Praseodymium

• 1. Introduction

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  Praseodymium gets its name from two Ancient Greek words: prasinos, meaning 'leek-green', and didymos, meaning 'twin'. The leek-green half describes its salts. The twin half hides a long confusion, because for decades praseodymium was not recognized as itself at all. Its symbol is Pr, its atomic number is 59, and it sits as the third member of the lanthanide series among the rare-earth metals. A soft, silvery, malleable metal, it is too reactive to be found in native form, and a pure sample slowly grows a green oxide coating in open air. So how did a metal hiding inside something called didymium finally earn its own name? Why does it tint glass and ceramics in shades of yellow and green? And why might a metal that is not actually rare become a prize in a world running on renewable energy?

• 2. A Twin Pulled Apart

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  In 1751, the Swedish mineralogist Axel Fredrik Cronstedt found a heavy mineral at the mine at Bastnas, later named cerite. Thirty years later, the fifteen-year-old Wilhelm Hisinger, from the family owning the mine, sent a sample to Carl Scheele, who found no new elements in it. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jons Jacob Berzelius and isolated a new oxide, named ceria after the dwarf planet Ceres. Ceria was independently isolated in Germany by Martin Heinrich Klaproth around the same time. Between 1839 and 1843, the Swedish surgeon and chemist Carl Gustaf Mosander showed that ceria was a mixture of oxides. Living in the same house as Berzelius, he separated out two further oxides, which he named lanthana and didymia, by roasting cerium nitrate in air and treating the result with dilute nitric acid. Lanthanum turned out to be a pure element, but didymium did not. It was a mixture of all the stable early lanthanides from praseodymium to europium. Marc Delafontaine had suspected this after spectroscopic analysis but lacked the time to separate it. The heavy pair of samarium and europium were removed in 1879 by Paul-Emile Lecoq de Boisbaudran. Only in 1885 did the Austrian chemist Carl Auer von Welsbach split the remaining didymium into two elements giving salts of different colours, which he named praseodymium and neodymium. His products were of relatively low purity, and because neodymium was the larger constituent, it kept the old name with disambiguation while praseodymium was distinguished by the leek-green colour of its salts. Bohuslav Brauner had already suggested in 1882 that didymium was composite, though he never pursued the separation himself.

• 3. The Shape Of An Atom

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  Neutral praseodymium's 59 electrons settle into the configuration Xe4f3 6s2. Like most lanthanides, it usually offers only three of these as valence electrons, because the remaining 4f electrons sit too tightly bound to join in bonding. The 4f orbitals penetrate most deeply through the inert xenon core toward the nucleus, followed by the 5d and 6s, and that penetration grows with higher ionic charge. Because praseodymium sits early in the lanthanide series, the nuclear charge is still low enough to let it lose a fourth valence electron in some compounds. Its calculated atomic radius is very large, at 247 pm, exceeded only by barium, rubidium and caesium, yet observationally it usually measures 185 pm. The metal is ductile, with a hardness comparable to that of silver. At room temperature it takes a double hexagonal close-packed structure called the alpha phase. At 795 degrees Celsius it transforms to a body-centered cubic allotrope known as beta, and it melts at 931 degrees Celsius. Praseodymium is paramagnetic at room temperature, like all the lanthanides. Unlike some rare-earth metals that order antiferromagnetically or ferromagnetically when chilled, praseodymium stays paramagnetic at every temperature above 1 kelvin.

• 4. The Reactive Surface

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  A centimetre-sized sample of praseodymium metal corrodes completely in about a year, shedding a spalling green oxide layer much like iron rust. Heated to 150 degrees Celsius, it burns to form a nonstoichiometric praseodymium(III,IV) oxide, which hydrogen gas can reduce to praseodymium(III) oxide. Praseodymium(IV) oxide is the most oxidised product of combustion, made by reacting the metal with pure oxygen at 400 degrees Celsius and 282 bar, or by disproportionation in boiling acetic acid. At 1000 degrees Celsius the oxides exist as disordered nonstoichiometric phases, but between 400 and 700 degrees Celsius the defects order into a family of phases labelled with Greek letters from beta through sigma. Praseodymium is electropositive. It reacts slowly with cold water and quite quickly with hot water to form praseodymium(III) hydroxide, and it combines with every stable halogen to make green trihalides. A tetrafluoride is also known, produced by reacting sodium fluoride and praseodymium(III) fluoride with fluorine gas, then stripping the sodium fluoride away with liquid hydrogen fluoride. Dissolved in dilute sulfuric acid, the metal gives chartreuse ions held as complexes. Praseodymium(IV) compounds, by contrast, will not survive in water: the high positive standard reduction potential of +3.2 volts makes those ions oxidise water itself and fall back to the +3 state.

