The lanthanide () or lanthanoid () series
of chemical elements comprises the 15 metallic
chemical elements with atomic numbers 57 through
71, from lanthanum through lutetium. These
elements, along with the chemically similar
elements scandium and yttrium, are often collectively
known as the rare earth elements.
The informal chemical symbol Ln is used in
general discussions of lanthanide chemistry
to refer to any lanthanide. All but one of
the lanthanides are f-block elements, corresponding
to the filling of the 4f electron shell; depending
on the source, either lanthanum or lutetium
is considered a d-block element, but is included
due to its chemical similarities with the
other 14. All lanthanide elements form trivalent
cations, Ln3+, whose chemistry is largely
determined by the ionic radius, which decreases
steadily from lanthanum to lutetium.
They are called lanthanides because the elements
in the series are chemically similar to lanthanum.
Both lanthanum and lutetium have been labeled
as group 3 elements, because they have a single
valence electron in the 5d shell. However,
both elements are often included in discussions
of the chemistry of lanthanide elements. Lanthanum
is the more often omitted of the two, because
its placement as a group 3 element is somewhat
more common in texts and for semantic reasons:
since "lanthanide" means "like lanthanum",
it has been argued that lanthanum cannot logically
be a lanthanide, but IUPAC acknowledges its
inclusion based on common usage.In presentations
of the periodic table, the lanthanides and
the actinides are customarily shown as two
additional rows below the main body of the
table, with placeholders or else a selected
single element of each series (either lanthanum
and actinium, or lutetium and lawrencium)
shown in a single cell of the main table,
between barium and hafnium, and radium and
rutherfordium, respectively. This convention
is entirely a matter of aesthetics and formatting
practicality; a rarely used wide-formatted
periodic table inserts the lanthanide and
actinide series in their proper places, as
parts of the table's sixth and seventh rows
(periods).
== Etymology ==
Together with the two elements at the top
of group 3, scandium and yttrium, the trivial
name "rare earths" is sometimes used to describe
all the lanthanides; a definition of rare
earths including the group 3, lanthanide,
and actinide elements is also occasionally
seen, and rarely Sc + Y + lanthanides + thorium.
The "earth" in the name "rare earths" arises
from the minerals from which they were isolated,
which were uncommon oxide-type minerals. However,
the use of the name is deprecated by IUPAC,
as the elements are neither rare in abundance
nor "earths" (an obsolete term for water-insoluble
strongly basic oxides of electropositive metals
incapable of being smelted into metal using
late 18th century technology). Group 2 is
known as the alkaline earth elements for much
the same reason.
The "rare" in the "rare earths" name has much
more to do with the difficulty of separating
out each of the individual lanthanide elements
than scarcity of any of them. By way of the
Greek "dysprositos" for "hard to get at,"
element 66, dysprosium was similarly named;
lanthanum itself is named after a word for
"hidden." The elements 57 (La) to 71 (Lu)
are very similar chemically to one another
and frequently occur together in nature, often
anywhere from three to all 15 of the lanthanides
(along with yttrium as a 16th) occur in minerals
such as samarskite, monazite and many others
which can also contain the other two group
3 elements as well as thorium and occasionally
other actinides as well. A majority of the
rare earths were discovered at the same mine
in Ytterby, Sweden and four of them are named
(yttrium, ytterbium, erbium, terbium) after
the city and a fifth *(holmium) after Stockholm;
scandium is named after Scandinavia, thulium
after the old name Thule, and the immediately-following
group 4 element (number 72) hafnium is named
for the Latin name of the city of Copenhagen.Samarskite
(a mineral which is the source of the name
of the element samarium) and other similar
minerals in particular also have these elements
in association with the nearby metals tantalum,
niobium, hafnium, zirconium, vanadium, and
titanium, from group 4 and group 5 often in
similar oxidation states. Monazite is a phosphate
of numerous group 3 + lanthanide + actinide
metals and mined especially for the thorium
content and specific rare earths especially
lanthanum, yttrium and cerium. Cerium and
lanthanum as well as other members of the
rare earth series are often produced as a
metal called mischmetal containing a variable
mixture of the these elements with cerium
and lanthanum predominating; it has direct
uses such as lighter flints and other spark
sources which do not require extensive purification
of one of these metals. There are also rare
earth-bearing minerals based on group 2 elements
such as yttrocalcite, yttrocerite, yttrofluorite
which vary in content of yttrium, cerium,
and lanthanum in a particular as well as varying
amounts of the others. Other lanthanide/rare
earth minerals include bastnäsite, florencite,
chernovite, perovskite, xenotime, cerite,
gadolinite, lanthanite, fergusonite, polycrase,
blomstrandine, håleniusite, miserite, loparite,
lepersonnite, euxenite, all of which have
a range of relative element concentration
and may have the symbol of a predominating
one such as monazite-ce; group 3 elements
do not occur as native element minerals in
the fashion of gold, silver, tantalum and
many others on earth but may in lunar regolith.
Very rare cerium, lanthanum, and presumably
other lanthanide/group 3 halides, feldspars
and garnets are also known to exist.All of
the this is the result of the order in which
the electron shells of these elements are
filled -- the outermost has the same configuration
for all of them, and a deeper shell is progressively
filled with electrons as the atomic number
increases from 57 towards 71. For many years,
mixtures of more than one rare earth were
considered to be single elements, such as
neodymium and praseodymium being thought to
be the single element didymium and so on.
