= Lanthanide =

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= Lanthanide =
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Introduction
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The lanthanide () or lanthanoid () series of chemical elements
comprises at least the 14 metallic chemical elements with atomic
numbers 57-70, from lanthanum through ytterbium. In the periodic
table, they fill the 4f orbitals. Lutetium (element 71) is also
sometimes considered a lanthanide, despite being a d-block element and
a transition metal.

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. Lutetium is a d-block element (thus also a
transition metal),
and on this basis its inclusion has been questioned; however, like
its congeners scandium and yttrium in group 3, it behaves similarly to
the other 14. The term rare-earth element or rare-earth metal is often
used to include the stable group 3 elements Sc, Y, and Lu in addition
to the 4f elements. All lanthanide elements form trivalent cations,
Ln3+, whose chemistry is largely determined by the ionic radius, which
decreases steadily from lanthanum (La) to lutetium (Lu).

These elements are called lanthanides because the elements in the
series are chemically similar to lanthanum. Because "lanthanide"
means "like lanthanum", it has been argued that lanthanum cannot
logically be a lanthanide, but the International Union of Pure and
Applied Chemistry (IUPAC) acknowledges its inclusion based on common
usage.

In presentations of the periodic table, the f-block elements are
customarily shown as two additional rows below the main body of the
table. This convention is entirely a matter of aesthetics and
formatting practicality; a rarely used wide-formatted periodic table
inserts the 4f and 5f series in their proper places, as parts of the
table's sixth and seventh rows (periods), respectively.

The 1985 IUPAC "Red Book" (p. 45) recommends using 'lanthanoid'
instead of 'lanthanide', as the ending ' normally indicates a negative
ion. However, owing to widespread current use, 'lanthanide' is still
allowed.

Etymology
======================================================================
The term "lanthanide" was introduced by Victor Goldschmidt in 1925.
Despite their abundance, the technical term "lanthanides" is
interpreted to reflect a sense of elusiveness on the part of these
elements, as it comes from the Greek λανθανειν ('lanthanein'), "to lie
hidden".

Rather than referring to their natural abundance, the word reflects
their property of "hiding" behind each other in minerals. The term
derives from lanthanum, first discovered in 1838, at that time a
so-called new rare-earth element "lying hidden" or "escaping notice"
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 gave its name to the whole series.

Together with the stable elements of group 3, scandium, yttrium, and
lutetium, the trivial name "rare earths" is sometimes used to describe
the set of lanthanides. The "earth" in the name "rare earths" arises
from the minerals from which they were isolated, which were uncommon
oxide-type minerals. However, these 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 name "rare earths" has more to do with the
difficulty of separating of the individual elements than the scarcity
of any of them. By way of the Greek 'dysprositos' for "hard to get
at", element 66, dysprosium was similarly named. The elements 57 (La)
to 71 (Lu) are very similar chemically to one another and frequently
occur together in nature. Often a mixture of three to all 15 of the
lanthanides (along with yttrium as a 16th) occur in minerals, such as
monazite and samarskite (for which samarium is named). These minerals
can also contain group 3 elements, and actinides such as uranium and
thorium. 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 village 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.

The properties of the lanthanides arise from the order in which the
electron shells of these elements are filled--the outermost (6s) has
the same configuration for all of them, and a deeper (4f) 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. Very
small differences in solubility are used in solvent and ion-exchange
purification methods for these elements, which require repeated
application to obtain a purified metal. The diverse applications of
refined metals and their compounds can be attributed to the subtle and
pronounced variations in their electronic, electrical, optical, and
magnetic properties.

By way of example of the term meaning "hidden" rather than "scarce",
cerium is almost as abundant as copper; on the other hand promethium,
with no stable or long-lived isotopes, is truly rare.

Physical properties of the elements
======================================================================
!Chemical
element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu
Atomic number |57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
|Image 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px
50px 50px 50px
|Density (g/cm3) |6.162 6.770 6.77 7.01 7.26 7.52 5.244 7.90 8.23
8.540 8.79 9.066 9.32 6.90 9.841
|Melting point (°C) |920 795 935 1024 1042 1072 826 1312 1356 1407
1461 1529 1545 824 1652
|Boiling point (°C) |3464 3443 3520 3074 3000 1794 1529 3273 3230
2567 2720 2868 1950 1196 3402
| Atomic electron configuration (gas
phase)*||**5d1**||4f1**5d1**||4f3||
4f4||4f5||4f6||4f7||4f7**5d1**||4f9||4f10||4f11||4f12||4f13||4f14||4f14**5d1**
|Metal lattice (RT) |dhcp fcc dhcp dhcp dhcp ** bcc hcp hcp hcp hcp
hcp hcp fcc hcp
|Metallic radius (pm) |162 181.8 182.4 181.4 183.4 180.4 208.4 180.4
177.3 178.1 176.2 176.1 175.9 193.3 173.8
|Resistivity at 25 °C (μΩ·cm) |57-80 20 °C 73 68 64 88 90 134 114 57
87 87 79 29 79
|Magnetic susceptibility χmol /10−6(cm3·mol−1) +95.9 +2500 (β) +5530
(α) +5930 (α) +1278 (α) +30900 +185000 (350 K) +170000 (α) +98000
+72900 +48000 +24700 +67 (β) +183
* Between initial Xe and final 6s2 electronic shells

