Terbium
Terbium is a chemical element with the atomic number 65 and chemical symbol Tb, classified as a rare-earth metal in the lanthanide series of the periodic table. It appears as a soft, silvery-gray metal that is malleable, ductile, and soft enough to be cut with a knife, with two known crystal modifications that transform at 1289°C.[1] Terbium has an electron configuration of [Xe] 4f⁹ 6s² and typically exhibits oxidation states of +3, though +4, +2, and +1 are also possible; its oxide, Tb₂O₃, is a weakly basic compound that forms a chocolate-brown powder.[1][2] Discovered in 1843 by Swedish chemist Carl Gustaf Mosander, terbium was isolated from yttrium compounds derived from the mineral yttria (yttrium oxide) found near the village of Ytterby, Sweden, after which the element is named.[1] It occurs naturally in minerals such as monazite (up to 0.03% terbia), xenotime, cerite, gadolinite, and euxenite (up to 1% terbia), from which it is extracted as a byproduct of rare-earth processing.[1][2] Terbium has 37 known isotopes, with terbium-159 being the only stable isotope and comprising 100% of natural samples; its atomic mass is 158.92535 u.[1][3][4] Key physical properties include a melting point of 1356°C, a boiling point of 3123°C, and a density of 8.234 g/cm³ at room temperature, where it exists as a solid reasonably stable in air but prone to oxidation over time.[1] Chemically, terbium is reactive with water and acids, forming salts, and it burns readily when ignited to produce terbia.[1] Notable applications leverage its optical and magnetic properties: terbium compounds, particularly green-emitting phosphors like terbium-doped yttrium aluminum garnet, are essential in fluorescent lamps, color television tubes, and LED displays as part of red-green-blue phosphor systems.[5] Additionally, sodium terbium borate is used in solid-state devices, and terbium oxide stabilizes zirconia in high-temperature fuel cells, while alloys with terbium enhance magnetostrictive materials for actuators and sensors in high-tech applications like lasers and batteries.[1][6] Terbium has no known biological role in living organisms and exhibits low toxicity, though inhalation or ingestion of its dust or compounds may cause fibrosis or irritation similar to other rare-earth metals; it is handled with standard precautions in laboratory settings.[2][3] As a critical rare-earth element, terbium's supply is vulnerable due to concentrated global production, primarily in China (over 70% as of 2024), with recent export controls in 2025 underscoring its importance in modern technologies from renewable energy to electronics.[6][7]Properties
Physical properties
Terbium (Tb) is a lanthanide element in the f-block of the periodic table, occupying period 6 and group 3 (IUPAC), with atomic number 65 and ground-state electron configuration [Xe] 4f⁹ 6s².[2] As a rare earth metal, terbium exhibits a silvery-white appearance and is malleable and ductile, allowing it to be shaped without fracturing. It is relatively soft, with a Mohs hardness of 2.3, and slowly tarnishes in air upon exposure due to surface oxidation.[8][9][10] Terbium has a high melting point of 1356 °C and boiling point of 3123 °C, reflecting strong metallic bonding typical of lanthanides, along with a density of 8.23 g/cm³ at room temperature. Its thermal conductivity is 11.1 W/(m·K), and electrical conductivity is approximately 8.7 × 10⁵ S/m (corresponding to a resistivity of 1.15 μΩ·m), values that are moderate for a rare earth metal and influenced by its electronic structure.[2][11][1] At room temperature, terbium is paramagnetic, but it transitions to ferromagnetic ordering below its Curie temperature of approximately 222 K, a behavior driven by the alignment of its 4f electrons. The mass magnetic susceptibility is 1.36 × 10⁻⁵ m³/kg, indicating strong response to external fields.[12][9] Terbium exhibits two allotropic forms: the alpha phase with a hexagonal close-packed (hcp) crystal structure stable at room temperature (lattice parameters a = 360.1 pm and c = 569.4 pm), transforming to the beta phase at 1289 °C, contributing to its metallic luster and mechanical properties.[13]Chemical properties
