Praseodymium

Characteristics
Physical

Praseodymium is the third member of the lanthanide series. In the periodic table, it appears between the lanthanides cerium to its left and neodymium to its right, and above the actinide protactinium. It is a ductile metal with a hardness comparable to that of silver.[5] Its 59 electrons are arranged in the configuration [Xe]4f36s2; theoretically, all five outer electrons can act as valence electrons, but the use of all five requires extreme conditions and normally, praseodymium only gives up three or sometimes four electrons in its compounds. Praseodymium is the first of the lanthanides to have an electron configuration conforming to the Aufbau principle, which predicts the 4f orbitals to have a lower energy level than the 5d orbitals; this does not hold for lanthanum and cerium, because the sudden contraction of the 4f orbitals does not happen until after lanthanum, and is not strong enough at cerium to avoid occupying the 5d subshell. Nevertheless, solid praseodymium takes on the [Xe]4f25d16s2 configuration, with one electron in the 5d subshell like all the other trivalent lanthanides (all but europium and ytterbium, which are divalent in the metallic state).[6]

Like most lanthanides, praseodymium usually only uses three electrons as valence electrons, as afterwards the remaining 4f electrons are too strongly bound: this is because the 4f orbitals penetrate the most through the inert xenon core of electrons to the nucleus, followed by 5d and 6s, and this increases with higher ionic charge. Praseodymium nevertheless can continue losing a fourth and even occasionally a fifth valence electron because it comes very early in the lanthanide series, where the nuclear charge is still low enough and the 4f subshell energy high enough to allow the removal of further valence electrons.[7] Thus, similarly to the other early trivalent lanthanides, praseodymium has a double hexagonal close-packed crystal structure at room temperature. At about 560 °C, it transitions to a face-centered cubic structure, and a body-centered cubic structure appears shortly before the melting point of 935 °C.[8]

Praseodymium, like all of the lanthanides (except lanthanum, ytterbium, and lutetium, which have no unpaired 4f electrons), is paramagnetic at room temperature.[9] Unlike some other rare-earth metals, which show antiferromagnetic or ferromagnetic ordering at low temperatures, praseodymium is paramagnetic at all temperatures above 1 K.[3]

Isotopes
Main article: Isotopes of praseodymium

Praseodymium has only one stable and naturally occurring isotope, ¹⁴¹Pr. It is thus a mononuclidic element, and its standard atomic weight can be determined with high precision. This isotope has 82 neutrons, a magic number that confers additional stability.^[10] This isotope is produced in stars through the s- and r-processes (slow and rapid neutron capture, respectively).^[11]

All other praseodymium isotopes have half-lives under a day (and most under a minute), with the single exception of ¹⁴³Pr with a half-life of 13.6 days. Both ¹⁴³Pr and ¹⁴¹Pr occur as fission products of uranium. The primary decay mode of isotopes lighter than ¹⁴¹Pr is inverse beta decay or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium.^[10]

Chemistry

Praseodymium metal tarnishes slowly in air, forming a spalling oxide layer like iron rust; a centimetre-sized sample of praseodymium metal corrodes completely in about a year.[12] It burns readily at 150 °C to form praseodymium(III,IV) oxide, a nonstoichiometric compound approximating to Pr₆O₁₁:[13]

12 Pr + 11 O₂ → 2 Pr₆O₁₁

This may be reduced to praseodymium(III) oxide (Pr₂O₃) with hydrogen gas.^[14] The dark-coloured praseodymium(IV) oxide, PrO₂, is the most oxidised product of the combustion of praseodymium and is only obtained by reaction of praseodymium metal with pure oxygen at 400 °C and 282 bar.^[14] The reactivity of praseodymium conforms to periodic trends, as it is one of the first and thus one of the largest lanthanides.^[7] At 1000 °C, many praseodymium oxides with composition PrO_2−x exist as disordered, nonstoichiometric phases with 0 < x < 0.25, but at 400–700 °C the oxide defects are instead ordered, creating phases of the general formula Pr_nO_2n−2 with n = 4, 7, 9, 10, 11, 12, and ∞. These phases PrO_y are sometimes labelled α and β′ (nonstoichiometric), β (y = 1.833), δ (1.818), ε (1.8), ζ (1.778), ι (1.714), θ, and σ.^[15]