** Sm has a close packed structure like most of the 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
======================================================================
!Chemical
element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu
Atomic number |57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
Ln3+ electron configuration* 4f0 4f1 4f2 4f3 4f4 4f5 4f6 4f7
4f8 4f9 4f10 4f11 4f12 4f13 4f14
Ln3+ radius (pm) 103 102 99 98.3 97 95.8 94.7 93.8 92.3
91.2 90.1 89 88 86.8 86.1
Ln4+ ion color in aqueous solution -- Orange-yellow Yellow
Blue-violet -- -- -- -- Red-brown Orange-yellow -- --
-- -- --
Ln3+ ion color in aqueous solution Colorless Colorless Green
Violet Pink Pale yellow Colorless Colorless V. pale pink
Pale yellow Yellow Rose Pale green Colorless Colorless
Ln2+ ion color in aqueous solution -- -- -- -- -- Blood
red Colorless -- -- -- -- -- Violet-red Yellow-green
--

* Not including initial [Xe] core

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.

Approximate colors of lanthanide ions in aqueous solution Oxidation
state 57 58 59 60 61 62 63 64 65 66 67 68 69
70 71
| **Sm2+** || **Eu2+**
| **Tm2+**
| **Yb2+**
+2
| +3|| **La3+** || **Ce3+**
| **Pr3+**
| **Nd3+**
| **Pm3+**
| **Sm3+**
| **Eu3+** || **Gd3+**
| **Tb3+**
| **Dy3+**
| **Ho3+**
| **Er3+**
| **Tm3+**
| **Yb3+** || **Lu3+**
| +4|| || **Ce4+**
| **Pr4+**
| **Nd4+** || || ||
| **Tb4+**
| **Dy4+** || || || ||

Effect of 4f orbitals
=======================
Viewing the lanthanides from left to right in the periodic table, the
seven 4f atomic orbitals become progressively more filled (see above
and ). The electronic configuration of most neutral gas-phase
lanthanide atoms is [Xe]6s24f'n', where 'n' is 56 less than the atomic
number 'Z'. Exceptions are La, Ce, Gd, and Lu, which have 4f'n'−15d1
(though even then 4f'n' is a low-lying excited state for La, Ce, and
Gd; for Lu, the 4f shell is already full, and the fifteenth electron
has no choice but to enter 5d). With the exception of lutetium, the 4f
orbitals are chemically active in all lanthanides and produce profound
differences between lanthanide chemistry and transition metal
chemistry. The 4f orbitals penetrate the [Xe] core and are isolated,
and thus they do not participate much 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 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]4f('n'−1).
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.

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
=============================
Ionization energies and reduction potentials of the elements
!Chemical
element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu
Atomic number |57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
| electron configuration above [Xe]
core||**4f05d1**6s2||**4f15d1**6s2||4f36s2||4f46s2||
4f56s2||4f66s2||4f76s2||**4f75d1**6s2||4f96s2||4f106s2||4f116s2||4f126s2||4f136s2||4f146s2||4f145d16s2
|E° Ln4+/Ln3+ 1.72 3.2 3.1
|E° Ln3+/Ln2+ −2.6 −1.55 −0.35 −2.5 −2.3 −1.05
|E° Ln3+/Ln −2.38 −2.34 −2.35 −2.32 −2.29 −2.30 −1.99
−2.28 −2.31 −2.29 −2.33 −2.32 −2.32 −2.22 −2.30
|1st Ionization energy (kJ·mol−1) |538 541 522 530 536 542 547 595
569 567 574 581 589 603 513
|2nd Ionization energy (kJ·mol−1) |1067 1047 1018 1034 1052 1068
1085 1172 1112 1126 1139 1151 1163 1175 1341
|1st + 2nd Ionization energy (kJ·mol−1) |1605 1588 1540 1564 1588
1610 1632 1767 1681 1693 1713 1732 1752 1778 1854
|3rd Ionization energy (kJ·mol−1) |1850 1940 2090 2128 2140 2285
2425 1999 2122 2230 2221 2207 2305 2408 2054
|1st + 2nd + 3rd Ionization energy (kJ·mol−1) |3455 3528 3630 3692
3728 3895 4057 3766 3803 3923 3934 3939 4057 4186 3908
|4th Ionization energy (kJ·mol−1) |4819 3547 3761 3900 3970 3990
4120 4250 3839 3990 4100 4120 4120 4203 4370
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. Also Tb has a well-known IV state, as
removing the 4th electron in this case produces a half-full 4f7
configuration.