Very small differences in solubility are used
in solvent and ion-exchange purification methods
for these elements which require a great deal
of repeating to get a purified metal. The
refined metals and their compounds have subtle
and stark differences amongst themselves in
electronic, electrical, optical, and magnetic
properties which account for their many niche
uses.By way of examples of the term meaning
the above considerations rather than their
scarcity, cerium is the 26th most abundant
element in the Earth's crust and more abundant
than copper, neodymium is more abundant than
gold; thulium (the second least common naturally
occurring lanthanide) is more abundant than
iodine, which is itself common enough for
biology to have evolved critical usages thereof,
and even the lone radioactive element in the
series, promethium, is more common than the
two rarest naturally occurring elements, francium
and astatine, combined. Despite their abundance,
even the technical term "lanthanides" could
be interpreted to reflect a sense of elusiveness
on the part of these elements, as it comes
from the Greek λανθανειν (lanthanein),
"to lie hidden". However, if not referring
to their natural abundance, but rather to
their property of "hiding" behind each other
in minerals, this interpretation is in fact
appropriate. The etymology of the term must
be sought in the first discovery of lanthanum,
at that time a so-called new rare earth element
"lying hidden" in a cerium mineral, and it
is an irony that lanthanum was later identified
as the first in an entire series of chemically
similar elements and could give name to the
whole series. The term "lanthanide" was introduced
by Victor Goldschmidt in 1925.
== Physical properties of the elements ==
* Between initial Xe and final 6s2 electronic
shells
** Sm has a close packed structure like the
other lanthanides but has an unusual 9 layer
repeat
Gschneider and Daane (1988) attribute the
trend in melting point which increases across
the series, (lanthanum (920 °C) – lutetium
(1622 °C)) to the extent of hybridization
of the 6s, 5d, and 4f orbitals. The hybridization
is believed to be at its greatest for cerium,
which has the lowest melting point of all,
795 °C.
The lanthanide metals are soft; their hardness
increases across the series. Europium stands
out, as it has the lowest density in the series
at 5.24 g/cm3 and the largest metallic radius
in the series at 208.4 pm. It can be compared
to barium, which has a metallic radius of
222 pm. It is believed that the metal contains
the larger Eu2+ ion and that there are only
two electrons in the conduction band. Ytterbium
also has a large metallic radius, and a similar
explanation is suggested.
The resistivities of the lanthanide metals
are relatively high, ranging from 29 to 134
μΩ·cm. These values can be compared to
a good conductor such as aluminium, which
has a resistivity of 2.655 μΩ·cm.
With the exceptions of La, Yb, and Lu (which
have no unpaired f electrons), the lanthanides
are strongly paramagnetic, and this is reflected
in their magnetic susceptibilities. Gadolinium
becomes ferromagnetic at below 16 °C (Curie
point). The other heavier lanthanides – terbium,
dysprosium, holmium, erbium, thulium, and
ytterbium – become ferromagnetic at much
lower temperatures.
== Chemistry and compounds ==
* Not including initial [Xe] core
The colors of lanthanide complexes originate
almost entirely from charge transfer interactions
between the metal and the ligand. f → f
transitions are symmetry forbidden (or Laporte-forbidden),
which is also true of transition metals. However,
transition metals are able to use vibronic
coupling to break this rule. The valence orbitals
in lanthanides are almost entirely non-bonding
and as such little effective vibronic coupling
takes, hence the spectra from f → f transitions
are much weaker and narrower than those from
d → d transitions. In general this makes
the colors of lanthanide complexes far fainter
than those of transition metal complexes.
f→f transitions are not possible for the
f1 and f13 configurations of Ce3+ and Yb3+
and thus these ions are colorless in aqueous
solution.
=== Effect of 4f orbitals ===
Going across the lanthanides in the periodic
table, the 4f orbitals are usually being filled.
The effect of the 4f orbitals on the chemistry
of the lanthanides is profound and is the
factor that distinguishes them from the transition
metals. There are seven 4f orbitals, and there
are two different ways in which they are depicted:
as a "cubic set" or as a general set. The
cubic set 
is fz3, fxz2, fyz2, fxyz, fz(x2−y2), fx(x2−3y2)
and fy(3x2−y2). The 4f orbitals penetrate
the [Xe] core and are isolated, and thus they
do not participate in bonding. This explains
why crystal field effects are small and why
they do not form π bonds. As there are seven
4f orbitals, the number of unpaired electrons
can be as high as 7, which gives rise to the
large magnetic moments observed for lanthanide
compounds. Measuring the magnetic moment can
be used to investigate the 4f electron configuration,
and this is a useful tool in providing an
insight into the chemical bonding. The lanthanide
contraction, i.e. the reduction in size of
the Ln3+ ion from La3+ (103 pm) to Lu3+ (86.1
pm), is often explained by the poor shielding
of the 5s and 5p electrons by the 4f electrons.
The electronic structure of the lanthanide
elements, with minor exceptions, is [Xe]6s24fn.
The chemistry of the lanthanides is dominated
by the +3 oxidation state, and in LnIII compounds
the 6s electrons and (usually) one 4f electron
are lost and the ions have the configuration
[Xe]4fm. All the lanthanide elements exhibit
the oxidation state +3. In addition, Ce3+
can lose its single f electron to form Ce4+
with the stable electronic configuration of
xenon. Also, Eu3+ can gain an electron to
form Eu2+ with the f7 configuration that has
the extra stability of a half-filled shell.