Praseodymium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form praseodymium(III) hydroxide:[13]

2 Pr (s) + 6 H₂O (l) → 2 Pr(OH)₃ (aq) + 3 H₂ (g)

Praseodymium metal reacts with all the halogens to form trihalides:[13]

2 Pr (s) + 3 F₂ (g) → 2 PrF₃ (s) [green]

2 Pr (s) + 3 Cl₂ (g) → 2 PrCl₃ (s) [green]

2 Pr (s) + 3 Br₂ (g) → 2 PrBr₃ (s) [green]

2 Pr (s) + 3 I₂ (g) → 2 PrI₃ (s)

The tetrafluoride, PrF₄, is also known, and is produced by reacting a mixture of sodium fluoride and praseodymium(III) fluoride with fluorine gas, producing Na₂PrF₆, following which sodium fluoride is removed from the reaction mixture with liquid hydrogen fluoride.^[16] Additionally, praseodymium forms a bronze diiodide; like the diiodides of lanthanum, cerium, and gadolinium, it is a praseodymium(III) electride compound.^[16]

Praseodymium dissolves readily in dilute sulfuric acid to form solutions containing the chartreuse Pr³⁺ ions, which exist as [Pr(H₂O)₉]³⁺ complexes:^[13][17]

2 Pr (s) + 3 H₂SO₄ (aq) → 2 Pr³⁺ (aq) + 3 SO2−
4 (aq) + 3 H₂ (g)

Dissolving praseodymium(IV) compounds in water results in solutions containing the yellow Pr⁴⁺ ions;^[18] because of the high positive standard reduction potential of the Pr⁴⁺/Pr³⁺ couple at +3.2 V, these ions are unstable in aqueous solution, oxidising water and being reduced to Pr³⁺. The value for the Pr³⁺/Pr couple is −2.35 V.^[6]

Although praseodymium(V) in the bulk state is unknown, the existence of praseodymium in its +5 oxidation state (with the stable electron configuration of the preceding noble gas xenon) under noble-gas matrix isolation conditions was reported in 2016. The species assigned to the +5 state were identified as [PrO₂]⁺, its O₂ and Ar adducts, and PrO₂(η²-O₂).^[19]

Organopraseodymium compounds are very similar to those of the other lanthanides, as they all share an inability to undergo π backbonding. They are thus mostly restricted to the mostly ionic cyclopentadienides (isostructural with those of lanthanum) and the σ-bonded simple alkyls and aryls, some of which may be polymeric.[20] The coordination chemistry of praseodymium is largely that of the large, electropositive Pr3+ ion, and is thus largely similar to those of the other early lanthanides La3+, Ce3+, and Nd3+. For instance, like lanthanum, cerium, and neodymium, praseodymium nitrates form both 4:3 and 1:1 complexes with 18-crown-6, whereas the middle lanthanides from promethium to gadolinium can only form the 4:3 complex and the later lanthanides from terbium to lutetium cannot successfully coordinate to all the ligands. Such praseodymium complexes have high but uncertain coordination numbers and poorly defined stereochemistry, with exceptions resulting from exceptionally bulky ligands such as the tricoordinate [Pr{N(SiMe3)2}3]. There are also a few mixed oxides and fluorides involving praseodymium(IV), but it does not have an appreciable coordination chemistry in this oxidation state like its neighbour cerium.[21]

History
Carl Auer von Welsbach (1858–1929), discoverer of praseodymium in 1885.

