Europium is achemical element; it hassymbolEu andatomic number 63. It is a silvery-white metal of thelanthanide 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 ofEurope.[10] Europium usually assumes theoxidation 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 slightlyreducing. Europium has no significant biological role but is relatively non-toxic compared to otherheavy metals. Most applications of europium exploit thephosphorescence of europium compounds. Europium is one of the rarest of therare-earth elements on Earth.[11]
About 300 g of dendritic sublimated 99.998% pure europium handled in a glove boxOxidized europium, coated with yellow europium(II) carbonate
Europium is aductile metal with a hardness similar to that oflead. It crystallizes in abody-centered cubic lattice.[12] 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.[12]: 1700
The chemistry of europium is broadlylanthanoid chemistry, but Europium is the most reactive lanthanoid.[12]: 1703 It rapidly oxidizes in air, so that bulk oxidation of a centimeter-sized sample occurs within several days.[13] Its reactivity with water is comparable to that ofcalcium, and the reaction is
2 Eu + 6 H2O → 2 Eu(OH)3 + 3 H2
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 formeuropium(III) oxide:[14]
4 Eu + 3 O2 → 2 Eu2O3
Europium dissolves readily in dilutesulfuric acid to form pale pink[15] solutions of[Eu(H2O)9]3+:
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 anelectron configuration 4f7 because the half-filledf-shell provides more stability. In terms of size andcoordination number, europium(II) andbarium(II) are similar. Thesulfates of bothbarium andeuropium(II) are also highly insoluble in water.[16] Divalent europium is a mild reducing agent, oxidizing in air to form Eu(III) 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 "negativeeuropium anomaly", the low europium content in many lanthanide minerals such asmonazite, relative to thechondritic 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 (trivalent) lanthanides made europium accessible even when present in low concentration, as it usually is.[17]
Europium(III) sulfate, Eu2(SO4)3Europium(III) sulfate fluorescing red under ultraviolet light
Europium compounds tend to exist in a trivalent oxidation state under most conditions. Commonly these compounds feature Eu(III) bound by 6–9 oxygenic ligands. The Eu(III) sulfates, nitrates and chlorides are soluble in water or polar organic solvents. Lipophilic europium complexes often featureacetylacetonate-like ligands, such asEuFOD.
This route gives white europium(III) fluoride (EuF3), yelloweuropium(III) chloride (EuCl3), gray[18]europium(III) bromide (EuBr3), and colorless europium(III) iodide (EuI3). Europium also forms the corresponding dihalides: yellow-green europium(II) fluoride (EuF2), colorlesseuropium(II) chloride (EuCl2) (although it has a bright blue fluorescence under UV light),[19] colorlesseuropium(II) bromide (EuBr2), and green europium(II) iodide (EuI2).[12]
Europium forms stable compounds with all of the chalcogens, but the heavier chalcogens (S, Se, and Te) stabilize the lower oxidation state. Threeoxides are known: europium(II) oxide (EuO),europium(III) oxide (Eu2O3), and themixed-valence oxide Eu3O4, consisting of both Eu(II) and Eu(III). Otherwise, the main chalcogenides areeuropium(II) sulfide (EuS), europium(II) selenide (EuSe) and europium(II) telluride (EuTe): all three of these are black solids. Europium(II) sulfide is prepared by sulfiding the oxide at temperatures sufficiently high to decompose the Eu2O3:[20]
Eu2O3 + 3 H2S → 2 EuS + 3 H2O + S
The mainnitride of europium is europium(III) nitride (EuN).
Naturally occurring europium is composed of twoisotopes,151Eu and153Eu, which occur in almost equal proportions;153Eu is slightly more abundant (52.2%natural abundance). While153Eu is stable,151Eu was found to be unstable toalpha decay with ahalf-life of4.6×1018 years,[21] giving about one alpha decay per two minutes in every kilogram of natural europium. Besides the natural radioisotope151Eu, 39 artificialradioisotopes have been characterized from130Eu to170Eu,[9][22] the most stable being150Eu 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, and155Eu 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 27meta states, with the most stable being150mEu (12.8 hours),152m1Eu (9.3116 hours) and152m5Eu (96 minutes).[9] The primarydecay mode for isotopes lighter than153Eu iselectron capture tosamarium isotopes, and the primary mode for heavier isotopes isbeta minus decay togadolinium isotopes.
151Eu is thebeta decay product ofsamarium-151 (not included in above yield), but since this has a long decay half-life and short mean time to neutron absorption, most151Sm instead ends up as152Sm.
152Eu (half-life 13.517 years) and154Eu (half-life 8.592 years) cannot be beta decay products because152Sm and154Sm are non-radioactive, but154Eu is the only long-lived "shielded"nuclide, other than134Cs, to have a fission yield of more than 2.5parts per million fissions.[24] A larger amount of154Eu is produced byneutron activation of a significant portion of the non-radioactive153Eu; however, as shown by the cross-sections, much of this is further converted to155Eu and156Eu, ending up as gadolinium.
