Lutetium

Lutetium
71Lu
Y ↑ Lu ↓ Lr Periodic table ytterbium ← lutetium → hafnium
Appearance
silvery white
General properties
Name, symbol, number	lutetium, Lu, 71
Pronunciation	/ lj uː ˈ t iː ʃ i ə m / lew-TEE-shee-əm
Element category	lanthanide sometimes considered a transition metal
Group, period, block	n/a, 6, d
Standard atomic weight	174.9668(4)
Electron configuration	[Xe] 6s2 4f14 5d1 2, 8, 18, 32, 9, 2
History
Discovery	Georges Urbain and Carl Auer von Welsbach (1906)
First isolation	Carl Auer von Welsbach (1906)
Physical properties
Phase	solid
Density (near r.t.)	9.841 g·cm−3
Liquid density at m.p.	9.3 g·cm−3
Melting point	1925 K, 1652 °C, 3006 °F
Boiling point	3675 K, 3402 °C, 6156 °F
Heat of fusion	ca. 22 kJ·mol−1
Heat of vaporization	414 kJ·mol−1
Molar heat capacity	26.86 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1906 2103 2346 (2653) (3072) (3663)
Atomic properties
Oxidation states	3, 2, 1 (weakly basic oxide)
Electronegativity	1.27 (Pauling scale)
Ionization energies	1st: 523.5 kJ·mol−1
	2nd: 1340 kJ·mol−1
	3rd: 2022.3 kJ·mol−1
Atomic radius	174 pm
Covalent radius	187±8 pm
Miscellanea
Crystal structure	hexagonal close-packed
Magnetic ordering	paramagnetic
Electrical resistivity	( r.t.) (poly) 582 nΩ·m
Thermal conductivity	16.4 W·m−1·K−1
Thermal expansion	( r.t.) (poly) 9.9 µm/(m·K)
Young's modulus	68.6 GPa
Shear modulus	27.2 GPa
Bulk modulus	47.6 GPa
Poisson ratio	0.261
Vickers hardness	1160 MPa
Brinell hardness	893 MPa
CAS registry number	7439-94-3
Most stable isotopes
Main article: Isotopes of lutetium
iso NA half-life DM DE ( MeV) DP 173Lu syn 1.37 y ε 0.671 173Yb 174Lu syn 3.31 y ε 1.374 174Yb 175Lu 97.41% 175Lu is stable with 104 neutrons 176Lu 2.59% 3.78×1010 y β− 1.193 176Hf

Lutetium is a chemical element with the symbol Lu and atomic number 71. It is a silvery white metal which resists corrosion in dry, but not moist, air. It is the last element in the lanthanide series, and traditionally counted among the rare earths.

Lutetium was independently discovered in 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, and American chemist Charles James. All of these men found lutetium as an impurity in the mineral ytterbia, which was previously thought to consist entirely of ytterbium. The dispute on the priority of the discovery occurred shortly after, with Urbain and von Welsbach accusing each other of publishing results influenced by the published research of the other; the naming honour went to Urbain as he published his results earlier. He chose the name lutecium for the new element but in 1949 the spelling of element 71 was changed to lutetium. In 1909, the priority was finally granted to Urbain and his names were adopted as official ones; however, the name cassiopeium (or later cassiopium) for element 71 proposed by von Welsbach was used by many German scientists until the 1950s.

Lutetium is not a particularly abundant element, though significantly more common than silver in the earth's crust; it has few specific uses. Lutetium-176 is a relatively abundant (2.5%) radioactive isotope with a half-life of about 38 billion years, and so used to determine the age of meteorites. Lutetium usually occurs in association with the element yttrium and is sometimes used in metal alloys and as a catalyst in various chemical reactions. ¹⁷⁷Lu- DOTA-TATE is used for radionuclide therapy (see Nuclear medicine) on neuroendocrine tumours.

