Europium is a chemical element; it has symbol Eu and atomic number 63. It is a soft, silvery-white lanthanide metal that tarnishes rapidly in air and is the most chemically reactive of the lanthanides. Europium has an unusually low density and large atomic volume for a lanthanide because the metal is effectively divalent, unlike the predominantly trivalent neighbouring elements. Its half-filled 4f7 electron configuration also gives elemental europium distinctive magnetic properties; it is paramagnetic at room temperature and becomes antiferromagnetic below about 91 K. In compounds, the +3 oxidation state is generally the most stable, but europium has the most readily accessible +2 state of any lanthanide.
Europium does not occur naturally as the free metal and is found only as a minor component of rare-earth minerals such as bastnäsite, monazite and xenotime. Its ability to form Eu(II) gives it unusual geochemical behaviour: europium can be preferentially incorporated into feldspar, producing positive or negative europium anomalies that are widely used in petrology and geochemistry. Naturally occurring europium consists of two isotopes in nearly equal proportions, Eu and Eu. Eu is observationally stable, while Eu is a very long-lived alpha emitter with a half-life of about .
The identification of europium formed part of the long 19th-century process of separating the rare-earth elements. William Crookes observed an unexplained spectral feature in rare-earth material in 1885, and Paul Émile Lecoq de Boisbaudran reported a further unidentified line in samarium in 1892. In 1896, the French chemist Eugène-Anatole Demarçay reported evidence for a new element associated with samarium; after repeated fractional crystallisation, he obtained it in sufficiently pure form in 1901 and named it europium after Europe. Elemental europium metal was prepared several decades later.
The major applications of europium exploit its luminescence. Eu(III) produces narrow orange-red emission lines, whereas Eu(II) gives broad, host-dependent emission that can span much of the visible spectrum. Europium phosphors became commercially important in the 1960s as red emitters in colour television and remain important in lighting, light-emitting diodes, display technologies, persistent luminescent materials and radiation detectors. Europium complexes are also used in anti-counterfeiting materials, time-resolved biochemical assays and specialised NMR applications. Europium has no known biological role, and toxicological data remain limited; the elemental metal is pyrophoric and must be protected from air and moisture.
Physical properties
Europium is a soft, silvery-white, ductile rare-earth metal. Its hardness is similar to that of lead. At room temperature it has a density of about 5.24 g/cm3 and a melting point of .
The ground-state electron configuration of a neutral europium atom is [Xe] 4f76s2, with the spectroscopic ground term 8So7/2. The seven 4f electrons form a particularly stable half-filled shell. In the metallic state the 4f electrons remain strongly localised, and europium is effectively divalent rather than trivalent, in contrast with most of the lanthanides. This behaviour is shared with ytterbium, whose filled 4f shell likewise favours a divalent metallic state. Consequently, europium and ytterbium have substantially larger atomic volumes than the neighbouring trivalent lanthanide metals.
At ambient pressure, europium crystallizes in the body-centered cubic (bcc) structure. Its unusually large atomic volume is associated with its divalent metallic state. Under pressure, europium undergoes a series of structural transformations. The bcc phase transforms to a hexagonal close-packed structure at about 12.5 GPa, which remains stable to about 31.5 GPa. Above this pressure europium adopts an incommensurately modulated monoclinic structure, known as Eu-IV, which is stable to about 37 GPa; another structural transition occurs at higher pressure.
The half-filled 4f shell also dominates the magnetic properties of elemental europium. The divalent 4f7 configuration has L = 0 and S = J = 7/2, giving a large predominantly spin magnetic moment. Europium is paramagnetic at room temperature, but below a Néel temperature of about 91 K it becomes antiferromagnetic. Neutron-diffraction measurements show that the ordered state has a helical spin structure, with the magnetic moments lying parallel to a cube face and rotating from one atomic plane to the next. The ordered magnetic moment was measured as 5.9 ± 0.4 μB per atom.
Chemical properties
Europium is a highly electropositive and reactive metal, and is the most reactive of the lanthanide metals. Fresh europium is silvery-white, but it rapidly tarnishes on exposure to air. When heated in oxygen it burns readily to form europium(III) oxide, Eu2O3:
Europium reacts slowly with cold water and more rapidly with hot water, liberating hydrogen and forming europium(III) hydroxide:
The metal also reacts readily with the halogens and combines with several other nonmetals on heating. It dissolves readily in dilute non-oxidising acids, evolving hydrogen and forming Eu(III) in solution. The net ionic reaction is:
Aqueous Eu(III) solutions are very pale pink, and the hydrated ion is commonly represented as . Europium ions generally adopt high coordination numbers. Eu(III) behaves as a hard Lewis acid and favours hard donor atoms, particularly oxygen, whereas the larger Eu(II) ion is softer and shows greater affinity for softer donors such as nitrogen and sulfur.
Oxidation states
The conventional chemistry of europium is dominated by the +3 and +2 oxidation states. Eu(III), with the electron configuration 4f6, is the more stable oxidation state under ordinary oxidising conditions. Europium is unusual among the lanthanides, however, because Eu(II) is also readily accessible. Eu(II) has the half-filled 4f7 configuration, which contributes to the relative stability of the divalent state; europium has the most accessible +2 oxidation state of any lanthanide.
