Europium

Europium earned its name from its continent of discovery, but its real claim to fame lies in what happens when energy strikes its electrons. No other element fluoresces as efficiently as europium, and the two oxidation states it adopts produce two completely different colors of light — a vivid red from the trivalent ion and a deep blue from the divalent ion. This dual luminescent personality made europium essential to color television technology, gave white LED lighting its warmth, and put it inside the security features of currency from the United States to the European Union. As the most reactive of the rare earth metals, europium corrodes readily in air and reacts with water, but its instability in bulk is no obstacle to its brilliance in phosphor matrices where it is locked safely in a crystal lattice.

Europium's primary industrial use centers on its luminescent properties. Eu3+ ions produce the intense red emission in the yttrium vanadate and yttrium oxide phosphors that gave color television its red channel, and they continue to serve in fluorescent lamps and LED phosphors. Eu2+ ions produce blue and blue-green emission in strontium aluminate and barium magnesium aluminate phosphors used in white LEDs, where combining blue europium emission with yellow or green phosphors generates the broad-spectrum light that appears white to the human eye. Euro banknotes and U.S. currency incorporate europium-containing phosphors in security inks that fluoresce under ultraviolet light, providing an authentication feature difficult to counterfeit. Nuclear reactor control rods sometimes use europium oxide as a neutron absorber because of europium's high neutron-capture cross section.

By the late nineteenth century, chemists working with rare-earth elements had grown accustomed to finding impurities hiding inside supposedly pure samples. Paul Lecoq de Boisbaudran and others had already separated samarium and gadolinium from what chemists once considered uniform materials. In 1896, Eugène-Anatole Demarcay, a French spectroscopist, noticed anomalous spectral lines in purified samarium samples that could not be explained by any known element. Systematic fractional crystallization of samarium magnesium nitrate over several years allowed him to concentrate the mysterious component, and by 1901 he had isolated enough to confirm it as a new element. He named it europium after Europe, making it one of several elements named for continents or regions. Its practical importance remained modest for decades until the phosphor chemistry underpinning color television displays in the 1960s revealed europium's unmatched red luminescence.

Europium is the least abundant of the naturally occurring lanthanides apart from the radioactive promethium, present in the Earth's crust at roughly two parts per million — comparable to the abundance of tin. It occurs entirely within mixed rare-earth mineral deposits, never in elemental form. Monazite and bastnäsite are the main commercial sources, though europium constitutes only about 0.05 percent of the total rare-earth content in typical deposits, making it the scarcest of the economically recoverable lanthanides. This relative scarcity, combined with its essential role in display and lighting technologies, has periodically made it a supply-chain concern. China supplies the vast majority of the world's europium. The element's high reactivity toward oxygen and moisture means that refined europium metal must be stored under inert gas or oil to prevent rapid surface oxidation.

Europium oxide (Eu2O3) is the most important commercial compound, serving as the starting material for phosphor production and as a neutron-absorbing material. Europium-doped yttrium oxide (Y2O3:Eu3+) and europium-doped yttrium vanadate (YVO4:Eu3+) are the classical red phosphors used in color television and fluorescent lighting. Europium-doped strontium aluminate (SrAl2O4:Eu2+) is a long-persistence phosphorescent pigment — commonly called 'glow in the dark' material — widely used in safety signs and novelty products. Europium(II) chloride and europium(III) chloride have been studied as precursors for thin-film deposition and optical coatings. Europium trifluoride and various europium beta-diketonate complexes are used in organic light-emitting diode (OLED) research and in luminescent sensor applications that exploit europium's long fluorescence lifetime for time-gated imaging.

• Europium is the most reactive rare earth metal, tarnishing almost immediately when exposed to air and reacting vigorously with water — a stark contrast to the stable, glowing roles it plays when safely embedded in phosphor crystals.
• The red you see on a color television or older computer monitor owes its vividness to europium: red phosphors based on europium emit a narrower, purer red than any competing technology, which is why screens could achieve such saturated colors.
• Euro banknotes glow in a pattern of stars and other features under ultraviolet light because of europium-based security phosphors embedded in the paper — a feature the European Central Bank has used since the currency's introduction in 2002.
• Europium has two dramatically different oxidation states that emit completely different colors: Eu3+ glows red, while Eu2+ glows blue — making a single element responsible for two of the three primary colors in phosphor-based lighting.
• Glow-in-the-dark materials that stay luminous for hours, such as those used on emergency exit signs and watch dials, typically use europium-doped strontium aluminate, which can store light energy and release it gradually over many hours.