The additional stable valences for Ce and Eu mean that their
abundances in rocks sometimes varies significantly relative to the
other rare earth elements: see cerium anomaly and europium anomaly.

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, Pr, Tb) or +2 (Sm, 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; with the exceptions 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. Lanthanides exist in many forms other than 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).

:Samples of lanthanide nitrates in their hexahydrate form. From left
to right: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

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. The
Ce(IV) is the exception owing to the tendency to form an unfilled f
shell. Otherwise tetravalent lanthanides are rare. However, recently
Tb(IV) and Pr(IV) complexes have been shown to exist.

Hydrides
==========
!Chemical
element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu
Atomic number |57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
|Metal lattice (RT) |dhcp fcc dhcp dhcp dhcp r bcc hcp hcp
hcp hcp hcp hcp hcp hcp
|Dihydride |LaH2+x CeH2+x PrH2+x NdH2+x SmH2+x EuH2 o "salt
like" GdH2+x TbH2+x DyH2+x HoH2+x ErH2+x TmH2+x YbH2+x o,
'fcc' "salt like" LuH2+x
|'Structure' |CaF2 CaF2 CaF2 CaF2 CaF2 CaF2 *PbCl2 CaF2 CaF2 CaF2
CaF2 CaF2 CaF2 CaF2
|'metal sub lattice' |'fcc' 'fcc' 'fcc' 'fcc' 'fcc' 'fcc' 'o' 'fcc'
'fcc' 'fcc' 'fcc' 'fcc' 'fcc' 'o' 'fcc' 'fcc'
|Trihydride |LaH3−x CeH3−x PrH3−x NdH3−x SmH3−x EuH3−x GdH3−x
TbH3−x DyH3−x HoH3−x ErH3−x TmH3−x LuH3−x
|'metal sub lattice' |'fcc' 'fcc' 'fcc' 'hcp' 'hcp' 'hcp' 'fcc' 'hcp'
'hcp' 'hcp' 'hcp' 'hcp' 'hcp' 'hcp' 'hcp'
|Trihydride properties transparent insulators (color where
recorded) |red bronze to grey PrH3−x 'fcc' NdH3−x 'hcp' golden
greenish EuH3−x 'fcc' GdH3−x 'hcp' TbH3−x 'hcp' DyH3−x 'hcp'
HoH3−x 'hcp' ErH3−x 'hcp' TmH3−x 'hcp' LuH3−x 'hcp'

