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= Praseodymium =
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Introduction
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Praseodymium is a chemical element; it has symbol Pr and atomic number
59. It is the third member of the lanthanide series and is considered
one of the rare-earth metals. It is a soft, silvery, malleable and
ductile metal, valued for its magnetic, electrical, chemical, and
optical properties. It is too reactive to be found in native form, and
pure praseodymium metal slowly develops a green oxide coating when
exposed to air.
Praseodymium always occurs naturally together with the other
rare-earth metals. It is the sixth-most abundant rare-earth element
and fourth-most abundant lanthanide, making up 9.1 parts per million
of the Earth's crust, an abundance similar to that of boron. In 1841,
Swedish chemist Carl Gustav Mosander extracted a rare-earth oxide
residue he called didymium from a residue he called "lanthana", in
turn separated from cerium salts. In 1885, the Austrian chemist Carl
Auer von Welsbach separated didymium into two elements that gave salts
of different colours, which he named praseodymium and neodymium. The
name praseodymium comes from the Ancient Greek (), meaning
'leek-green', and () 'twin'.
Like most rare-earth elements, praseodymium most readily forms the +3
oxidation state, which is the only stable state in aqueous solution,
although the +4 oxidation state is known in some solid compounds and,
uniquely among the lanthanides, the +5 oxidation state is attainable
at low temperatures. The 0, +1, and +2 oxidation states are rarely
found. Aqueous praseodymium ions are yellowish-green, and similarly,
praseodymium results in various shades of yellow-green when
incorporated into glasses. Many of praseodymium's industrial uses
involve its ability to filter yellow light from light sources.
Physical properties
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Praseodymium is the third member of the lanthanide series, and a
member of the rare-earth metals. 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. Praseodymium is calculated to
have a very large atomic radius; with a radius of 247 pm, barium,
rubidium and caesium are larger. However, observationally, it is
usually 185 pm.
Neutral praseodymium's 59 electrons are arranged in the configuration
[Xe]4f36s2.
Like most other lanthanides, praseodymium usually uses only three
electrons as valence electrons, as the remaining 4f electrons are too
strongly bound to engage in bonding: 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 penetration increases with
higher ionic charge. Even so, praseodymium can in some compounds lose
a fourth valence electron because it is 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.
Similarly to the other early lanthanides, praseodymium has a double
hexagonal close-packed crystal structure at room temperature, called
the alpha phase (α-Pr). At 795 C it transforms to a different
allotrope that has a body-centered cubic structure (β-Pr), and it
melts at 931 C.
Praseodymium, like all of the lanthanides, is paramagnetic at room
temperature. Unlike some other rare-earth metals, which show
antiferromagnetic or ferromagnetic ordering at low temperatures,
praseodymium is paramagnetic at all temperatures above 1 K.
Chemical properties
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Praseodymium metal tarnishes slowly in air, forming a spalling green
oxide layer like iron rust; a centimetre-sized sample of praseodymium
metal corrodes completely in about a year. It burns readily at 150 °C
to form praseodymium(III,IV) oxide, a nonstoichiometric compound
approximating to :
:
This may be reduced to praseodymium(III) oxide with hydrogen gas.
Praseodymium(IV) oxide, , is the most oxidised product of the
combustion of praseodymium and can be obtained by either reaction of
praseodymium metal with pure oxygen at 400 °C and 282 bar or by
disproportionation of in boiling acetic acid. The reactivity of
praseodymium conforms to periodic trends, as it is one of the first
and thus one of the largest lanthanides. At 1000 °C, many praseodymium
oxides with composition PrO2−'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 {{chem2|Pr_{'n'}O_{2'n'−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.
Praseodymium is an electropositive element and reacts slowly with cold
water and quite quickly with hot water to form praseodymium(III)
hydroxide:
:
Praseodymium metal reacts with all the stable halogens to form
trihalides:
: [green]
: [green]
: [green]
:
The tetrafluoride, PrF4, is also known, and is produced by reacting a
mixture of sodium fluoride and praseodymium(III) fluoride with
fluorine gas, producing , following which sodium fluoride is removed
from the reaction mixture with liquid hydrogen fluoride. Additionally,
praseodymium forms a bronze diiodide; like the diiodides of lanthanum,
cerium, and gadolinium, it is a praseodymium(III) electride compound.
Praseodymium dissolves readily in dilute sulfuric acid to form
solutions containing the chartreuse ions, which exist as complexes:
:
Dissolving praseodymium(IV) compounds in water does not result in
solutions containing the yellow ions; because of the high positive
standard reduction potential of the/ couple at +3.2 V, these ions are
unstable in aqueous solution, oxidising water and being reduced to .
The value for the /Pr couple is −2.35 V. However, in highly basic
aqueous media, ions can be generated by oxidation with ozone.
Praseodymium(V) has been observed by matrix isolation (in 2016) and in
the bulk state (in 2025). 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 , its and Ar adducts, and {{chem2|PrO2(\h{2}O2)}}.