Other than Ce(IV) and Eu(II), none of the
lanthanides are stable in oxidation states
other than +3 in aqueous solution. Promethium
is effectively a man-made element, as all
its isotopes are radioactive with half-lives
shorter than 20 years.
In terms of reduction potentials, the Ln0/3+
couples are nearly the same for all lanthanides,
ranging from −1.99 (for Eu) to −2.35 V
(for Pr). Thus these metals are highly reducing,
with reducing power similar to alkaline earth
metals such as Mg (−2.36 V).
=== Lanthanide oxidation states ===
All of the lanthanide elements are commonly
known to have the +3 oxidation state and it
was thought that only samarium, europium,
and ytterbium had the +2 oxidation readily
accessible in solution. Now, it is known that
all of the lanthanides can form +2 complexes
in solution.The ionization energies for the
lanthanides can be compared with aluminium.
In aluminium the sum of the first three ionization
energies is 5139 kJ·mol−1, whereas the
lanthanides fall in the range 3455 – 4186
kJ·mol−1. This correlates with the highly
reactive nature of the lanthanides.
The sum of the first two ionization energies
for europium, 1632 kJ·mol−1 can be compared
with that of barium 1468.1 kJ·mol−1 and
europium's third ionization energy is the
highest of the lanthanides. The sum of the
first two ionization energies for ytterbium
are the second lowest in the series and its
third ionization energy is the second highest.
The high third ionization energy for Eu and
Yb correlate with the half filling 4f7 and
complete filling 4f14 of the 4f subshell,
and the stability afforded by such configurations
due to exchange energy. Europium and ytterbium
form salt like compounds with Eu2+ and Yb2+,
for example the salt like dihydrides. Both
europium and ytterbium dissolve in liquid
ammonia forming solutions of Ln2+(NH3)x again
demonstrating their similarities to the alkaline
earth metals.The relative ease with which
the 4th electron can be removed in cerium
and (to a lesser extent praseodymium) indicates
why Ce(IV) and Pr(IV) compounds can be formed,
for example CeO2 is formed rather than Ce2O3
when cerium reacts with oxygen.
=== Separation of lanthanides ===
The similarity in ionic radius between adjacent
lanthanide elements makes it difficult to
separate them from each other in naturally
occurring ores and other mixtures. Historically,
the very laborious processes of cascading
and fractional crystallization were used.
Because the lanthanide ions have slightly
different radii, the lattice energy of their
salts and hydration energies of the ions will
be slightly different, leading to a small
difference in solubility. Salts of the formula
Ln(NO3)3·2NH4NO3·4H2O can be used. Industrially,
the elements are separated from each other
by solvent extraction. Typically an aqueous
solution of nitrates is extracted into kerosene
containing tri-n-butylphosphate. The strength
of the complexes formed increases as the ionic
radius decreases, so solubility in the organic
phase increases. Complete separation can be
achieved continuously by use of countercurrent
exchange methods. The elements can also be
separated by ion-exchange chromatography,
making use of the fact that the stability
constant for formation of EDTA complexes increases
for log K ≈ 15.5 for [La(EDTA)]− to log
K ≈ 19.8 for [Lu(EDTA)]−.
=== Coordination chemistry and catalysis ===
When in the form of coordination complexes,
lanthanides exist overwhelmingly in their
+3 oxidation state, although particularly
stable 4f configurations can also give +4
(Ce, Tb) or +2 (Eu, Yb) ions. All of these
forms are strongly electropositive and thus
lanthanide ions are hard Lewis acids. The
oxidation states are also very stable and
with the exception of SmI2 and cerium(IV)
salts lanthanides are not used for redox chemistry.
4f electrons have a high probability of being
found close to the nucleus and are thus strongly
affected as the nuclear charge increases across
the series; this results in a corresponding
decrease in ionic radii referred to as the
lanthanide contraction.
The low probability of the 4f electrons existing
at the outer region of the atom or ion permits
little effective overlap between the orbitals
of a lanthanide ion and any binding ligand.
Thus lanthanide complexes typically have little
or no covalent character and are not influenced
by orbital geometries. The lack of orbital
interaction also means that varying the metal
typically has little effect on the complex
(other than size), especially when compared
to transition metals. Complexes are held together
by weaker electrostatic forces which are omni-directional
and thus the ligands alone dictate the symmetry
and coordination of complexes. Steric factors
therefore dominate, with coordinative saturation
of the metal being balanced against inter-ligand
repulsion. This results in a diverse range
of coordination geometries, many of which
are irregular, and also manifests itself in
the highly fluxional nature of the complexes.
As there is no energetic reason to be locked
into a single geometry, rapid intramolecular
and intermolecular ligand exchange will take
place. This typically results in complexes
that rapidly fluctuate between all possible
configurations.
Many of these features make lanthanide complexes
effective catalysts. Hard Lewis acids are
able to polarise bonds upon coordination and
thus alter the electrophilicity of compounds,
with a classic example being the Luche reduction.
The large size of the ions coupled with their
labile ionic bonding allows even bulky coordinating
species to bind and dissociate rapidly, resulting
in very high turnover rates; thus excellent
yields can often be achieved with loadings
of only a few mol%. The lack of orbital interactions
combined with the lanthanide contraction means
that the lanthanides change in size across
the series but that their chemistry remains
much the same. This allows for easy tuning
of the steric environments and examples exist
where this has been used to improve the catalytic
activity of the complex and change the nuclearity
of metal clusters.Despite this, the use of
lanthanide coordination complexes as homogeneous
catalysts is largely restricted to the laboratory
and there are currently few examples them
being used on an industrial scale. It should
be noted however, that lanthanides exist in
many forms other that coordination complexes
and many of these are industrially useful.
