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Terbium
Terbium (Tb, atomic number 65) is a silvery-white, malleable heavy lanthanide — HCP, melting point 1,356 °C, 4f⁹6s² — notable for being one of only two lanthanides (with Ce) that show a stable +4 oxidation state in the solid phase, giving the air-formed oxide Tb₄O₇ (mixed Tb³⁺/Tb⁴⁺), and for its uniquely strong green luminescence from the Tb³⁺ ⁵D₄→⁷F₅ transition at 543 nm. ¹⁵⁹Tb is the only natural isotope (monoisotopic, 100%, I = 3/2, NMR-active). Tb is a heavy rare-earth element (HREE), extracted primarily from ion-adsorption clay deposits and bastnäsite, with China supplying >90% of global production; it is classified as critical on EU and US materials lists.
Tb is the second-most-important element (after Dy) for enhancing NdFeB magnet coercivity at elevated temperatures: grain boundary diffusion of TbFe alloy or TbH₂ hydride into pre-sintered Nd₂Fe₁₄B magnets raises the magnetocrystalline anisotropy field at grain boundaries, enabling coercivity of 2–3 T at 150 °C — essential for EV traction motors and wind turbine generators operating above NdFeB's thermal stability limit. Terfenol-D (Tb₀.₃Dy₀.₇Fe₂), the commercial giant magnetostrictive material with λ_s ≈ 1,600 ppm, is used in high-power sonar transducers, ultrasonic cleaners, and precision actuators. Each tonne of Terfenol-D requires ~270 kg Tb.
Tb³⁺ 543 nm emission (⁵D₄→⁷F₅, ms-range lifetime) is the brightest and most efficient green luminescence available from any lanthanide, used in tri-band fluorescent lamp green phosphors (LaPO₄:Ce,Tb; GdMgB₅O₁₀:Ce,Tb), in Tb-based TRFIA immunoassay labels, and in Tb³⁺-chelate biological probes for time-gated fluorescence microscopy. Tb-doped garnet (TbIG, Tb₃Fe₅O₁₂) has the largest Faraday rotation of any known transparent material and is used in magneto-optical isolators protecting high-power laser sources from back-reflection.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 65 | 4f⁹6s²; +3 dominant in solution; +4 stable in Tb₄O₇ and TbO₂ solids. ¹⁵⁹Tb (I = 3/2) NMR-active: ¹⁵⁹Tb NMR characterises Tb³⁺ coordination in TRFIA chelate complexes, phosphor host lattices, and magneto-optical garnet films. |
| Atomic Mass | 158.925 u | Monoisotopic: ¹⁵⁹Tb (100%, stable). Single ICP-MS signal at m/z 159. ¹⁶⁰Tb (t½ = 72.3 days, β⁻ + γ, produced by neutron activation of ¹⁵⁹Tb) is used as a radiotracer and is under investigation as a theranostic radioisotope (β⁻ therapy + γ imaging) alongside ¹⁴⁹Tb (α), ¹⁵²Tb (EC/β⁺ PET imaging), and ¹⁵⁵Tb (Auger/γ SPECT) in the CERN-MEDICIS matched-quartet radioisotope platform. |
| Density (20 °C) | 8.229 g/cm³ | Heavy lanthanide density; Terfenol-D (Tb₀.₃Dy₀.₇Fe₂) has density ~9.25 g/cm³, relevant to transducer mass calculations for sonar and ultrasonic actuator design. |
| Melting Point | 1,356 °C (1,629 K) | High melting point for a heavy lanthanide. Tb metal requires Ar or vacuum processing; produced by Ca-reduction of TbF₃. |
| Boiling Point | 3,123 °C | High boiling point; Tb evaporation from TbIG and Tb-doped garnet melts must be controlled during Czochralski growth. |
| Thermal Conductivity | 11.1 W/m·K | Low conductivity typical of heavy lanthanides. Relevant to thermal management of TbIG magneto-optical isolator elements in high-power laser systems. |