• 5. Reaching The Fifth State

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  Praseodymium(V) carries the stable electron configuration of xenon, the noble gas just before it, and chasing that state has pushed chemists to extreme conditions. In 2016 it was observed by matrix isolation, where species trapped under noble-gas conditions were assigned to the +5 state, including argon adducts. In 2025 a neutral compound, formally praseodymium(V) but with an inverted ligand field, was isolated and characterized crystallographically at low temperatures. This +5 state is unique among the lanthanides, none of which otherwise reach so high. The more familiar chemistry stays modest. Organopraseodymium compounds resemble those of the other lanthanides, sharing an inability to undergo pi backbonding, which restricts them mostly to ionic cyclopentadienides and sigma-bonded alkyls and aryls. The coordination chemistry is that of a large, electropositive ion, giving complexes with high but uncertain coordination numbers and poorly defined stereochemistry. Praseodymium nitrates form both 4:3 and 1:1 complexes with 18-crown-6, a versatility shared with lanthanum, cerium and neodymium but lost to the middle and later lanthanides. The first example of a molecular complex of praseodymium(IV) has only recently been reported.

• 6. One Stable Isotope

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  Praseodymium-141 is the element's only stable, naturally occurring isotope, which makes praseodymium both mononuclidic and monoisotopic. Its standard atomic weight is therefore a constant of nature, fixed with high precision. That isotope carries 82 neutrons, a magic number that grants extra stability. Thirty-eight other radioisotopes have been synthesized, almost all with half-lives under a day and most under a minute. The lone exception is praseodymium-143, with a half-life of 13.57 days. Both 141Pr and 143Pr appear as fission products of uranium, alongside heavier isotopes. Isotopes lighter than 141Pr decay mainly by positron emission or electron capture into cerium, while heavier ones undergo beta decay into neodymium.

• 7. Mined With Its Neighbours

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  Praseodymium makes up about 9.1 parts per million of the Earth's crust, an abundance close to that of boron and far from genuinely rare. It is the sixth-most abundant rare-earth element and the fourth-most abundant lanthanide. Its rare-earth label comes from scarcity relative to common earths like lime and magnesia, the few commercially viable minerals that hold it, and the length and complexity of extraction. It is never the dominant rare earth in its host minerals, always preceded by cerium and lanthanum and usually by neodymium. The Pr3+ ion shares a size with the early cerium-group lanthanides, so it travels with them through phosphate, silicate and carbonate minerals such as monazite and bastnasite. Bastnasite is easier to work, usually lacking thorium and the heavy lanthanides. Monazite demands more steps, including electromagnetic separation, treatment with hot concentrated sulfuric acid, and the removal of thorium as a hydroxide near pH 3 to 4. Some residues must be handled with care because they contain radium-228, a strong gamma emitter and the daughter of thorium-232. Praseodymium is finally teased apart from the other lanthanides by ion-exchange chromatography, where the cations descend a column and elute from heaviest to lightest, or with a solvent such as tributyl phosphate. Worldwide production was estimated at 2,000 tonnes in 2020 and rose to roughly 3,700 tonnes in 2022, with China producing more than 80 percent of global output, followed at a distance by the United States and Australia.

• 8. From Glass To Absolute Zero

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  Leo Moser, son of the founder of the Moser Glassworks in what is now Karlovy Vary in the Czech Republic, investigated praseodymium glass coloration in the late 1920s. His yellow-green glass, named Prasemit, lost out to far cheaper colorants and survives in extremely rare examples. He had more luck blending praseodymium with neodymium to make a glass called Heliolite. The first enduring commercial use, still active today, is a yellow-orange Praseodymium Yellow stain for ceramics, a solid solution in the zircon lattice with no hint of green. Pr3+ as a dopant ion has long excited-state lifetimes and high luminescence yields, thanks to its shielded f-orbitals, which carries it into lasers, single-mode fiber optical amplifiers, upconverting nanoparticles, and red, green, blue and ultraviolet phosphors. Silicate crystals doped with praseodymium have slowed a light pulse to a few hundred metres per second. Paired with neodymium, it builds high-power permanent magnets prized for strength and durability, and the demand for these NdPr alloys in electric vehicles and wind turbines is driving production growth. A praseodymium-nickel intermetallic, PrNi5, has so strong a magnetocaloric effect that it has let scientists approach within one thousandth of a degree of absolute zero. The element also alloys with magnesium for aircraft engines, sits in the fluoride core of carbon arc lights used for studio lighting, and tints didymium glass for welder's and glass blower's goggles. Some early lanthanides, praseodymium among them, even prove essential to methanotrophic bacteria living in volcanic mudpots, where lanthanum, cerium, praseodymium and neodymium serve about equally well.