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= Europium =
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Introduction
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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
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Its discoverer, Eugène-Anatole Demarçay, named the element after the
continent of Europe.
Physical properties
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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
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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(III) oxide:
:
Europium dissolves readily in dilute sulfuric acid to form pale pink
solutions of :
:
Eu(II) vs. Eu(III)
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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 4'f'7 because the half-filled 'f'-shell
provides more stability. In terms of size and coordination number,
europium(II) and barium(II) are similar. The sulfates of both barium
and europium(II) are also highly insoluble in water. 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 "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 (trivalent) lanthanides made europium
accessible even when present in low concentration, as it usually is.
Compounds
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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 feature acetylacetonate-like ligands, such as EuFOD.
Halides
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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), gray europium(III) bromide (EuBr3),
and colorless europium(III) iodide (EuI3). Europium also forms the
corresponding dihalides: yellow-green europium(II) fluoride (EuF2),
colorless europium(II) chloride (EuCl2) (although it has a bright blue
fluorescence under UV light), colorless europium(II) bromide (EuBr2),
and green europium(II) iodide (EuI2).
Chalcogenides and pnictides
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Europium forms stable compounds with all of the chalcogens, but the
heavier chalcogens (S, Se, and Te) stabilize the lower oxidation
state. Three oxides are known: europium(II) oxide (EuO), europium(III)
oxide (Eu2O3), and the mixed-valence oxide Eu3O4, consisting of both
Eu(II) and Eu(III). Otherwise, the main chalcogenides are europium(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:
:Eu2O3 + 3 H2S → 2 EuS + 3 H2O + S
The main nitride of europium is europium(III) nitride (EuN).
Isotopes
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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 , 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 as a nuclear fission product
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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.
Thermal neutron capture cross sections
!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 (not included in above
yield), but since this has a long decay half-life and short mean time
to neutron absorption, most 151Sm instead ends up as 152Sm.
152Eu (half-life 13.517 years) and 154Eu (half-life 8.592 years)
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
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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. 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
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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-(Ce), xenotime, and monazite in
mineable quantities. Bastnäsite is a group of related
fluorocarbonates, Ln(CO3)(F,OH). Monazite is a group of related of
orthophosphate minerals (Ln denotes a mixture of all the lanthanides
except promethium), loparite-(Ce) is an oxide, and xenotime is an
orthophosphate (Y,Yb,Er,...)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(III) to cerium(IV) 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(III) to europium(II). Europium(II)
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 (or CaCl2) 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 (La-Gd, Sc, and Y) 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.
History
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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.
Applications
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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. Europium
fluorescence is used to interrogate biomolecular interactions in
drug-discovery screens. It is also used in the anti-counterfeiting
phosphors in euro banknotes.
An application that has almost fallen out of use with the introduction
of affordable superconducting magnets is the use of europium
complexes, such as Eu(fod)3, as shift reagents in NMR spectroscopy.
Chiral shift reagents, such as Eu(hfc)3, are still used to determine
enantiomeric purity.
Europium compounds are used to label antibodies for sensitive
detection of antigens in body fluids, a form of immunoassay. When
these europium-labeled antibodies bind to specific antigens, the
resulting complex can be detected with laser excited fluorescence.
Precautions
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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.
External links
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* [
http://education.jlab.org/itselemental/ele063.html It's Elemental -
Europium]
License
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Original Article:
http://en.wikipedia.org/wiki/Europium