In particular lanthanide metal oxides are
used as heterogeneous catalysts in various
industrial processes.
==== Ln(III) compounds ====
The trivalent lanthanides mostly form ionic
salts. The trivalent ions are hard acceptors
and form more stable complexes with oxygen-donor
ligands than with nitrogen-donor ligands.
The larger ions are 9-coordinate in aqueous
solution, [Ln(H2O)9]3+ but the smaller ions
are 8-coordinate, [Ln(H2O)8]3+. There is some
evidence that the later lanthanides have more
water molecules in the second coordination
sphere. Complexation with monodentate ligands
is generally weak because it is difficult
to displace water molecules from the first
coordination sphere. Stronger complexes are
formed with chelating ligands because of the
chelate effect, such as the tetra-anion derived
from 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic
acid (DOTA).
==== Ln(II) and Ln(IV) compounds ====
The most common divalent derivatives of the
lanthanides are for Eu(II), which achieves
a favorable f7 configuration. Divalent halide
derivatives are known for all of the lanthanides.
They are either conventional salts or are
Ln(III) "electride"-like salts. The simple
salts include YbI2, EuI2, and SmI2. The electride-like
salts, described as Ln3+, 2I−, e−, include
LaI2, CeI2 and GdI2. Many of the iodides form
soluble complexes with ethers, e.g. TmI2(dimethoxyethane)3.
Samarium(II) iodide is a useful reducing agent.
Ln(II) complexes can be synthesized by transmetalation
reactions. The normal range of oxidation states
can be expanded via the use of sterically
bulky cyclopentadienyl ligands, in this way
many lanthanides can be isolated as Ln(II)
compounds.Ce(IV) in ceric ammonium nitrate
is a useful oxidizing agent. Otherwise tetravalent
lanthanides are rare. The Ce(IV) is the exception
owing to the tendency to form an unfilled
f shell.
==== Hydrides ====
Lanthanide metals react exothermically with
hydrogen to form LnH2, dihydrides. With the
exception of Eu and Yb which resemble the
Ba and Ca hydrides (non conducting, transparent
salt like compounds) they form black pyrophoric,
conducting compounds where the metal sub-lattice
is face centred cubic and the H atoms occupy
tetrahedral sites. Further hydrogenation produces
a trihydride which is non-stoichiometric,
non-conducting, more salt like. The formation
of trihydride is associated with and increase
in 8–10% volume and this is linked to greater
localization of charge on the hydrogen atoms
which become more anionic (H− hydride anion)
in character.
==== Halides ====
The only tetrahalides known are the tetrafluorides
of cerium, praseodymium, terbium, neodymium
and dysprosium, the last two known only under
matrix isolation conditions.
All of the lanthanides form trihalides with
fluorine, chlorine, bromine and iodine. They
are all high melting and predominantly ionic
in nature. The fluorides are only slightly
soluble in water and are not sensitive to
air, and this contrasts with the other halides
which are air sensitive, readily soluble in
water and react at high temperature to form
oxohalides.
The trihalides were important as pure metal
can be prepared from them. In the gas phase
the trihalides are planar or approximately
planar, the lighter lanthanides have a lower
% of dimers, the heavier lanthanides a higher
proportion. The dimers have a similar structure
to Al2Cl6.Some of the dihalides are conducting
while the rest are insulators. The conducting
forms can be considered as LnIII electride
compounds where the electron is delocalised
into a conduction band, Ln3+ (X−)2(e−).
All of the diodides have relatively short
metal-metal separations. The CuTi2 structure
of the lanthanum, cerium and praseodymium
diodides along with HP-NdI2 contain 44 nets
of metal and iodine atoms with short metal-metal
bonds (393-386 La-Pr). these compounds should
be considered to be two-dimensional metals
(two-dimensional in the same way that graphite
is). The salt-like dihalides include those
of Eu, Dy, Tm, and Yb. The formation of a
relatively stable +2 oxidation state for Eu
and Yb is usually explained by the stability
(exchange energy) of half filled (f7) and
fully filled f14. GdI2 possesses the layered
MoS2 structure, is ferromagnetic and exhibits
colossal magnetoresistance
The sesquihalides Ln2X3 and the Ln7I12 compounds
listed in the table contain metal clusters,
discrete Ln6I12 clusters in Ln7I12 and condensed
clusters forming chains in the sesquihalides.
Scandium forms a similar cluster compound
with chlorine, Sc7Cl12 Unlike many transition
metal clusters these lanthanide clusters do
not have strong metal-metal interactions and
this is due to the low number of valence electrons
involved, but instead are stabilised by the
surrounding halogen atoms.LaI is the only
known monohalide. Prepared from the reaction
of LaI3 and La metal, it has a NiAs type structure
and can be formulated La3+ (I−)(e−)2.