Characteristics

Physical properties

A lutetium atom has 71 electrons, arranged in the configuration [Xe] 4f¹⁴5d¹6s². When entering a chemical reaction, the atom loses its two outermost electrons and the single 5d-electron; this is unusual since reactions of the other lanthanides invariably involve f-shell electrons. The lutetium atom is the smallest among the lanthanide atoms, due to the lanthanide contraction, and as a result lutetium has the highest density, melting point, and hardness of the lanthanides.

Chemical properties and compounds

Lutetium's compounds always contain the element in the oxidation state +3. Aqueous solutions of most lutetium salts are colorless and form white crystalline solids upon drying, with the common exception of the iodide. The soluble salts, such as nitrate, sulfate and acetate form hydrates upon crystallization. The oxide, hydroxide, fluoride, carbonate, phosphate and oxalate are insoluble in water.

Lutetium metal is slightly unstable in air at standard conditions, but it burns readily at 150 °C to form lutetium oxide. The resulting compound is known to adsorb water and carbon dioxide, and may be used to remove vapors of these compounds from closed atmospheres. Similar observations are made during reaction between lutetium and water (slow when cold and fast when hot); lutetium hydroxide is formed in the reaction. Lutetium metal is known to react with the four lightest halogens to form tri halides; all of them (except the fluoride) are soluble in water.

Lutetium dissolves readily in weak acids and dilute sulfuric acid to form solutions containing the colorless lutetium ions, which exist as a [Lu(H₂O)₉]³⁺ complex:

2 Lu + 3 H₂SO₄ + 18 H₂O → 2 [Lu(H₂O)₉]³⁺ + 3 SO2–
4 + 3 H₂↑

Isotopes

Lutetium occurs on the Earth in form of two isotopes: lutetium-175 and lutetium-176. Out of these two, only the former is stable, making the element monoisotopic. The latter one, lutetium-176, decays via beta decay with a half-life of 3.78×10¹⁰ years; it makes up about 2.5% of natural lutetium. To date, 32 synthetic radioisotopes of the element have been characterized, ranging in mass from 149.973 (lutetium-150) to 183.961 (lutetium-184); the most stable such isotopes are lutetium-174 with a half-life of 3.31 years, and lutetium-173 with a half-life of 1.37 years. All of the remaining radioactive isotopes have half-lives that are less than 9 days, and the majority of these have half-lives that are less than half an hour. Isotopes lighter than the stable lutetium-175 decay via electron capture (to produce isotopes of ytterbium), with some alpha and positron emission); the heavier isotopes decay primarily via beta decay, producing hafnium isotopes.

The element also has 42 nuclear isomers, with masses of 150, 151, 153—162, 166—180 (not every mass number corresponds to only one isomer). The most stable of them are lutetium-177m, with half-life of 160.4 days and lutetium-174m, with half-life of 142 days; this is longer than half-lives of the ground states of all radioactive lutetium isotopes, except only for lutetium-173, 174, and 176.

History

Lutetium, derived from the Latin Lutetia (Paris), was independently discovered in 1907 by French scientist Georges Urbain, Austrian mineralogist Baron Carl Auer von Welsbach, and American chemist Charles James. They found it as an impurity in ytterbia, which was thought by Swiss chemist Jean Charles Galissard de Marignac to consist entirely of ytterbium. The scientists proposed different names for the elements: Urbain chose neoytterbium and lutecium, while Welsbach chose aldebaranium and cassiopeium. Both of these articles accused each other of publishing results based on the other party.

The Commission on Atomic Mass, which was then responsible for the attribution of new element names, settled the dispute in 1909 by granting priority to Urbain and adopting his names as official ones, based on the fact that the separation of lutetium from Marignac's ytterbium was first described by Urbain; after Urbain's names were recognized, neoytterbium was reverted to ytterbium. Until the 1950s, some German chemists called lutetium by von Welsbach's name cassiopeium; in 1949, the spelling of element 71 was changed to lutetium. However, Welsbach's 1907 samples of lutetium had been pure, while Urbain's 1907 samples only contained traces of lutetium. This later misled Urbain into thinking that he had discovered element 72, which he named celtium, which was actually very pure lutetium. Charles James, who stayed out of the priority argument, worked on a much larger scale and possessed the largest supply of lutetium at the time. Pure lutetium metal was first produced in 1953.

