Terbium

Terbium
65Tb
- ↑ Tb ↓ Bk Periodic table gadolinium ← terbium → dysprosium
Appearance
silvery white
General properties
Name, symbol, number	terbium, Tb, 65
Pronunciation	/ ˈ t ɜr b i ə m / TUR-bee-əm
Element category	lanthanide
Group, period, block	n/a, 6, f
Standard atomic weight	158.92535
Electron configuration	[Xe] 4f9 6s2 2, 8, 18, 27, 8, 2
History
Discovery	Carl Gustaf Mosander (1842)
First isolation	Carl Gustaf Mosander (1842)
Physical properties
Phase	solid
Density (near r.t.)	8.23 g·cm−3
Liquid density at m.p.	7.65 g·cm−3
Melting point	1629 K, 1356 °C, 2473 °F
Boiling point	3503 K, 3230 °C, 5846 °F
Heat of fusion	10.15 kJ·mol−1
Heat of vaporization	293 kJ·mol−1
Molar heat capacity	28.91 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1789 1979 (2201) (2505) (2913) (3491)
Atomic properties
Oxidation states	4, 3, 2, 1 (weakly basic oxide)
Electronegativity	? 1.2 (Pauling scale)
Ionization energies	1st: 565.8 kJ·mol−1
	2nd: 1110 kJ·mol−1
	3rd: 2114 kJ·mol−1
Atomic radius	177 pm
Covalent radius	194±5 pm
Miscellanea
Crystal structure	hexagonal close-packed
Magnetic ordering	paramagnetic at 300 K
Electrical resistivity	( r.t.) (α, poly) 1.150 µΩ·m
Thermal conductivity	11.1 W·m−1·K−1
Thermal expansion	( r.t.) (α, poly) 10.3 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 2620 m·s−1
Young's modulus	(α form) 55.7 GPa
Shear modulus	(α form) 22.1 GPa
Bulk modulus	(α form) 38.7 GPa
Poisson ratio	(α form) 0.261
Vickers hardness	863 MPa
Brinell hardness	677 MPa
CAS registry number	7440-27-9
Most stable isotopes
Main article: Isotopes of terbium
iso NA half-life DM DE ( MeV) DP 157Tb syn 71 y ε 0.060 157Gd 158Tb syn 180 y ε 1.220 158Gd β− 0.937 158Dy 159Tb 100% 159Tb is stable with 94 neutrons

Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.

Terbium is used to dope calcium fluoride, calcium tungstate and strontium molybdate, materials that are used in solid-state devices, and as a crystal stabilizer of fuel cells which operate at elevated temperatures. As a component of Terfenol-D (an alloy that expands and contracts when exposed to magnetic fields more than any other alloy), terbium is of use in actuators, in naval sonar systems and in sensors.

Most of the world's terbium supply is used in green phosphors. Terbium oxide is in fluorescent lamps and TV tubes. Terbium green phosphors are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide "trichromatic" lighting technology, a high-efficiency white light used for standard illumination in indoor lighting.

Characteristics

Physical properties

Terbium is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. It is relatively stable in air as compared to other lanthanides. Terbium exists in two crystal allotropes with a transformation temperature of 1289 °C between them.

The terbium(III) cation is brilliantly fluorescent, in a bright lemon-yellow colour that is the result of a strong green emission line in combination with other lines in the orange and red. The yttrofluorite variety of the mineral fluorite owes its creamy-yellow fluorescence in part to terbium. Terbium easily oxidizes, and is therefore used in its elemental form specifically for research. Single Tb atoms have been isolated by implanting them into fullerene molecules.

Terbium has a simple ferromagnetic ordering at temperatures below 219 K. Above 219 K, it turns into a helical antiferromagnetic state in which all of the atomic moments in a particular basal plane layer are parallel, and oriented at a fixed angle to the moments of adjacent layers. This unusual antiferromagnetism transforms into a disordered paramagnetic state at 230 K.

Chemical properties

The most common valence state of terbium is +3, as in Tb₂O₃. The +4 state is known in TbO₂ and TbF₄. Terbium burns readily to form a mixed terbium(III,IV) oxide:

8 Tb + 7 O₂ → 2 Tb₄O₇

In solution, terbium forms only trivalent ions. Terbium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form terbium hydroxide:

2 Tb + 6 H₂O → 2 Tb(OH)₃ + 3 H₂↑

Terbium metal reacts with all the halogens, forming white trihalides:

2 Tb + 3 X₂ → 2 TbX₃ (X = F, Cl, Br, I)

Terbium dissolves readily in dilute sulfuric acid to form solutions containing the pale pink terbium(III) ions, which exist as a [Tb(OH₂)₉]³⁺ complexes:

2 Tb (s) + 3 H₂SO₄ → 2 Tb³⁺ + 3 SO2−
4 + 3 H₂↑

Compounds

Terbium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming various binary compounds such as TbH₂, TbH₃, TbB₂, Tb₂S₃, TbSe, TbTe and TbN. In those compounds, Tb mostly exhibit oxidation states +3 and sometimes +2. Terbium(II) halogenides are obtained by annealing Tb(III) halogenides in presence of metallic Tb in tantalum containers. Terbium also forms sesquichloride Tb₂Cl₃, which can be further reduced to TbCl by annealing at 800 °C. This terbium(I) chloride forms platelets with layered graphite-like structure.