==== Oxides and hydroxides ====
All of the lanthanides form sesquioxides,
Ln2O3. The lighter/larger lanthanides adopt
a hexagonal 7-coordinate structure while the
heavier/smaller ones adopt a cubic 6-coordinate
"C-M2O3" structure. All of the sesquioxides
are basic, and absorb water and carbon dioxide
from air to form carbonates, hydroxides and
hydroxycarbonates. They dissolve in acids
to form salts.Cerium forms a stoichiometric
dioxide, CeO2, where cerium has an oxidation
state of +4. CeO2 is basic and dissolves with
difficulty in acid to form Ce4+ solutions,
from which CeIV salts can be isolated, for
example the hydrated nitrate Ce(NO3)4.5H2O.
CeO2 is used as an oxidation catalyst in catalytic
converters. Praseodymium and terbium form
non-stoichiometric oxides containing LnIV,
although more extreme reaction conditions
can produce stoichiometric (or near stoichiometric)
PrO2 and TbO2.Europium and ytterbium form
salt-like monoxides, EuO and YbO, which have
a rock salt structure. EuO is ferromagnetic
at low temperatures, and is a semiconductor
with possible applications in spintronics.
A mixed EuII/EuIII oxide Eu3O4 can be produced
by reducing Eu2O3 in a stream of hydrogen.
Neodymium and samarium also form monoxides,
but these are shiny conducting solids, although
the existence of samarium monoxide is considered
dubious.All of the lanthanides form hydroxides,
Ln(OH)3. With the exception of lutetium hydroxide,
which has a cubic structure, they have the
hexagonal UCl3 structure. The hydroxides can
be precipitated from solutions of LnIII. They
can also be formed by the reaction of the
sesquioxide, Ln2O3, with water, but although
this reaction is thermodynamically favorable
it is kinetically slow for the heavier members
of the series. Fajans' rules indicate that
the smaller Ln3+ ions will be more polarizing
and their salts correspondingly less ionic.
The hydroxides of the heavier lanthanides
become less basic, for example Yb(OH)3 and
Lu(OH)3 are still basic hydroxides but will
dissolve in hot concentrated NaOH.
==== Chalcogenides (S, Se, Te) ====
All of the lanthanides form Ln2Q3 (Q= S, Se,
Te). The sesquisulfides can be produced by
reaction of the elements or (with the exception
of Eu2S3) sulfidizing the oxide (Ln2O3) with
H2S. The sesquisulfides, Ln2S3 generally lose
sulfur when heated and can form a range of
compositions between Ln2S3 and Ln3S4. The
sesquisulfides are insulators but some of
the Ln3S4 are metallic conductors (e.g. Ce3S4)
formulated (Ln3+)3 (S2−)4 (e−), while
others (e.g. Eu3S4 and Sm3S4) are semiconductors.
Structurally the sesquisulfides adopt structures
that vary according to the size of the Ln
metal. The lighter and larger lanthanides
favoring 7-coordinate metal atoms, the heaviest
and smallest lanthanides (Yb and Lu) favoring
6 coordination and the rest structures with
a mixture of 6 and 7 coordination. Polymorphism
is common amongst the sesquisulfides. The
colors of the sesquisulfides vary metal to
metal and depend on the polymorphic form.
The colors of the γ-sesquisulfides are La2S3,
white/yellow; Ce2S3, dark red; Pr2S3, green;
Nd2S3, light green; Gd2S3, sand; Tb2S3, light
yellow and Dy2S3, orange. The shade of γ-Ce2S3
can be varied by doping with Na or Ca with
hues ranging from dark red to yellow, and
Ce2S3 based pigments are used commercially
and are seen as low toxicity substitutes for
cadmium based pigments.All of the lanthanides
form monochalcogenides, LnQ, (Q= S, Se, Te).
The majority of the monochalcogenides are
conducting, indicating a formulation LnIIIQ2−(e-)
where the electron is in conduction bands.
The exceptions are SmQ, EuQ and YbQ which
are semiconductors or insulators but exhibit
a pressure induced transition to a conducting
state.
Compounds LnQ2 are known but these do not
contain LnIV but are LnIII compounds containing
polychalcogenide anions.Oxysulfides Ln2O2S
are well known, they all have the same structure
with 7-coordinate Ln atoms, and 3 sulfur and
4 oxygen atoms as near neighbours.
Doping these with other lanthanide elements
produces phosphors. As an example, gadolinium
oxysulfide, Gd2O2S doped with Tb3+ produces
visible photons when irradiated with high
energy X-rays and is used as a scintillator
in flat panel detectors.
When mischmetal, an alloy of lanthanide metals,
is added to molten steel to remove oxygen
and sulfur, stable oxysulfides are produced
that form an immiscible solid.
==== Pnictides (group 15) ====
All of the lanthanides form a mononitride,
LnN, with the rock salt structure. The mononitrides
have attracted interest because of their unusual
physical properties. SmN and EuN are reported
as being "half metals". NdN, GdN, TbN and
DyN are ferromagnetic, SmN is antiferromagnetic.
Applications in the field of spintronics are
being investigated.
CeN is unusual as it is a metallic conductor,
contrasting with the other nitrides also with
the other cerium pnictides. A simple description
is Ce4+N3− (e–) but the interatomic distances
are a better match for the trivalent state
rather than for the tetravalent state. A number
of different explanations have been offered.
The nitrides can be prepared by the reaction
of lanthanum metals with nitrogen. Some nitride
is produced along with the oxide, when lanthanum
metals are ignited in air. Alternative methods
of synthesis are a high temperature reaction
of lanthanide metals with ammonia or the decomposition
of lanthanide amides, Ln(NH2)3. Achieving
pure stoichiometric compounds, and crystals
with low defect density has proved difficult.
