Europium is a chemical element with symbol
Eu and atomic number 63. It was isolated in
1901 and is named after the continent of Europe.
It is a moderately hard, silvery metal which
readily oxidizes in air and water. Being a
typical member of the lanthanide series, europium
usually assumes the oxidation state +3, but
the oxidation state +2 is also common. All
europium compounds with oxidation state +2
are slightly reducing. Europium has no significant
biological role and is relatively non-toxic
compared to other heavy metals. Most applications
of europium exploit the phosphorescence of
europium compounds. Europium is one of the
least abundant elements in the universe; only
about 5×10−8% of all matter in the universe
is europium.
== Characteristics ==
=== 
Physical properties ===
Europium is a ductile metal with a hardness
similar to that of lead. It crystallizes in
a body-centered cubic lattice. Some properties
of europium are strongly influenced by its
half-filled electron shell. Europium has the
second lowest melting point and the lowest
density of all lanthanides.Europium becomes
a superconductor when it is cooled below 1.8
K and compressed to above 80 GPa. This is
because europium is divalent in the metallic
state, and is converted into the trivalent
state by the applied pressure. In the divalent
state, the strong local magnetic moment (J
= 7/2) suppresses the superconductivity, which
is induced by eliminating this local moment
(J = 0 in Eu3+).
=== Chemical properties ===
Europium is the most reactive rare-earth element.
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
2 Eu + 6 H2O → 2 Eu(OH)3 + 3 H2Because 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:
4 Eu + 3 O2 → 2 Eu2O3Europium dissolves
readily in dilute sulfuric acid to form pale
pink solutions of the hydrated Eu(III), which
exist as 
a nonahydrate:
2 Eu + 3 H2SO4 + 18 H2O → 2 [Eu(H2O)9]3+
+ 3 SO2−4 + 3 H2
==== Eu(II) vs. Eu(III) ====
Although usually trivalent, europium readily
forms divalent compounds. This behavior is
unusual to 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 gives more 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) 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.
=== Isotopes ===
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 years, giving about 1 alpha
decay per two minutes in every kilogram of
natural europium. This value is in reasonable
agreement with theoretical predictions. Besides
the natural radioisotope 151Eu, 35 artificial
radioisotopes have been characterized, 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 the remaining radioactive isotopes
have half-lives shorter than 4.7612 years,
and the majority of these have half-lives
shorter than 12.2 seconds. This element also
has 8 meta states, with the most stable being
150mEu (t1/2=12.8 hours), 152m1Eu (t1/2=9.3116
hours) and 152m2Eu (t1/2=96 minutes).The primary
decay mode for isotopes lighter than 153Eu
is electron capture, and the primary mode
for heavier isotopes 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).
==== Europium as a nuclear fission product
====
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 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 (half-life 13.516 years) and 154Eu (half-life
8.593 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, much of this
is further converted to 155Eu.
155Eu (half-life 4.7612 years) has a fission
yield of 330 parts per million (ppm) for uranium-235
and thermal neutrons; most of it is transmuted
to non-radioactive and nonabsorptive gadolinium-156
by the end of fuel burnup.
Overall, europium is overshadowed by caesium-137
and strontium-90 as a radiation hazard, and
by samarium and others as a neutron poison.
=== 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-(Ce). No europium-dominant
minerals are known yet, despite of a single
find of a tiny possible Eu–O or Eu–O–C
system phase in the Moon's regolith.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.
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.
== Production ==
Europium is associated with the other rare-earth
elements and is, therefore, mined together
with them. Separation of the rare-earth elements
is a step in the 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
LnPO4 (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 and subsequent 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 carbonate
or is 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 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. 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.
== Compounds ==
Europium compounds tend to exist trivalent
oxidation state under most conditions. Commonly
these compounds feature Eu(III) bound by 6–9
oxygenic ligands, typically water. These compounds,
the chlorides, sulfates, nitrates, are soluble
in water or polar organic solvent. Lipophilic
europium complexes often feature acetylacetonate-like
ligands, e.g., Eufod.
=== 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), 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), colorless europium(II) bromide
(EuBr2), and green europium(II) iodide (EuI2).
=== Chalcogenides and pnictides ===
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. EuS is prepared
by sulfiding the oxide at temperatures sufficiently
high to decompose the Eu2O3:
Eu2O3 + 3 H2S → 2 EuS + 3 H2O + SThe main
nitride is europium(III) nitride (EuN).
== History ==
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
observed the phosphorescent spectra of the
rare elements including those eventually assigned
to europium.Europium was first found in 1892
by Paul Émile Lecoq de Boisbaudran, who obtained
basic fractions 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
it in 1901; he then named it europium.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, as
the 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 to be in use in the TV industry
ever since as well as 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 he was lecturing
on the rare earths in the 1930s, 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 Herbert Newby McCoy, who had developed
a famous method of europium purification involving
redox chemistry.
== Applications ==
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.
Europium is also used in the manufacture of
fluorescent glass. 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.A recent (2015) application of europium
is in quantum memory chips which can reliably
store information for days at a time; these
could allow sensitive quantum data to be stored
to a hard disk-like device and shipped around
the country.
== Precautions ==
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