Further, in 2025, a neutral compound
{{chem2|[Pr(NP^{'t'}Bu3)4]+[PF6]-}}, formally Pr(V) but with an
inverted ligand field, was isolated and characterized
crystallographically at low temperatures.
Organopraseodymium compounds
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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. The coordination chemistry
of praseodymium is largely that of the large, electropositive ion,
and is thus largely similar to those of the other early lanthanides ,
, and . 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
{{chem2|[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. However, the first example of a molecular complex of
praseodymium(IV) has recently been reported.
Isotopes
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Praseodymium has only one stable and naturally occurring isotope,
141Pr. It is thus a mononuclidic and monoisotopic element, and its
standard atomic weight can be determined with high precision as it is
a constant of nature. This isotope has 82 neutrons, which is a magic
number that confers additional stability. This isotope is produced in
stars through the s- and r-processes (slow and rapid neutron capture,
respectively). Thirty-eight other radioisotopes have been synthesized.
All of these 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 positron emission
or electron capture to isotopes of cerium, while that of heavier
isotopes is beta decay to isotopes of neodymium.
History
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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 Wilhelm 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. Ceria was
simultaneously and independently isolated in Germany by Martin
Heinrich Klaproth. 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'. 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'.
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. Von Welsbach
confirmed the separation by spectroscopic analysis, but the products
were of relatively low purity. 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"). The composite
nature of didymium had previously been suggested in 1882 by Bohuslav
Brauner, who did not experimentally pursue its separation.
Occurrence and production
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Praseodymium is not particularly rare, despite it being in the
rare-earth metals, making up 9.2 mg/kg of the Earth's crust.
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. Although
not particularly rare, praseodymium is never found as a dominant rare
earth in praseodymium-bearing minerals. It is always preceded by
cerium and lanthanum and usually also by neodymium.
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 (MIIIPO4) and bastnäsite (MIIICO3F), 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). 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.
The procedure for monazite, which usually contains all the rare earth,
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 earth. The
acidic filtrates are partially neutralized with sodium hydroxide to pH
3-4, during which thorium precipitates as hydroxide and is removed.
The solution is treated with ammonium oxalate to convert rare earth 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. Care must be taken when handling some of the residues as they
contain 228Ra, the daughter of 232Th, which is a strong gamma emitter.
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 . The Gibbs free energy of formation for
Ln(edta·H) complexes increases along with 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
metals.
Applications
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Leo Moser (not to be confused with the mathematician of the same
name), son of Ludwig Moser, founder of the Moser Glassworks in what is
now Karlovy Vary in the Czech Republic, investigated the use of
praseodymium in glass coloration in the late 1920s, yielding a
yellow-green glass given the name "Prasemit". However, at that time
far cheaper colorants could give a similar color, 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.
Like many other lanthanides, praseodymium's shielded f-orbitals allow
for long excited state lifetimes and high luminescence yields. Pr3+ as
a dopant ion therefore sees many applications in optics and photonics.
These include DPSS-lasers, single-mode fiber optical amplifiers, fiber
lasers, upconverting nanoparticles as well as activators in red,
green, blue, and ultraviolet phosphors. Silicate crystals doped with
praseodymium ions have also been used to slow a light pulse down to a
few hundred meters per second.
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:
* In combination with neodymium, another rare-earth element,
praseodymium is used to create high-power magnets notable for their
strength and durability. 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.
*Praseodymium-nickel intermetallic (PrNi5) has such a strong
magnetocaloric effect that it has allowed scientists to approach
within one thousandth of a degree of absolute zero.
* As an alloying agent with magnesium to create high-strength metals
that are used in aircraft engines; yttrium and neodymium are suitable
substitutes.
* 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.
* Praseodymium compounds give glasses, enamels and ceramics a yellow
color.
* Praseodymium is a component of didymium glass, which is used to make
certain types of welder's and glass blower's goggles.
* Praseodymium oxide in solid solution with ceria or ceria-zirconia
has been used as an oxidation catalyst.
Due to its role in permanent magnets used for wind turbines, it has
been argued that praseodymium will be one of the main objects of
geopolitical competition in a world running on renewable energy.
However, this perspective has been criticized for failing to recognize
that most wind turbines do not use permanent magnets and for
underestimating the power of economic incentives for expanded
production.
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Biological role and precautions
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The early lanthanides have been found to be essential to some
methanotrophic bacteria living in volcanic mudpots, such as
'Methylacidiphilum fumariolicum': lanthanum, cerium, praseodymium, and
neodymium are about equally effective. Praseodymium is otherwise not
known to have a biological role in any other organisms, but it 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.
Further reading
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* R. J. Callow, 'The Industrial Chemistry of the Lanthanons, Yttrium,
Thorium, and Uranium', Pergamon Press, 1967.
*
External links
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* [
http://www.webelements.com/webelements/elements/text/Pr/index.html
WebElements.com--Praseodymium]
* [
http://education.jlab.org/itselemental/ele059.html It's
Elemental--The Element Praseodymium]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Praseodymium