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
=========
Lanthanide halides
!Chemical
element!!La!!Ce!!Pr!!Nd!!Pm!!Sm!!Eu!!Gd!!Tb!!Dy!!Ho!!Er!!Tm!!Yb!!Lu
Atomic number |57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
|**Tetrafluoride**
| ||**'CeF4**'||**'PrF4**'||**'NdF4**'|| || || ||
||**'TbF4**'||**'DyF4**'|| || || ||
|Color m.p. °C white dec white dec white dec
|Structure C.N. |UF4 8 UF4 8 UF4 8
|**Trifluoride**
|**'LaF3**'||**'CeF3**'||**'PrF3**'||**'NdF3**'||**'PmF3**'||**'SmF3**'||**'EuF3**'||**'GdF3**'||**'TbF3**'||**'DyF3**'||**'HoF3**'||**'ErF3**'||**'TmF3**'||**'YbF3**'||**'LuF3**'
|Color m.p. °C |white 1493 white 1430 green 1395 violet 1374 green
1399 white 1306 white 1276 white 1231 white 1172 green 1154 pink 1143
pink 1140 white 1158 white 1157 white 1182 |Structure C.N. LaF3 9
LaF3 9 LaF3 9 LaF3 9 LaF3 9 YF3 8 YF3 8 YF3 8 YF3 8 YF3 8 YF3 8
YF3 8 YF3 8 YF3 8 YF3 8
|**Trichloride**
|**'LaCl3**'||**'CeCl3**'||**'PrCl3**'||**'NdCl3**'||**'PmCl3**'||**'SmCl3**'||**'EuCl3**'||**'GdCl3**'||**'TbCl3**'||**'DyCl3**'||**'HoCl3**'||**'ErCl3**'||**'TmCl3**'||**'YbCl3**'||**'LuCl3**'
|Color m.p. °C |white 858 white 817 green 786 mauve 758 green 786
yellow 682 yellow dec white 602 white 582 white 647 yellow 720 violet
776 yellow 824 white 865 white 925 |Structure C.N. |UCl3 9 UCl3 9 UCl3
9 UCl3 9 UCl3 9 UCl3 9 UCl3 9 UCl3 9 PuBr3 8 PuBr3 8 YCl3 6 YCl3 6
YCl3 6 YCl3 6 YCl3 6
|**Tribromide**
|**'LaBr3**'||**'CeBr3**'||**'PrBr3**'||**'NdBr3**'||**'PmBr3**'||**'SmBr3**'||**'EuBr3**'||**'GdBr3**'||**'TbBr3**'||**'DyBr3**'||**'HoBr3**'||**'ErBr3**'||**'TmBr3**'||**'YbBr3**'||**'LuBr3**'
|Color m.p. °C |white 783 white 733 green 691 violet 682 green 693
yellow 640 grey dec white 770 white 828 white 879 yellow 919 violet
923 white 954 white dec white 1025 |Structure C.N. |UCl3 9 UCl3 9 UCl3
9 PuBr3 8 PuBr3 8 PuBr3 8 PuBr3 8 6 6 6 6 6 6 6 6
|**Triiodide**
|**'LaI3**'||**'CeI3**'||**'PrI3**'||**'NdI3**'||**'PmI3**'||**'SmI3**'||**'EuI3**'||**'GdI3**'||**'TbI3**'||**'DyI3**'||**'HoI3**'||**'ErI3**'||**'TmI3**'||**'YbI3**'||**'LuI3**'
|Color m.p. °C |yellow-green 772 yellow 766 green 738 green 784 green
737 orange 850 colorless dec. yellow 925 brown 957 green 978 yellow
994 violet 1015 yellow 1021 white dec brown 1050 |Structure C.N.
|PuBr3 8 PuBr3 8 PuBr3 8 PuBr3 8 BiI3 6 BiI3 6 BiI3 6 BiI3 6 BiI3 6
BiI3 6 BiI3 6 BiI3 6 BiI3 6 BiI3 6
|**Difluoride**
| || || || || ||**'SmF2**'||**'EuF2**'|| || || || || ||
**'TmF2**'||**'YbF2**'
|Color m.p. °C purple 1417 yellow 1416
grey |Structure C.N. CaF2 8 CaF2 8 CaF2 8
|**Dichloride**
| || || ||**'NdCl2**'|| ||**'SmCl2**'||**'EuCl2**'|| ||
||**'DyCl2**'|| || ||**'TmCl2**'||**'YbCl2**'
|Color m.p. °C green 841 brown 859 white 731 black
dec. green 718 green 720 |Structure C.N. PbCl2 9 PbCl2 9
PbCl2 9 SrBr2 SrI2 7 SrI2 7
|**Dibromide**
| || || ||**'NdBr2**'|| ||**'SmBr2**'||**'EuBr2**'|| ||
||**'DyBr2**'|| || ||**'TmBr2**'||**'YbBr2**'
|Color m.p. °C green 725 brown 669 white 731 black
green yellow 673 |Structure C.N. PbCl2 9 SrBr2 8 SrBr2 8
SrI2 7 SrI2 7 SrI2 7
|**Diiodide**
|**'LaI2**' metallic||**'CeI2**' metallic ||**'PrI2**' metallic
||**'NdI2**' high pressure metallic || ||**'SmI2**'||**'EuI2**'
||**'GdI2**' metallic || ||**'DyI2**'|| ||
||**'TmI2**'||**'YbI2**'
|Color m.p. °C bronze 808 bronze 758 violet 562 green 520 green
580 bronze 831 purple 721 black 756 yellow 780 Lu |Structure
C.N. |CuTi2 8 CuTi2 8 CuTi2 8 SrBr2 8 CuTi2 8 EuI2 7 EuI2 7
2H-MoS2 6 CdI2 6 CdI2 6
|**Ln7I12**
|**'La7I12**'|| ||**'Pr7I12**'|| || || || || ||**'Tb7I12**'||
|| || || ||
|**Sesquichloride**
|**'La2Cl3**' || || || || || || ||**'Gd2Cl3**' ||**'Tb2Cl3**'|| ||
|| **'Er2Cl3**' || **'Tm2Cl3**' || ||**'Lu2Cl3**'
|Structure Gd2Cl3 Gd2Cl3
|**Sesquibromide**
| || || || || || || ||**'Gd2Br3**' ||**'Tb2Br3**' || || || || ||
|Structure Gd2Cl3 Gd2Cl3
|**Monoiodide**
| **'LaI**'|| || || || || || || || || || || ||**'TmI**'||
|Structure NiAs type
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 diiodides have relatively short metal-metal
separations. The CuTi2 structure of the lanthanum, cerium and
praseodymium diiodides 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 and TmI are the only known monohalides. LaI, prepared from the
reaction of LaI3 and La metal, it has a NiAs type structure and can be
formulated La3+ (I−)(e−)2. TmI is a true Tm(I) compound, however it is
not isolated in a pure state.