In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite. Thirty years later, the fifteen-year-old Vilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide, which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier.[22] Ceria was simultaneously and independently isolated in Germany by Martin Heinrich Klaproth.[23] Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius; he separated out two other oxides, which he named lanthana and didymia.[24] He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid. The metals that formed these oxides were thus named lanthanum and didymium.[25] While lanthanum turned out to be a pure element, didymium was not and turned out to be only a mixture of all the stable early lanthanides from praseodymium to europium, as had been suspected by Marc Delafontaine after spectroscopic analysis, though he lacked the time to pursue its separation into its constituents. The heavy pair of samarium and europium were only removed in 1879 by Paul-Émile Lecoq de Boisbaudran and it was not until 1885 that Carl Auer von Welsbach separated didymium into praseodymium and neodymium.[26] Since neodymium was a larger constituent of didymium than praseodymium, it kept the old name with disambiguation, while praseodymium was distinguished by the leek-green colour of its salts (Greek πρασιος, "leek green").[27]

Occurrence and production

Praseodymium is not particularly rare, making up 9.1 mg/kg of the Earth's crust. This value is between those of lead (13 mg/kg) and boron (9 mg/kg), and makes praseodymium the fourth-most abundant of the lanthanides, behind cerium (66 mg/kg), neodymium (40 mg/kg), and lanthanum (35 mg/kg); it is less abundant than the rare-earth elements yttrium (31 mg/kg) and scandium (25 mg/kg).[27] Instead, praseodymium's classification as a rare-earth metal comes from its rarity relative to "common earths" such as lime and magnesia, the few known minerals containing it for which extraction is commercially viable, as well as the length and complexity of extraction.[28]

The Pr³⁺ ion is similar in size to the early lanthanides of the cerium group (those from lanthanum up to samarium and europium) that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite (M^IIIPO₄) and bastnäsite (M^IIICO₃F), where M refers to all the rare-earth metals except scandium and the radioactive promethium (mostly Ce, La, and Y, with somewhat less Nd and Pr).^[27] Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.^[27]

The procedure for monazite, which usually contains all the rare earths, as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralised with sodium hydroxide to pH 3–4, during which thorium precipitates as a hydroxide and is removed. The solution is treated with ammonium oxalate to convert rare earths to their insoluble oxalates, the oxalates are converted to oxides by annealing, and the oxides are dissolved in nitric acid. This last step excludes one of the main components, cerium, whose oxide is insoluble in HNO₃.^[28] Care must be taken when handling some of the residues as they contain ²²⁸Ra, the daughter of ²³²Th, which is a strong gamma emitter.^[27]

Praseodymium may then be separated from the other lanthanides via ion-exchange chromatography, or by using a solvent such as tributyl phosphate where the solubility of Ln³⁺ increases as the atomic number increases. If ion-exchange chromatography is used, the mixture of lanthanides is loaded into one column of cation-exchange resin and Cu²⁺ or Zn²⁺ or Fe³⁺ is loaded into the other. An aqueous solution of a complexing agent, known as the eluant (usually triammonium edtate), is passed through the columns, and Ln³⁺ is displaced from the first column and redeposited in a compact band at the top of the column before being re-displaced by NH+
4. The Gibbs free energy of formation for Ln(edta·H) complexes increases along the lanthanides by about one quarter from Ce³⁺ to Lu³⁺, so that the Ln³⁺ cations descend the development column in a band and are fractionated repeatedly, eluting from heaviest to lightest. They are then precipitated as their insoluble oxalates, burned to form the oxides, and then reduced to the metals.^[27]

Applications

Leo Moser (son of Ludwig Moser, founder of the Moser Glassworks in what is now Karlovy Vary in the Czech Republic, not to be confused with the mathematician of the same name) investigated the use of praseodymium in glass colouration in the late 1920s, yielding a yellow-green glass given the name "Prasemit". However, at that time far cheaper colourants could give a similar colour, so Prasemit was not popular, few pieces were made, and examples are now extremely rare. Moser also blended praseodymium with neodymium to produce "Heliolite" glass ("Heliolit" in German), which was more widely accepted. The first enduring commercial use of purified praseodymium, which continues today, is in the form of a yellow-orange "Praseodymium Yellow" stain for ceramics, which is a solid solution in the zircon lattice. This stain has no hint of green in it; by contrast, at sufficiently high loadings, praseodymium glass is distinctly green rather than pure yellow.[29]