Occurrence and production

Found with almost all other rare-earth metals but never by itself, lutetium is very difficult to separate from other elements. The principal commercially viable ore of lutetium is the rare earth phosphate mineral monazite: (Ce,La,etc.)PO₄ which contains 0.0001% of the element. The abundance of lutetium in the Earth crust is only about 0.5 mg/kg. The main mining areas are China, United States, Brazil, India, Sri Lanka and Australia. The world production of lutetium (in the form of oxide) is about 10 tonnes per year. Pure lutetium metal is very difficult to prepare. It is one of the rarest and most expensive of the rare earth metals with the price about US$10,000 per kilogram, or about one-fourth that of gold.

Crushed minerals are treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. Thorium precipitates out of solution as hydroxide and is removed. After that the solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO₃. Several rare earth metals, including lutetium, are separated as a double salt with ammonium nitrate by crystallization. Lutetium is separated by ion exchange. In this process, rare-earth ions are sorbed onto suitable ion-exchange resin by exchange with hydrogen, ammonium or cupric ions present in the resin. Lutetium salts are then selectively washed out by suitable complexing agent. Lutetium metal is then obtained by reduction of anhydrous LuCl₃ or LuF₃ by either an alkali metal or alkaline earth metal.

2 LuCl₃ + 3 Ca → 2 Lu + 3 CaCl₂

Applications

Because of the rarity and high price, lutetium has very few commercial uses. However, stable lutetium can be used as catalysts in petroleum cracking in refineries and can also be used in alkylation, hydrogenation, and polymerization applications.

Lutetium aluminium garnet (Al₅Lu₃O₁₂) has been proposed for use as a lens material in high refractive index immersion lithography. Additionally, a tiny amount of lutetium is added as a dopant to gadolinium gallium garnet (GGG), which is used in magnetic bubble memory devices. Cerium-doped lutetium oxyorthosilicate (LSO) is currently the preferred compound for detectors in positron emission tomography (PET). Lutetium is used as a phosphor in LED light bulbs.

Aside from stable lutetium, its radioactive isotopes have several specific uses. The suitable half-life and decay mode made lutetium-176 used as a pure beta emitter, using lutetium which has been exposed to neutron activation, and in lutetium–hafnium dating to date meteorites. The synthetic isotope lutetium-177 bound to octreotate (a somatostatin analogue), is used experimentally in targeted radionuclide therapy for neuroendocrine tumors.

Lutetium tantalate (LuTaO₄) is the densest known stable white material (density 9.81 g/cm³) and therefore is an ideal host for X-ray phosphors. The only denser white material is thorium dioxide, with density of 10 g/cm³, but the thorium it contains is radioactive.

Precautions

Like other rare-earth metals, lutetium is regarded as having a low degree of toxicity, but its compounds should be handled with care nonetheless: for example, lutetium fluoride inhalation is dangerous and the compound irritates skin. Lutetium nitrate may be dangerous as it may explode and burn once heated. Lutetium oxide powder is toxic as well if inhaled or ingested.

Similarly to other group 3 elements and lanthanides, lutetium has no known biological role, but it is found even in humans, concentrating in bones, and to a lesser extent in the liver and kidneys. Lutetium salts are known to occur together with other lanthanide salts in nature; the element is the least abundant in the human body of all lanthanides. Human diets have not been monitored for lutetium content, so it is not known how much the average human takes in, but estimations show the amount is only about several micrograms per year, all coming from tiny amounts taken by plants. Soluble lutetium salts are mildly toxic, but insoluble ones are not.