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, where Q =
P, As, Sb or Bi. 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 C-C bond is less elongated than in the dicarbides, with the
exception of Ce2C3, indicating that the delocalized metal electrons do
not fill C-C antibonding orbitals.

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). These hydrolyze to methane.

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.

Magnetic and spectroscopic
============================
All the trivalent lanthanide ions, except lanthanum and lutetium, have
unpaired f electrons. (Ligand-to-metal charge transfer can nonetheless
produce a nonzero f-occupancy even in La(III) compounds.) 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
======================================================================
Samarskite and similar minerals contain lanthanides in association
with the elements such as 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 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 lanthanide-bearing minerals based on group-2 elements,
such as yttrocalcite, yttrocerite and yttrofluorite, which vary in
content of yttrium, cerium, lanthanum and others. Other
lanthanide-bearing 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 be denoted by a predominating
one, as in 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 occur in lunar soil. Very rare halides
of cerium, lanthanum, and presumably other lanthanides, feldspars and
garnets are also known to exist.

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. Promethium is
effectively a man-made element, as all its isotopes are radioactive
with half-lives shorter than 20 years.

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 s 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 applications of lanthanides !Application !Percentage
|Catalytic converters |45%
|Petroleum refining catalysts |25%
|Permanent magnets |12%
|Glass polishing and ceramics |7%
|Metallurgical |7%
|Phosphors |3%
|Other |1%

The complex Gd(DOTA) is used in magnetic resonance imaging.

Mixtures containing all of the lanthanides operating as a single-atom
catalysts have been proposed for the electroreduction of carbon
dioxide (CO2) to carbon monoxide (CO) with a faradaic efficiency
greater than 90%.

Life science
==============
Lanthanide complexes can be used for optical imaging. Applications are
limited by the lability of the complexes.

Some applications depend on the unique luminescence properties of
lanthanide 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 Förster resonance energy
transfer (FRET) offers a powerful tool for drug discovery researchers:
Time-Resolved Förster 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).
align="center" |+ Life Science lanthanide Donor-Acceptor pairings
!Donor !Excitation⇒Emission λ (nm) !Acceptor !Excitation⇒Emission λ
(nm) !Stokes Shift (nm)
|Eu3+ |340⇒615 |Allophycocyanin |615⇒660 |320
|Tb3+ |340⇒545 |Phycoerythrin |545⇒575 |235

Possible medical uses
=======================
Currently there is research showing that lanthanide 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.
In the field of magnetic resonance imaging (MRI), compounds containing
gadolinium are utilized extensively.
The photobiological characteristics, anticancer, anti-leukemia, and
anti-HIV activities of the lanthanides with coumarin and its related
compounds are demonstrated by the biological activities of the
complex.
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.
In the field of magnetic resonance imaging (MRI), compounds containing
gadolinium are utilized extensively.

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 Verrucomicrobiota, '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. The same nutritional requirement has also been
observed in 'Methylorubrum extorquens' and 'Methylobacterium
radiotolerans'.

See also
======================================================================
* Actinides, the heavier congeners of the lanthanides
* Group 3 element
* Lanthanide probes

External links
======================================================================
*
[https://web.archive.org/web/20181014043641/http://www.sparkle.pro.br/
lanthanide Sparkle Model], used in the computational chemistry of
lanthanide complexes
* [http://minerals.usgs.gov/minerals/pubs/commodity/rare_earths/ USGS
Rare Earths Statistics and Information]
*
[https://web.archive.org/web/20080415085514/http://www.chem.unr.edu/faculty/abd/
Ana de Bettencourt-Dias: Chemistry of the lanthanides and
lanthanide-containing materials]
* Eric Scerri, 2007, 'The periodic table: Its story and its
significance,' Oxford University Press, New York,

License
=========
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Original Article: http://en.wikipedia.org/wiki/Lanthanide