Depletion or enrichment of europium in minerals relative to other rare-earth elements is known as theeuropium anomaly.[26] Europium is commonly included in trace element studies ingeochemistry andpetrology to understand the processes that formigneous rocks (rocks that cooled frommagma orlava). The nature of the europium anomaly found helps reconstruct the relationships within a suite of igneous rocks. Themedian crustal abundance of europium is 2 ppm; values of the less abundant elements may vary with location by several orders of magnitude.[27]
Divalent europium (Eu2+) in small amounts is the activator of the bright bluefluorescence of some samples of the mineralfluorite (CaF2). The reduction from Eu3+ to Eu2+ is induced by irradiation with energetic particles.[28] The most outstanding examples of this originated aroundWeardale 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.[29][30][31][32]
Inastrophysics, the signature of europium in stellarspectra can be used toclassify 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 starLAMOST J112456.61+453531.3 to propose that theaccretion process for the star occurred late.[33]
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 mineralsbastnäsite,loparite-(Ce),xenotime, andmonazite in mineable quantities. Bastnäsite is a group of relatedfluorocarbonates, Ln(CO3)(F,OH). Monazite is a group of related of orthophosphate mineralsLnPO 4 (Ln denotes a mixture of all the lanthanides exceptpromethium), loparite-(Ce) is an oxide, and xenotime is an orthophosphate (Y,Yb,Er,...)PO4. Monazite also containsthorium andyttrium, 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(III) to cerium(IV) and then precipitated. Further separation bysolvent extractions orion 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(III) to europium(II). Europium(II) reacts in a way similar to that ofalkaline earth metals and therefore it can be precipitated as a carbonate or co-precipitated with barium sulfate.[34] Europium metal is available through the electrolysis of a mixture of molten EuCl3 and NaCl (or CaCl2) in a graphite cell, which serves as cathode, using graphite as anode. The other product ischlorine gas.[25][34][35][36][37]
A few large deposits produce or produced a significant amount of the world production. TheBayan Obo iron ore deposit inInner 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.[38][39][40] 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 theMountain Pass rare earth mine in California. The bastnäsite mined there is especially rich in the light rare-earth elements (La-Gd, Sc, and Y) and contains only 0.1% of europium. Another large source for rare-earth elements is the loparite found on theKola peninsula. It contains besides niobium, tantalum and titanium up to 30% rare-earth elements and is the largest source for these elements in Russia.[25][41]
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.[42]: 936 In 1892,Paul Émile Lecoq de Boisbaudran obtained basic fractions from samarium-gadolinium concentrates which had spectral lines not accounted for by samarium orgadolinium.FrenchchemistEugène-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 iteuropium.[43][44][45] Crookes confirmed the discovery in 1905 and observed the phosphorescent spectra of the rare elements including those eventually assigned to europium.[46][42]
Europium is one of the elements involved in emitting red light in CRT televisions.
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 adopant in some types ofglass inlasers and other optoelectronic devices. Europium oxide (Eu2O3) is widely used as a redphosphor intelevision sets andfluorescent lamps, and as an activator foryttrium-based phosphors.[47][48] Color TV screens contain between 0.5 and 1 g of europium oxide.[49] Whereas trivalent europium gives red phosphors,[50] the luminescence of divalent europium depends strongly on the composition of the host structure. UV to deep red luminescence can be achieved.[51][52] The two classes of europium-based phosphor (red and blue), combined with the yellow/greenterbium 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,[47] but as an additive, it can be particularly effective in improving the intensity of red phosphor.[11] Europium is also used in the manufacture of fluorescent glass, increasing the general efficiency of fluorescent lamps.[11] One of the more common persistent after-glow phosphors besides copper-doped zinc sulfide is europium-dopedstrontium aluminate.[53] Europium fluorescence is used to interrogate biomolecular interactions in drug-discovery screens. It is also used in the anti-counterfeiting phosphors ineuro banknotes.[54][55]
An application that has almost fallen out of use with the introduction of affordable superconducting magnets is the use of europium complexes, such asEu(fod)3, as shift reagents inNMR spectroscopy.Chiral shift reagents, such as Eu(hfc)3, are still used to determineenantiomeric purity.[56]
Europium compounds are used to labelantibodies for sensitive detection ofantigens in body fluids, a form ofimmunoassay. When these europium-labeled antibodies bind to specific antigens, the resulting complex can be detected with laser excited fluorescence.[57]
There are no clear indications that europium is particularly toxic compared to otherheavy 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.[59][60] The metal dust presents a fire and explosion hazard.[61]
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