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Lanthanide

From Wikipedia, the free encyclopedia

The lanthanide or lanthanoid (IUPAC nomenclature)[1] series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium.[2][3][4]

These fifteen lanthanide 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. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell; lutetium, a d-block element, is also generally considered to be a lanthanide due to its chemical similarities with the other fourteen. All lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium.

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
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
Atomic electron configuration* 5d1 4f15d1 4f3 4f4 4f5 4f6 4f7 4f75d1 4f9 4f10 4f11 4f12 4f13 4f14 4f145d1
Ln3+ electron configuration*[5] 4f0[6] 4f1 4f2 4f3 4f4 4f5 4f6 4f7 4f8 4f9 4f10 4f11 4f12 4f13

4f14

Ln3+ radius (pm)[7] 103 102 99 98.3 97 95.8 94.7 93.8 92.3 91.2 90.1 89 88 86.8 86.1
  • Between initial [Xe] and final 6s2 electronic shells

The lanthanide elements are the group of elements with atomic number increasing from 57 (lanthanum) to 71 (lutetium). They are termed lanthanide because the lighter elements in the series are chemically similar to lanthanum. Strictly speaking, both lanthanum and lutetium have been labeled as group 3 elements, because they both have a single valence electron in the d shell. However, both elements are often included in any general discussion of the chemistry of the lanthanide elements.

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,[2] with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) 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 scandium and yttrium, the trivial name "rare earths" is sometimes used to describe all the lanthanides. This name 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) [citation needed]. Cerium is the 26th most abundant element in the Earth's crust, neodymium is more abundant than gold and even thulium (the least common naturally occurring lanthanide) is more abundant than iodine.[8] 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 probably introduced by Victor Goldschmidt in 1925.[9]

Chemical compounds

Lanthanide oxides: clockwise from top center: praseodymium, cerium, lanthanum, neodymium, samarium and gadolinium.

The electronic structure of the lanthanide elements, with minor exceptions is [Xe]6s24fn. In their compounds, the 6s electrons are lost and the ions have the configuration [Xe]4fm.[10] The chemistry of the lanthanides differs from main group elements and transition metals because of the nature of the 4f orbitals. These orbitals are "buried" inside the atom and are shielded from the atom's environment by the 4d and 5p electrons. As a consequence of this, the chemistry of the elements is largely determined by their size, which decreases gradually from 102 pm (La3+) with increasing atomic number to 86 pm (Lu3+), the so-called lanthanide contraction. 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 which has the extra stability of a half-filled shell. Promethium is effectively a man-made element as all its isotopes are radioactive with half-lives shorter than 20 y.

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).[7]

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)]–.[7][11]

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.[12] 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 Ln2+, 2I–, e–, include NdI2, DyI2 and TmI2. Many of the iodides form soluble complexes with ethers, e.g. TmI2(dimethoxyethane)3.[13] Samarium(II) iodide is a useful reducing agent.

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.

Organometallic compounds

Lanthanide-carbon σ bonds are well known but they exhibit carbanion-like behavior, unlike 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].[14] Similarly complexes of cyclopentadienyl anion (Cp–), e.g. [Ln(C5H5)3], are far less common than the corresponding pentamethylcyclopentadienyl, e.g. [Ln(C5Me5)3Cl]. Analogues of uranocene are derived from dilithiocyclooctatetraene, Li2C8H8. Organic lanthanide(II) compounds are also known, such as Cp*2Eu.[13]

Magnetic and spectroscopic properties

All the trivalent lanthanide ions, except 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.

A solution of 4% holmium oxide in 10% perchloric acid, permanently fused into a quartz cuvette as a wavelength calibration standard

Crystal field splitting is rather small for the lanthanide ions and is less important than spin-orbit coupling in regard to energy levels.[15] 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,[16] and are available commercially.[17]

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.[18] 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.[19]

Geochemistry

Main article: Rare earth element § Geological distribution

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.[20]

Biological effects

Compared to most other nondietary elements, non-radioactive lanthanides are classified as having low toxicity.[21]

Technological applications

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.[21]

Of the many technological devices, including 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).[22] 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 welding, replacing thorium, which was mildly hazardous to work with. Many defense-related products also use lanthanide elements such as night vision goggles, rangefinders, the SPY-1 radar used in some Aegis equipped warships, and the propulsion system of Arleigh Burke-class destroyers all use rare earth elements in critical capacities.[23] The price of lanthanum oxide used in fluid catalytic cracking has risen from $5 per kilogram in early 2010 to $140 per kilogram in June 2011.[24]

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:[8]

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.

