosphors. Color TV screens contain between 0.5 and 1 g of europium oxide. Whereas trivalent europium gives red phosphors, the luminescence of divalent europium depends strongly on the composition of the host structure. UV to deep red luminescence can be achieved. The two classes of europium-based phosphor (red and blue), combined with the yellow/green terbium phosphors give "white" light, the color temperature of which can be varied by altering the proportion or specific composition of the individual phosphors. This phosphor system is typically encountered in helical fluorescent light bulbs. Combining the same three classes is one way to make trichromatic systems in TV and computer screens, but as an additive, it can be particularly effective in improving the intensity of red phosphor. Europium is also used in the manufacture of fluorescent glass, increasing the general efficiency of fluorescent lamps. One of the more common persistent after-glow phosphors besides copper-doped zinc sulfide is europium-doped strontium aluminate. …
Naturally occurring europium is composed of two isotopes, 151Eu and 153Eu, which occur in almost equal proportions; 153Eu is slightly more abundant (52.2% natural abundance). While 153Eu is stable, 151Eu was found to be unstable to alpha decay with a half-life of 4.6×1018 years, 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 (12.8 hours), 152m1Eu (9.3116 hours) and 152m5Eu (96 minutes). 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 is produced by nuclear fission: 155Eu (half-life 4.742 years) 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. …
There are no clear indications that europium is particularly toxic compared to other heavy metals. Europium chloride, nitrate and oxide have been tested for toxicity: europium chloride shows an acute intraperitoneal LD50 toxicity of 550 mg/kg and the acute oral LD50 toxicity is 5000 mg/kg. Europium nitrate shows a slightly higher intraperitoneal LD50 toxicity of 320 mg/kg, while the oral toxicity is above 5000 mg/kg. The metal dust presents a fire and explosion hazard.