Praseodymium

Praseodymium was isolated in 1885 by the Austrian chemist Carl Auer von Welsbach, who split a substance long thought to be a single element, didymium, into two. He named one half praseodymium, from the Greek prasios didymos, green twin, after the green colour of its salts. It is a soft, silvery rare-earth metal used in strong magnets, in the yellow-green glass of welders' goggles, and in high-strength alloys. It is used with neodymium in strong permanent magnets and in the didymium glass that shields the eyes of welders and glassblowers from glare, and it lends a clear yellow colour to ceramics and glass.

Auer von Welsbach was one of the most commercially successful chemists of his era: besides splitting didymium, he invented the incandescent gas mantle that bears his name, still recognisable in camping lanterns today, developed the flint alloy used in modern cigarette lighters, and pioneered an early metal-filament electric lamp, so the same rare-earth chemistry that yielded praseodymium and neodymium also quietly lit homes and streets across turn-of-the-century Europe. His Vienna-area factory at Treibach became one of the first industrial centres for rare-earth extraction anywhere in the world, processing the same monazite sands that would later also yield thorium.

Praseodymium's name preserves a long chapter of confusion. In 1841 Mosander had drawn from cerium ores a substance he named didymium, from the Greek for twin, believing it a single new element and a near-twin of lanthanum. For over forty years didymium was listed among the elements, until Auer von Welsbach demonstrated that it was in fact two elements intimately mixed. He named them for their appearance and their history: praseodymium, the green twin, for its leek-green salts, and neodymium, the new twin. The episode is emblematic of the rare earths, whose stubborn similarity repeatedly disguised two or more elements as one. Praseodymium has no ancient tradition; its poetry is the botanical green of its compounds and the twinship written into its name.

Today praseodymium sits, with neodymium, on most governments' critical-minerals lists, since China controls a large majority of world rare-earth mining and, more decisively, of the specialised separation and refining capacity the two green-and-pink twins still require, a modern industrial dependency that echoes, in miniature, the nineteenth-century laboratories that first had to tease the pair apart. Efforts to build alternative supply chains in North America and Australia remain, as of the mid-2020s, small relative to demand.