Other compounds include

• Chlorides: TbCl₃
• Bromides: TbBr₃
• Iodides: TbI₃
• Fluorides: TbF₃, TbF₄

Terbium(IV) fluoride is a strong fluorinating agent, emitting relatively pure atomic fluorine when heated rather than the mixture of fluoride vapors emitted from CoF₃ or CeF₄.

Isotopes

Naturally occurring terbium is composed of its only stable isotope, terbium-159; the element is thus called mononuclidic and monoisotopic. 36 radioisotopes have been characterized, with the heaviest being terbium-171 (with atomic mass of 170.95330(86) u) and lightest being terbium-135 (exact mass unknown). The most stable synthetic radioisotope of terbium are terbium-158, with a half-life of 180 years, and terbium-157, with a half-life of 71 years. All of the remaining radioactive isotopes have half-lives that are well less than quarter a year, and the majority of these have half-lives that are less than half a minute. The primary decay mode before the most abundant stable isotope, ¹⁵⁹Tb, is electron capture, which results in production of gadolinium isotopes, and the primary mode after is beta minus decay, resulting in dysprosium isotopes.

The element also has 27 nuclear isomers, with masses of 141—154, 156, and 158 (not every mass number corresponds to only one isomer). The most stable of them are terbium-156m, with half-life of 24.4 hours and terbium-156m2, with half-life of 22.7 hours; this is longer than half-lives of most ground states of radioactive terbium isotopes, except only those with mass numbers 155—161.

History

Terbium was discovered in 1843 by Swedish chemist Carl Gustaf Mosander, who detected it as an impurity in Yttrium oxide, Y₂O₃, and named after the village Ytterby in Sweden. It was not isolated in pure form until the recent advent of ion exchange techniques.

When Mosander first partitioned "yttria" into three fractions, "terbia" was the fraction that contained the pink colour (due to what is now known as erbium), and "erbia" was the fraction that was essentially colorless in solution, but gave a brown-tinged oxide. Later workers had difficulty in observing the latter, but the pink fraction was impossible to miss. Arguments went back and forth as to whether "erbia" even existed. In the confusion, the original names got reversed, and the exchange of names stuck. It is now thought that those workers who used the double sodium or potassium sulfates to remove "ceria" from "yttria" inadvertently lost the terbium content of the system into the ceria-containing precipitate. In any case, what is now known as terbium was only about 1% of the original yttria, but that was sufficient to impart a yellowish colour to the oxide. Thus, terbium was a minor component in the original terbium fraction, dominated by its immediate neighbors, gadolinium and dysprosium. Thereafter, whenever other rare earths were teased apart from this mixture, whichever fraction gave the brown oxide retained the terbium name, until at last it was pure. The 19th century investigators did not have the benefit of fluorescence technology, wherewith to observe the brilliant fluorescence that would have made this element much easier to track in mixtures.

Occurrence

Terbium is never found in nature by itself, but is contained along with other rare earth elements in many minerals, including monazite ((Ce,La,Th,Nd,Y)PO₄ with up to 0.03% terbium), xenotime (YPO₄) and euxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)₂O₆ with 1% or more terbium). The crust abundance of terbium is estimated as 1.2 mg/kg.

Currently, the richest commercial sources of terbium are the ion-adsorption clays of southern China; the concentrates with about two-thirds yttrium oxide by weight have about 1% terbia. Small amounts of terbium occur in bastnäsite and monazite; when these are processed by solvent extraction to recover the valuable heavy lanthanides as samarium-europium-gadolinium concentrate, terbium is recovered therein. Due to the large volumes of bastnäsite processed relative to the ion-adsorption clays, a significant proportion of the world's terbium supply comes from bastnäsite.

Production

Crushed terbium-containing minerals are treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with caustic soda to pH 3-4. 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. Terbium is separated as a double salt with ammonium nitrate by crystallization.

The most efficient separation routine for terbium salt from the rare-earth salt solution is 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. The rare earth ions are then selectively washed out by suitable complexing agent. As with other rare earths, terbium metal is produced by reducing the anhydrous chloride or fluoride with calcium metal. Calcium and tantalum impurities can be removed by vacuum remelting, distillation, amalgam formation or zone melting.

Applications

Terbium is used as a Dopant in calcium fluoride, calcium tungstate and strontium molybdate, materials that are used in solid-state devices, and as a crystal stabilizer of fuel cells which operate at elevated temperatures, together with ZrO₂.

Terbium is also used in alloys and in the production of electronic devices. As a component of Terfenol-D, terbium is of use in actuators, in naval sonar systems, sensors, in the SoundBug device (its first commercial application), and other magnetomechanical devices. Terfenol-D is an alloy that expands or contracts in the presence of a magnetic field. It has the highest magnetostriction of any alloy.

Terbium oxide is used in green phosphors in fluorescent lamps and colour TV tubes. Sodium terbium borate is used in solid state devices. The brilliant fluorescence allows terbium to be used as a probe in biochemistry, where it somewhat resembles calcium in its behaviour. Terbium "green" phosphors (which fluoresce a brilliant lemon-yellow) are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide the " trichromatic" lighting technology which is by far the largest consumer of the world's terbium supply. Trichromatic lighting provides much higher light output for a given amount of electrical energy than does incandescent lighting.

Precautions

As with the other lanthanides, terbium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Terbium has no known biological role.