Terbium is a highly reactive lanthanide metal that tarnishes slowly in air due to the formation of a thin protective oxide layer, primarily Tb₂O₃, which limits further oxidation under ambient conditions.[14] When heated or ignited in air, however, it burns vigorously to produce terbium heptaoxide (Tb₄O₇) according to the reaction 8Tb + 7O₂ → 2Tb₄O₇.[14] The metal also exhibits significant reactivity with water, reacting slowly with cold water and more rapidly with hot water to form terbium(III) hydroxide and hydrogen gas: 2Tb + 6H₂O → 2Tb(OH)₃ + 3H₂.[14] Similarly, terbium dissolves readily in dilute acids, such as sulfuric acid, liberating hydrogen and yielding Tb³⁺ ions: 2Tb + 3H₂SO₄ → 2Tb³⁺ + 3SO₄²⁻ + 3H₂.[14] As an electropositive element, terbium readily loses electrons to form the stable +3 oxidation state, which dominates its chemistry, though +4 is possible under oxidizing conditions.[15] This electropositivity is reflected in its highly negative standard reduction potential of -2.31 V for the Tb³⁺/Tb couple, indicating a strong tendency to oxidize.[16] Terbium reacts directly with halogens at elevated temperatures to form trihalides, for example, 2Tb + 3F₂ → 2TbF₃.[14] In coordination chemistry, Tb³⁺ ions favor high coordination numbers ranging from 8 to 12, facilitated by the relatively large ionic radius of 1.04 Å (for coordination number 8), allowing for stable complexes with multiple ligands typical of lanthanides.[17] In aqueous solutions, terbium salts are soluble in dilute acids but insoluble in alkalis, where hydroxide precipitation occurs. The oxide Tb₂O₃ displays amphoteric properties, dissolving in strong acids to form salts and in concentrated bases under certain conditions.[18]Isotopes
Terbium consists of one stable isotope in nature, ¹⁵⁹Tb, which accounts for 100% of its natural abundance. This isotope has an atomic mass of 158.925 3547(19) u, giving terbium a standard atomic weight of 158.925 35(2) u. The nucleus of ¹⁵⁹Tb possesses a nuclear spin of I = 3/2 and a magnetic moment of +2.014 μ<sub>N</sub>. Terbium has 39 known radioactive isotopes, spanning mass numbers from 135 to 183, with half-lives ranging from microseconds to centuries. The most stable radioactive isotope is ¹⁵⁸Tb, with a half-life of 180 years, decaying primarily by electron capture (84%) to stable ¹⁵⁸Gd and to a lesser extent by beta minus decay (16%) to ¹⁵⁸Dy. Another long-lived isotope, ¹⁶⁰Tb, has a half-life of 72.3 days and undergoes beta minus decay to stable ¹⁶⁰Dy. Shorter-lived isotopes include ¹⁶¹Tb, with a half-life of 6.91 days, which decays by beta minus emission (average energy 148 keV) to ¹⁶¹Dy; this isotope is produced artificially for potential use in targeted radionuclide therapy due to its emission of conversion and Auger electrons alongside gamma rays suitable for imaging. Radioactive terbium isotopes are synthesized through neutron capture reactions in nuclear reactors or charged-particle bombardments in cyclotrons and accelerators. For instance, neutron-rich isotopes like ¹⁶¹Tb are generated via the indirect route **¹⁶⁰Gd(n,γ)**¹⁶¹Gd → β⁻ ¹⁶¹Tb, using enriched gadolinium targets irradiated with thermal neutrons. Neutron-deficient isotopes, such as ¹⁵⁵Tb, are produced by proton-induced spallation or (p,n) reactions on heavier targets like tantalum or gadolinium. The nuclear binding energy per nucleon for ¹⁵⁹Tb is 8.189 MeV, consistent with values for nearby lanthanide isotopes and contributing to its stability. Terbium isotopes exhibit high neutron capture cross-sections, with ¹⁵⁹Tb having a thermal neutron capture cross-section of 23.4 ± 0.4 barns for the **¹⁵⁹Tb(n,γ)**¹⁶⁰Tb reaction, making it relevant for neutron absorption in reactor design and shielding applications. Fission cross-sections for terbium isotopes are negligible, as the element lacks the heavy nuclei required for induced fission under typical neutron fluxes.| Isotope | Half-life | Decay mode | Principal daughter | Natural abundance |
|---|---|---|---|---|
| ¹⁵⁸Tb | 180 y | EC (84%), β⁻ (16%) | ¹⁵⁸Gd, ¹⁵⁸Dy | None |
| ¹⁵⁹Tb | Stable | - | - | 100% |
| ¹⁶⁰Tb | 72.3 d | β⁻ | ¹⁶⁰Dy | None |
| ¹⁶¹Tb | 6.91 d | β⁻ | ¹⁶¹Dy | None (artificial) |