See also

References

  1. ^ The current IUPAC recommendation is that the name lanthanoid be used rather than lanthanide, as the suffix "-ide" is preferred for negative ions whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. However, lanthanide is still favored in most (~90%) scientific articles and is currently adopted on Wikipedia. In the older literature, the name "lanthanon" was often used.
  2. ^ a b Gray, Theodore (2009). The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog & Leventhal Publishers. p. 240. ISBN 978-1-57912-814-2.
  3. ^ Lanthanide, Encyclopædia Britannica on-line
  4. ^ Holden, Norman E. and Coplen, Tyler (January–February 2004). "The Periodic Table of the Elements". Chemistry International. 26 (1). IUPAC: 8. Retrieved March 23, 2010.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Walter Koechner (2006). Solid-state laser engineering. Springer. pp. 47–. ISBN 978-0-387-29094-2. Retrieved 15 January 2012.
  6. ^ Lanthanum – Chemistry Encyclopedia – reaction, water, elements, metal, gas, name, atom. Chemistryexplained.com. Retrieved on 2012-01-15.
  7. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1233. ISBN 978-0-08-037941-8.
  8. ^ a b Helen C. Aspinall (2001). Chemistry of the f-block elements. CRC Press. p. 8. ISBN 905699333X.
  9. ^ Hakala, Reino W. (1952). "Letters". Journal of Chemical Education. 29 (11): 581. Bibcode:1952JChEd..29..581H. doi:10.1021/ed029p581.2.
  10. ^ Mark Winter. Lanthanum ionisation energies. WebElements Ltd, UK. Retrieved 02-09-2010. {{cite book}}: Check date values in: |accessdate= (help)
  11. ^ L. Pettit and K. Powell, SC-database. Acadsoft.co.uk. Retrieved on 2012-01-15.
  12. ^ Burgess,, J. (1978). 'Metal ions in solution'. , New York: Ellis Horwood. ISBN 0853120277.{{cite book}}: CS1 maint: extra punctuation (link)
  13. ^ a b F. Nief "Non-classical divalent lanthanide complexes" Dalton Trans., 2010, 39, 6589–6598. doi:10.1039/c001280g
  14. ^ Cotton, S.A. (1997). "Aspects of the lanthanide-carbon σ-bond". Coord. Chem. Revs. 160: 93–127. doi:10.1016/S0010-8545(96)01340-9.
  15. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p 1242
  16. ^ R. P. MacDonald (1964). "Uses for a Holmium Oxide Filter in Spectrophotometry" (PDF). Clinical Chemistry. 10 (12): 1117. PMID 14240747.
  17. ^ "Holmium Glass Filter for Spectrophotometer Calibration". Retrieved 2009-06-06.
  18. ^ Levine, Albert K.; Palilla, Frank C. (1964). "A new, highly efficient red-emitting cathodoluminescent phosphor (YVO4:Eu) for color television". Applied Physics Letters. 5 (6): 118. Bibcode:1964ApPhL...5..118L. doi:10.1063/1.1723611.
  19. ^ Werts, M.H.V.; Making Sense of Lanthanide Luminescence, Science Progress, 2005, 88, 101–131.
  20. ^ There exist other naturally occurred radioactive isotopes of lanthanides with long half-lives (144Nd, 150Nd, 148Sm, 151Eu, 152Gd) but they are not used as chronometers.
  21. ^ a b Ian McGill "Rare Earth Elements" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH , Weinheim. doi:10.1002/14356007.a22 607.
  22. ^ "Haxel G, Hedrick J, Orris J. 2006. Rare earth elements critical resources for high technology. Reston (VA): United States Geological Survey. USGS Fact Sheet: 087‐02" (PDF). Retrieved 2008-04-19.
  23. ^ Livergood R. (2010). "Rare Earth Elements: A Wrench in the Supply Chain" (PDF). Center for Strategic and International Studies. Retrieved 2010-10-22.
  24. ^ Chu, Steven. Critical Materials Strategy p. 17 United States Department of Energy, December 2011. Accessed: 23 December 2011.
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