Europium

About Europium

Europium is the oddball of the lanthanide row. Its half-filled 4f⁷ shell makes the +2 state unusually accessible — in fact, Eu²⁺ is stable in aqueous solution, which is almost unheard of for a rare earth. That accessible redox couple is also why europium is so soft (a knife will cut it) and why it tarnishes faster than any other lanthanide, oxidizing within hours in moist air. The chemistry that put europium on the map is luminescence. Eu²⁺ in a barium magnesium aluminate host emits a clean blue around 450 nm, while Eu³⁺ in yttrium oxysulfide gives the deep red that defined NTSC color television and still drives the red subpixel in trichromatic LED phosphors. Europium-doped strontium aluminate is the long-persistence green-yellow glow in safety signage and watch dials. The element is one of the more expensive lanthanides per gram, mostly because the +2/+3 separation chemistry happens to be easier than separating its neighbors, but demand from solid-state lighting keeps the market tight.