The lanthanide nitrides are sensitive to air
and hydrolyse producing ammonia.The other
pnictides phosphorus, arsenic, antimony and
bismuth also react with the lanthanide metals
to form monopnictides, LnQ. Additionally a
range of other compounds can be produced with
varying stoichiometries, such as LnP2, LnP5,
LnP7, Ln3As, Ln5As3 and LnAs2.
==== Carbides ====
Carbides of varying stoichiometries are known
for the lanthanides. Non-stoichiometry is
common. All of the lanthanides form LnC2 and
Ln2C3 which both contain C2 units. The dicarbides
with exception of EuC2, are metallic conductors
with the calcium carbide structure and can
be formulated as Ln3+C22−(e–). The C-C
bond length is longer than that in CaC2, which
contains the C22− anion, indicating that
the antibonding orbitals of the C22− anion
are involved in the conduction band. These
dicarbides hydrolyse to form hydrogen and
a mixture of hydrocarbons. EuC2 and to a lesser
extent YbC2 hydrolyse differently producing
a higher percentage of acetylene (ethyne).
The sesquicarbides, Ln2C3 can be formulated
as Ln4(C2)3. These compounds adopt the Pu2C3
structure which has been described as having
C22− anions in bisphenoid holes formed by
eight near Ln neighbours. The lengthening
of the C-C bond is less marked in the sesquicarbides
than in the dicarbides, with the exception
of Ce2C3.
Other carbon rich stoichiometries are known
for some lanthanides. Ln3C4 (Ho-Lu) containing
C, C2 and C3 units; Ln4C7 (Ho-Lu) contain
C atoms and C3 units and Ln4C5 (Gd-Ho) containing
C and C2 units.
Metal rich carbides contain interstitial C
atoms and no C2 or C3 units. These are Ln4C3
(Tb and Lu); Ln2C (Dy, Ho, Tm) and Ln3C (Sm-Lu).
==== Borides ====
All of the lanthanides form a number of borides.
The "higher" borides (LnBx where x > 12) are
insulators/semiconductors whereas the lower
borides are typically conducting. The lower
borides have stoichiometries of LnB2, LnB4,
LnB6 and LnB12. Applications in the field
of spintronics are being investigated. The
range of borides formed by the lanthanides
can be compared to those formed by the transition
metals. The boron rich borides are typical
of the lanthanides (and groups 1–3) whereas
for the transition metals tend to form metal
rich, "lower" borides. The lanthanide borides
are typically grouped together with the group
3 metals with which they share many similarities
of reactivity, stoichiometry and structure.
Collectively these are then termed the rare
earth borides.Many methods of producing lanthanide
borides have been used, amongst them are direct
reaction of the elements; the reduction of
Ln2O3 with boron; reduction of boron oxide,
B2O3, and Ln2O3 together with carbon; reduction
of metal oxide with boron carbide, B4C. Producing
high purity samples has proved to be difficult.
Single crystals of the higher borides have
been grown in a low melting metal (e.g. Sn,
Cu, Al).Diborides, LnB2, have been reported
for Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
All have the same, AlB2, structure containing
a graphitic layer of boron atoms. Low temperature
ferromagnetic transitions for Tb, Dy, Ho and
Er. TmB2 is ferromagnetic at 7.2 K.Tetraborides,
LnB4 have been reported for all of the lanthanides
except EuB4, all have the same UB4 structure.
The structure has a boron sub-lattice consists
of chains of octahedral B6 clusters linked
by boron atoms. The unit cell decreases in
size successively from LaB4 to LuB4. The tetraborides
of the lighter lanthanides melt with decomposition
to LnB6. Attempts to make EuB4 have failed.
The LnB4 are good conductors and typically
antiferromagnetic.Hexaborides, LnB6 have been
reported for all of the lanthanides. They
all have the CaB6 structure, containing B6
clusters. They are non-stoichiometric due
to cation defects. The hexaborides of the
lighter lanthanides (La – Sm) melt without
decomposition, EuB6 decomposes to boron and
metal and the heavier lanthanides decompose
to LnB4 with exception of YbB6 which decomposes
forming YbB12. The stability has in part been
correlated to differences in volatility between
the lanthanide metals. In EuB6 and YbB6 the
metals have an oxidation state of +2 whereas
in the rest of the lanthanide hexaborides
it is +3. This rationalises the differences
in conductivity, the extra electrons in the
LnIII hexaborides entering conduction bands.
EuB6 is a semiconductor and the rest are good
conductors. LaB6 and CeB6 are thermionic emitters,
used, for example, in scanning electron microscopes.Dodecaborides,
LnB12, are formed by the heavier smaller lanthanides,
but not by the lighter larger metals, La – Eu.
With the exception YbB12 (where Yb takes an
intermediate valence and is a Kondo insulator),
the dodecaborides are all metallic compounds.
They all have the UB12 structure containing
a 3 dimensional framework of cubooctahedral
B12 clusters.The higher boride LnB66 is known
for all lanthanide metals. The composition
is approximate as the compounds are non-stoichiometric.
They all have similar complex structure with
over 1600 atoms in the unit cell. The boron
cubic sub lattice contains super icosahedra
made up of a central B12 icosahedra surrounded
by 12 others, B12(B12)12. Other complex higher
borides LnB50 (Tb, Dy, Ho Er Tm Lu) and LnB25
are known (Gd, Tb, Dy, Ho, Er) and these contain
boron icosahedra in the boron framework.