As the lanthanides are so similar, praseodymium can substitute for most other lanthanides without significant loss of function, and indeed many applications such as mischmetal and ferrocerium alloys involve variable mixes of several lanthanides, including small quantities of praseodymium. The following more modern applications involve praseodymium specifically, or at least praseodymium in a small subset of the lanthanides:[30]

• In combination with neodymium, another rare-earth element, praseodymium is used to create high-power magnets notable for their strength and durability.^[31]
• As an alloying agent with magnesium to create high-strength metals that are used in aircraft engines; yttrium and neodymium are also viable substitutes.^[32][33]
• Praseodymium is present in the rare-earth mixture whose fluoride forms the core of carbon arc lights, which are used in the motion picture industry for studio lighting and projector lights.^[34]
• Praseodymium compounds give glasses and enamels a yellow color.^[5]
• Praseodymium is used to color ceramics yellow.^[30]
• Praseodymium is a component of didymium glass, which is used to make certain types of welder's and glass blower's goggles.^[5]
• Silicate crystals doped with praseodymium ions have been used to slow a light pulse down to a few hundred meters per second.^[35]
• Praseodymium alloyed with nickel (PrNi₅) has such a strong magnetocaloric effect that it has allowed scientists to approach within one thousandth of a degree of absolute zero.^[34] In general, most alloys of the cerium-group rare earths (lanthanum through samarium) with 3d transition metals give extremely stable magnets that are often used in small equipment, such as motors, printers, watches, headphones, loudspeakers, and magnetic storage.^[30]
• Doping praseodymium in fluoride glass allows it to be used as a single-mode fiber optical amplifier.^[36]
• Praseodymium oxide in solid solution with ceria or ceria-zirconia has been used as an oxidation catalyst.^[37]
• Pr³⁺ ions are used as activators in some red, green, blue, and ultraviolet phosphors.^[30]

Biological role and precautions

The early lanthanides act as essential cofactors for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV, for which lanthanum, cerium, praseodymium, and neodymium alone are about equally effective.[38] Praseodymium is otherwise not known to have a biological role in any other organisms, but is not very toxic either. Intravenous injection of rare earths into animals has been known to impair liver function, but the main side effects from inhalation of rare-earth oxides in humans come from radioactive thorium and uranium impurities.[30]

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(Berzelius) (1839) "Nouveau métal" (New metal), Comptes rendus, 8 : 356–357. From p. 356: "L'oxide de cérium, extrait de la cérite par la procédé ordinaire, contient à peu près les deux cinquièmes de son poids de l'oxide du nouveau métal qui ne change que peu les propriétés du cérium, et qui s'y tient pour ainsi dire caché. Cette raison a engagé M. Mosander à donner au nouveau métal le nom de Lantane." (The oxide of cerium, extracted from cerite by the usual procedure, contains almost two fifths of its weight in the oxide of the new metal, which differs only slightly from the properties of cerium, and which is held in it so to speak "hidden". This reason motivated Mr. Mosander to give to the new metal the name Lantane.)
(Berzelius) (1839) "Latanium — a new metal," Philosophical Magazine, new series, 14 : 390–391.

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Pol, Arjan; Barends, Thomas R. M.; Dietl, Andreas; Khadem, Ahmad F.; Eygensteyn, Jelle; Jetten, Mike S. M.; Op Den Camp, Huub J. M. (2013). "Rare earth metals are essential for methanotrophic life in volcanic mudpots". Environmental Microbiology. 16 (1): 255–64. PMID 24034209. doi:10.1111/1462-2920.12249.

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Further reading

R. J. Callow, The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium, Pergamon Press, 1967.