In aqueous solution, the standard reduction potential of the Eu(III)/Eu(II) couple is about −0.34 V relative to the standard hydrogen electrode. Eu(III) can therefore be reduced to Eu(II) substantially more readily than the corresponding trivalent ions of most other lanthanides. Eu(II) is a mild reducing agent and is readily oxidised by molecular oxygen to Eu(III), although it is sufficiently persistent under oxygen-free conditions for an extensive aqueous and coordination chemistry to be developed.
The Eu(II) ion is substantially larger than Eu(III) and in size closely resembles Sr(II). The ionic radii of Eu(II) and Sr(II) are very similar, although Eu(II) is slightly softer and consequently shows differences in solvation and coordination; for example, X-ray absorption measurements found lower coordination numbers for Eu(II) than for Sr(II) in several oxygen-donor solvents. The large difference in ionic radius and electronic configuration between Eu(II) and Eu(III) gives the two oxidation states markedly different coordination, magnetic and optical properties.
Compounds
Europium forms a wide variety of inorganic, coordination and organometallic compounds. Unlike most lanthanides, for which the +3 oxidation state overwhelmingly dominates ordinary chemistry, europium has extensive chemistry in both the +3 and +2 states. Eu(III) is generally more stable under oxidising conditions, but the half-filled 4f7 configuration makes Eu(II) unusually accessible. The two oxidation states consequently have markedly different structural, magnetic and optical properties.
Oxides and chalcogenides
Europium forms three well-established binary oxides. Europium(II) oxide, EuO, contains divalent europium and has the cubic rock salt structure. Eu3O4 is a mixed-valence oxide containing both Eu(II) and Eu(III). Europium(III) oxide, Eu2O3, contains only trivalent europium and is the stable sesquioxide under ordinary oxidising conditions. At ambient conditions Eu2O3 has the cubic C-type rare-earth sesquioxide structure; monoclinic and trigonal polymorphs occur at elevated temperature or pressure. Eu3O4 has an orthorhombic structure related to the CaFe2O4 type, with Eu(II) and Eu(III) occupying distinct crystallographic sites.
EuO is a ferromagnetic semiconductor below a Curie temperature of about 69 K, and has been extensively studied because of the strong coupling between its magnetic and electronic properties. The heavier monochalcogenides EuS, EuSe and EuTe likewise contain Eu(II) and crystallize in the rock-salt structure. They form a classic series of magnetic semiconductors: EuS is ferromagnetic at low temperature, EuSe exhibits competing ferro- and antiferromagnetic interactions, and EuTe is antiferromagnetic.
Europium also forms mixed-anion chalcogenides. Europium oxysulfide, Eu2O2S, contains Eu(III) and exhibits the characteristic sharp visible emission of the trivalent ion.
Halides
Europium forms all four trihalides, EuF3, EuCl3, EuBr3 and EuI3, as well as the complete corresponding series of divalent halides EuF2, EuCl2, EuBr2 and EuI2. The existence of a complete Eu(II) halide series reflects the exceptional stability of divalent europium. EuF3 is white, EuCl3 yellow, EuBr3 grey and EuI3 colourless; the divalent halides are generally white to yellow solids.
The crystal structures vary considerably across the series. EuF2 adopts the cubic fluorite structure, whereas EuCl2 has the orthorhombic PbCl2-type structure; several structural forms of EuI2 are known. The structural resemblance of many Eu(II) halides to alkaline-earth halides reflects the similar size of Eu2+ and the heavier alkaline-earth ions.
The Eu(II) halides also show characteristic 4f–5d luminescence. Solid EuCl2, EuBr2 and EuI2 fluoresce under ultraviolet excitation, with broad emission bands whose positions depend strongly on the halide ligand; EuCl2, for example, emits in the blue region.
Hydrides
The binary hydride europium dihydride, EuH2, is an ionic compound containing Eu(II). Neutron-diffraction measurements on the corresponding deuteride show that it adopts the orthorhombic PbCl2-type structure, with nine-coordinate europium. Although earlier pressure–composition measurements suggested appreciable hydrogen deficiency, diffraction measurements on equilibrated samples indicate essentially fully occupied hydrogen sites.
Europium hydride chemistry extends beyond the binary hydride. EuLiH3 contains Eu(II) and crystallizes in the cubic inverse-perovskite structure, with twelve-coordinate europium. Complex ternary hydrides including EuMg2H6 and EuMgH4 are also known. Structural, electrical and magnetic measurements indicate that europium remains divalent in the europium hydrides studied, in contrast to samarium, which can readily become trivalent in its hydrides.
Borides and carbides
Among the europium borides, europium hexaboride, EuB6, is particularly well studied. It has the cubic CaB6-type structure, in which a three-dimensional framework of linked B6 octahedra surrounds the europium ions. Unlike most rare-earth hexaborides, europium is divalent in EuB6. EuB6 is a ferromagnetic semimetal and undergoes two closely spaced electronic and magnetic transitions near 15.5 and 12.6 K. Its unusual transport properties, including large magnetoresistance, have been associated with the formation and subsequent overlap of magnetic polarons.