==== Organometallic compounds ====
Lanthanide-carbon σ bonds are well known;
however as the 4f electrons have a low probability
of existing at the outer region of the atom
there is little effective orbital overlap,
resulting in bonds with significant ionic
character. As such organo-lanthanide compounds
exhibit carbanion-like behavior, unlike the
behavior in transition metal organometallic
compounds. Because of their large size, lanthanides
tend to form more stable organometallic derivatives
with bulky ligands to give compounds such
as Ln[CH(SiMe3)3]. Analogues of uranocene
are derived from dilithiocyclooctatetraene,
Li2C8H8. Organic lanthanide(II) compounds
are also known, such as Cp*2Eu.
== Physical 
properties ==
=== Magnetic and spectroscopic ===
All the trivalent lanthanide ions, except
lanthanum and lutetium, have unpaired f electrons.
However, the magnetic moments deviate considerably
from the spin-only values because of strong
spin-orbit coupling. The maximum number of
unpaired electrons is 7, in Gd3+, with a magnetic
moment of 7.94 B.M., but the largest magnetic
moments, at 10.4–10.7 B.M., are exhibited
by Dy3+ and Ho3+. However, in Gd3+ all the
electrons have parallel spin and this property
is important for the use of gadolinium complexes
as contrast reagent in MRI scans.
Crystal field splitting is rather small for
the lanthanide ions and is less important
than spin-orbit coupling in regard to energy
levels. Transitions of electrons between f
orbitals are forbidden by the Laporte rule.
Furthermore, because of the "buried" nature
of the f orbitals, coupling with molecular
vibrations is weak. Consequently, the spectra
of lanthanide ions are rather weak and the
absorption bands are similarly narrow. Glass
containing holmium oxide and holmium oxide
solutions (usually in perchloric acid) have
sharp optical absorption peaks in the spectral
range 200–900 nm and can be used as a wavelength
calibration standard for optical spectrophotometers,
and are available commercially.As f-f transitions
are Laporte-forbidden, once an electron has
been excited, decay to the ground state will
be slow. This makes them suitable for use
in lasers as it makes the population inversion
easy to achieve. The Nd:YAG laser is one that
is widely used. Europium-doped yttrium vanadate
was the first red phosphor to enable the development
of color television screens. Lanthanide ions
have notable luminescent properties due to
their unique 4f orbitals. Laporte forbidden
f-f transitions can be activated by excitation
of a bound "antenna" ligand. This leads to
sharp emission bands throughout the visible,
NIR, and IR and relatively long luminescence
lifetimes.
== Occurrence ==
The lanthanide contraction is responsible
for the great geochemical divide that splits
the lanthanides into light and heavy-lanthanide
enriched minerals, the latter being almost
inevitably associated with and dominated by
yttrium. This divide is reflected in the first
two "rare earths" that were discovered: yttria
(1794) and ceria (1803). The geochemical divide
has put more of the light lanthanides in the
Earth's crust, but more of the heavy members
in the Earth's mantle. The result is that
although large rich ore-bodies are found that
are enriched in the light lanthanides, correspondingly
large ore-bodies for the heavy members are
few. The principal ores are monazite and bastnäsite.
Monazite sands usually contain all the lanthanide
elements, but the heavier elements are lacking
in bastnäsite. The lanthanides obey the Oddo-Harkins
rule – odd-numbered elements are less abundant
than their even-numbered neighbors.
Three of the lanthanide elements have radioactive
isotopes with long half-lives (138La, 147Sm
and 176Lu) that can be used to date minerals
and rocks from Earth, the Moon and meteorites.
== Applications ==
=== 
Industrial ===
Lanthanide elements and their compounds have
many uses but the quantities consumed are
relatively small in comparison to other elements.
About 15000 ton/year of the lanthanides are
consumed as catalysts and in the production
of glasses. This 15000 tons corresponds to
about 85% of the lanthanide production. From
the perspective of value, however, applications
in phosphors and magnets are more important.The
devices lanthanide elements are used in include
superconductors, samarium-cobalt and neodymium-iron-boron
high-flux rare-earth magnets, magnesium alloys,
electronic polishers, refining catalysts and
hybrid car components (primarily batteries
and magnets). Lanthanide ions are used as
the active ions in luminescent materials used
in optoelectronics applications, most notably
the Nd:YAG laser. Erbium-doped fiber amplifiers
are significant devices in optical-fiber communication
systems. Phosphors with lanthanide dopants
are also widely used in cathode ray tube technology
such as television sets. The earliest color
television CRTs had a poor-quality red; europium
as a phosphor dopant made good red phosphors
possible. Yttrium iron garnet (YIG) spheres
can act as tunable microwave resonators. Lanthanide
oxides are mixed with tungsten to improve
their high temperature properties for TIG
welding, replacing thorium, which was mildly
hazardous to work with. Many defense-related
products also use lanthanide elements such
as night vision goggles and rangefinders.
The SPY-1 radar used in some Aegis equipped
warships, and the hybrid propulsion system
of Arleigh Burke-class destroyers all use
rare earth magnets in critical capacities.
The price for lanthanum oxide used in fluid
catalytic cracking has risen from $5 per kilogram
in early 2010 to $140 per kilogram in June
2011.Most lanthanides are widely used in lasers,
and as (co-)dopants in doped-fiber optical
amplifiers; for example, in Er-doped fiber
amplifiers, which are used as repeaters in
the terrestrial and submarine fiber-optic
transmission links that carry internet traffic.
