Europium ( /jʊˈroʊpiəm/ ew-ROH-pee-əm) is a chemical element with the symbol Eu and atomic number 63. It was named after the continent of Europe.
Contents
[hide]
- 1 Characteristics
- 1.1 Physical properties
- 1.2 Chemical properties
- 1.3 Isotopes
- 1.3.1 Europium as a nuclear fission product
- 1.4 Occurrence
- 2 Production
- 3 Compounds
- 3.1 Eu(II) vs Eu(III)
- 3.2 Halides
- 3.3 Chalcogenides and pnictides
- 4 History
- 5 Applications
- 6 Precautions
- 7 See also
- 8 References
- 9 External links
[edit] Characteristics
[edit] Physical properties
dendritic sublimated Eu handled in a glovebox (~300 g; purity 99.998%)
oxidized europium, coated with yellow europium(II) carbonate
Europium is a ductile metal that is hard as lead. It crystallizes in a body-centered cubic habit.[2]
It becomes a superconductor when it is simultaneously at both high pressure (80 GPa) and at low temperature (1.8 K). The occurrence of superconductivity is due to the applied pressure driving europium from a divalent (J = 7/2) state into a trivalent (J = 0) state. In the divalent state, the strong local magnetic moment is thought to play a role in suppressing the superconductivity and so through eliminating this local moment the opportunity for superconductivity arises.[3]
[edit] Chemical properties
Europium is the most reactive of the rare earth elements. It rapidly oxidizes in air: bulk oxidation of a centimeter-sized sample occurs within several days.[4]) It resembles calcium in its reaction with water:
2 Eu + 6 H2O → 2 Eu(OH)3 + 3 H2
Samples 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 150 to 180 °C to form europium(III) oxide:
4 Eu + 3 O2 → 2 Eu2O3
Similarly, europium metal dissolves readily in dilute sulfuric acid to form pale pink coloured solutions of the hydrated Eu(III), which exist as a nonahydrate:[5]
2 Eu + 3 H2SO4 + 18 H2O → 2 [Eu(OH2)9]3+ + 3 SO2−
4 + 3 H2
[edit] Isotopes
Main article: Isotopes of europium
Naturally occurring europium is composed of 2 isotopes, 151Eu and 153Eu, with 153Eu being the most abundant (52.2% natural abundance). While 153Eu is stable, 151Eu was recently found to be unstable to alpha decay with half-life of 5+11
−3×1018
year[6] (in reasonable agreement with theoretical predictions), giving about 1 alpha decay per two minutes in every kilogram of natural europium. Besides natural radioisotope 151Eu, 35 artificial 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 (T½=12.8 hours), 152m1Eu (T½=9.3116 hours) and 152m2Eu (T½=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).
[edit] Europium as a nuclear fission product
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
Prop:
Unit: t½
a Yield
% Q *
KeV βγ
*
155Eu 4.76 .0803 252 βγ
85Kr 10.76 .2180 687 βγ
113mCd 14.1 .0008 316 β
90Sr 28.9 4.505 2826 β
137Cs 30.23 6.337 1176 βγ
121mSn 43.9 .00005 390 βγ
151Sm 90 .5314 77 β
Europium is produced by nuclear fission, but the fission product yields of europium isotopes are low near the top of the mass range for fission products.
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 half-life and short mean time to neutron absorption, most 151Sm instead winds up as 152Sm.
152Eu (half-life 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.[7] 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.
[edit] Occurrence
Monazite
Europium is not found in nature as a free element. Many minerals contain europium, with the most important sources being bastnäsite and monazite. Depletion or enrichment of europium in minerals relative to other rare earth elements is known as the europium anomaly.
Europium has also been identified in the spectra of the sun and certain stars.
Europium has no known biological role.
Divalent europium in small amounts is the activator of the bright blue fluorescence of some samples of the mineral fluorite (CaF2). 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.
[edit] Production
Main article: Monazite
Europium is found in minerals xenotime, monazite, and bastnäsite. The first two are orthophosphate minerals LnPO4 (Ln denotes a mixture of all the lanthanides except promethium), and the third is a fluorocarbonate LnCO3F. Monazite also contains thorium and yttrium, which complicates handling because thorium and its decay products are radioactive. For the isolation of individual lanthanoids, the metals are extracted from the ores with acids and separated by solvent extractions and ion exchange chromatography.[8] 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 is chlorine gas.
[edit] Compounds
See also: Category:Europium compounds
[edit] Eu(II) vs Eu(III)
Europium commonly forms divalent compounds, in contrast to most lanthanides, which almost exclusively form compounds with an oxidation state of +3. The 2+ state has a configuration 4f7, the half-filled shell being known to confer stability. In terms of size and coordination number, europium(II) and barium(II) are similar. For example, the sulfates of both barium and europium(II) also highly insoluble in water.[9] Divalent europium is, however, a mild reducing agent, oxidizing in air to Eu(III) compounds. 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 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 accessible divalency of europium has always made it one of the easiest lanthanides to extract and purify, even when present in low concentration, as it usually is.
[edit] Halides
Europium metal reacts with all the halogens:
2 Eu + 3 X2 → 2 EuX3 (X = F, Cl, Br, I)
This route gives white europium(III) fluoride (EuF3), yellow europium(III) chloride (EuCl3), and gray europium(III) bromide (EuBr3), and colourless europium(III) iodide (EuI3). Europium also forms the corresponding dihalides including yellow-green europium(II) fluoride (EuF2), colourless europium(II) chloride (EuCl2), colourless europium(II) bromide (EuBr2), and green europium(II) iodide (EuI2).[2]
[edit] Chalcogenides and pnictides
Europium forms stable compounds with all of the chalcogenides, but the heavier chalcogenides stabilize the lower oxidation state. Three oxides are known: europium(II) oxide (EuO), europium(III) oxide (Eu2O3), and the mixed oxide (Eu3O4). Otherwise the following are the main chalcogenide with the formulae EuX (X = S, Se, Te), all three of which are black solids. EuS is pepared by sulfiding the oxide at temperatures sufficiently high to decompose the Eu2S3:[10]
Eu2O3 + 3 H2S → 2 EuS + 3 H2O + S
The main nitrides is europium(III) nitride (EuN).
[edit] 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 scramble for the limited supply of europium on hand among the monazite processors. (Typical europium content in monazite is about 0.05%.) However, the Molycorp bastnäsite deposit at the Mountain Pass rare earth mine, California, whose lanthanides had an unusually high 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. Californian 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-exchange technology that revolutionized the rare earth industry in the mid-1950s once related the story of how, in the 1930s, 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 Dr. McCoy who had developed a famous method of europium purification involving redox chemistry.
[edit] Applications
Europium is one of the elements used to make the red color in CRT televisions.
There are many commercial applications for europium metal: 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, the luminescence of divalent europium depends on the host lattice, but tends to be on the blue side. The two europium phosphor classes (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 is the phosphor system typically encountered in the helical fluorescent lightbulbs. Combining the same three classes is one way to make trichromatic systems in TV and computer screens. 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.[11]
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.
[edit] 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.
引用出處：
http://en.wikipedia.org/wiki/Europium
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