These elements deflect ultraviolet and infrared
radiation and are commonly used in the production
of sunglass lenses. Other applications are
summarized in the following table:
The complex Gd(DOTA) is used in magnetic resonance
imaging.
=== Life science ===
As mentioned in the industrial applications
section above, lanthanide metals are particularly
useful in technologies that take advantage
of their reactivity to specific wavelengths
of light. Certain life science applications
take advantage of the unique luminescence
properties of lanthanide ion complexes (Ln(III)
chelates or cryptates). These are well-suited
for this application due to their large Stokes
shifts and extremely long emission lifetimes
(from microseconds to milliseconds) compared
to more traditional fluorophores (e.g., fluorescein,
allophycocyanin, phycoerythrin, and rhodamine).
The biological fluids or serum commonly used
in these research applications contain many
compounds and proteins which are naturally
fluorescent. Therefore, the use of conventional,
steady-state fluorescence measurement presents
serious limitations in assay sensitivity.
Long-lived fluorophores, such as lanthanides,
combined with time-resolved detection (a delay
between excitation and emission detection)
minimizes prompt fluorescence interference.
Time-resolved fluorometry (TRF) combined with
fluorescence resonance energy transfer (FRET)
offers a powerful tool for drug discovery
researchers: Time-Resolved Fluorescence Resonance
Energy Transfer or TR-FRET. TR-FRET combines
the low background aspect of TRF with the
homogeneous assay format of FRET. The resulting
assay provides an increase in flexibility,
reliability and sensitivity in addition to
higher throughput and fewer false positive/false
negative results.
This method involves two fluorophores: a donor
and an acceptor. Excitation of the donor fluorophore
(in this case, the lanthanide ion complex)
by an energy source (e.g. flash lamp or laser)
produces an energy transfer to the acceptor
fluorophore if they are within a given proximity
to each other (known as the Förster’s radius).
The acceptor fluorophore in turn emits light
at its characteristic wavelength.
The two most commonly used lanthanides in
life science assays are shown below along
with their corresponding acceptor dye as well
as their excitation and emission wavelengths
and resultant Stokes shift (separation of
excitation and emission wavelengths).
=== Upcoming Medical Uses ===
Currently there is research showing that lanthanides
elements can be used as anticancer agents.
The main role of the lanthanides in these
studies is to inhibit proliferation of the
cancer cells. Specifically cerium and lanthanum
have been studied for their role as anti-cancer
agents.
One of the specific elements from the lanthanide
group that has been tested and used is cerium
(Ce). There have been studies that use a protein-cerium
complex to observe the effect of cerium on
the cancer cells. The hope was to inhibit
cell proliferation and promote cytotoxicity.
Transferrin receptors in cancer cells, such
as those in breast cancer cells and epithelial
cervical cells, promote the cell proliferation
and malignancy of the cancer. Transferrin
is a protein used to transport iron into the
cells and is needed to aid the cancer cells
in DNA replication. Transferrin acts as a
growth factor for the cancerous cells and
is dependent on iron. Cancer cells have much
higher levels of transferrin receptors than
normal cells and are very dependent on iron
for their proliferation. Cerium has shown
results as an anti-cancer agent due to its
similarities in structure and biochemistry
to iron. Cerium may bind in the place of iron
on to the transferrin and then be brought
into the cancer cells by transferrin-receptor
mediated endocytosis. The cerium binding to
the transferrin in place of the iron inhibits
the transferrin activity in the cell. This
creates a toxic environment for the cancer
cells and causes a decrease in cell growth.
This is the proposed mechanism for cerium’s
effect on cancer cells, though the real mechanism
may be more complex in how cerium inhibits
cancer cell proliferation. Specifically in
HeLa cancer cells studied in vitro, cell viability
was decreased after 48 to 72 hours of cerium
treatments. Cells treated with just cerium
had decreases in cell viability, but cells
treated with both cerium and transferrin had
more significant inhibition for cellular activity.Another
specific element that has been tested and
used as an anti-cancer agent is lanthanum,
more specifically lanthanum chloride (LaCl3
). The lanthanum ion is used to affect the
levels of let-7a and microRNAs miR-34a in
a cell throughout the cell cycle. When the
lanthanum ion was introduced to the cell in
vivo or in vitro, it inhibited the rapid growth
and induced apoptosis of the cancer cells
(specifically cervical cancer cells). This
effect was caused by the regulation of the
let-7a and microRNAs by the lanthanum ions.
The mechanism for this effect is still unclear
but it is possible that the lanthanum is acting
in a similar way as the cerium and binding
to a ligand necessary for cancer cell proliferation.
== Biological effects ==
Due to their sparse distribution in the earth's
crust and low aqueous solubility, the lanthanides
have a low availability in the biosphere,
and for a long time were not known to naturally
form part of any biological molecules. In
2007 a novel methanol dehydrogenase that strictly
uses lanthanides as enzymatic cofactors was
discovered in a bacterium from the phylum
Verrucomicrobia, Methylacidiphilum fumariolicum.
This bacterium was found to survive only if
there are lanthanides present in the environment.
Compared to most other nondietary elements,
non-radioactive lanthanides are classified
as having low toxicity.
== See also ==
Actinide
Group 3 element
Lanthanide contraction
Rare earth element
Lanthanide probes