| Electrical Resistivity | 115 nΩ·m (20 °C) | High resistivity; anomalous magnetoresistance below T_N (ferromagnetic below 219 K, helimagnetic 219–230 K, paramagnetic above ~230 K) is studied in fundamental rare-earth magnetism research. |
| Crystal Structure | HCP, a = 3.601 Å, c = 5.694 Å | HCP at RT; helimagnetic spin structure below 219 K with ferromagnetic order below ~220 K makes Tb the only lanthanide ferromagnet with a helimagnetic intermediate phase, extensively studied in neutron diffraction and thin-film spintronics research. |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Tensile Strength | 220 MPa | Moderate for a heavy lanthanide. Tb is malleable and can be rolled and machined under inert atmosphere. |
| Young's Modulus | 56 GPa | Low-moderate modulus. Terfenol-D has a strongly field-dependent modulus (ΔE effect, softening by ~50% in zero field), relevant to resonant actuator and harvester design. |
| Hardness | ~55–65 HB (annealed) | Moderate hardness for a heavy lanthanide; harder than light lanthanides La/Ce but softer than Lu. Tb can be machined under Ar atmosphere. |
| Elongation at Break | ~25% | Good ductility. Tb foil (99.9%+) is used as sputtering target for TbFe and TbIG thin-film deposition and as GBD alloy precursor. |
| Poisson's Ratio | 0.26 | Typical for an HCP heavy lanthanide. |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Oxidation States | +3 dominant (TbCl₃, Tb(NO₃)₃); +4 in solids (Tb₄O₇, TbO₂) | Tb⁴⁺ is a very strong oxidant in solution but is stabilised in the solid state by crystal-field effects. Tb₄O₇ (the air-stable oxide) contains both Tb³⁺ and Tb⁴⁺; reduction to Tb₂O₃ occurs above ~1,000 °C in reducing atmospheres. Tb³⁺ green phosphors require strictly +3 oxidation state; Tb₄O₇ is calcined under slightly reducing conditions before use as phosphor precursor. |
| Corrosion Resistance | Moderate; oxidises in air within days; reacts slowly with water; dissolves in dilute acids | More stable than light lanthanides; should be stored under Ar or mineral oil. Tb powder is flammable. |
| Surface Oxide | Tb₄O₇ (mixed Tb³⁺/Tb⁴⁺, dark brown) forms in air at RT | Tb₄O₇ is the thermodynamically stable air-formed oxide. Calcination above ~1,000 °C in H₂/Ar gives Tb₂O₃ (cubic); re-oxidation gives Tb₄O₇. Both forms are used as phosphor and garnet precursors; the specific phase affects stoichiometry calculations in batch compositions. |
| Identifier | Value |
|---|---|
| Symbol | Tb |
| Atomic Number | 65 |
| CAS Number | 7440-27-9 |
| UN Number | UN3089 (powder) |
| EINECS Number | 231-137-6 |
| Isotope | Type | Notes |
|---|---|---|
| ¹⁵⁹Tb | Stable | 100%; I = 3/2, NMR-active. Monoisotopic. ¹⁵⁹Tb NMR characterises Tb³⁺ coordination in TRFIA chelate complexes, green phosphor host lattices (LaPO₄:Ce,Tb), and TbIG magneto-optical garnet films. σ(thermal) = 23.4 barn. Neutron activation produces ¹⁶⁰Tb (t½ = 72.3 days, β⁻ + γ), a radiotracer and candidate theranostic isotope. The CERN-MEDICIS facility also produces ¹⁴⁹Tb (α, t½ = 4.1 hr), ¹⁵²Tb (EC/β⁺, t½ = 17.5 hr, PET imaging), and ¹⁵⁵Tb (EC + Auger, t½ = 5.32 days, SPECT), enabling a unique four-isotope matched theranostic platform for the same Tb-DOTANOC targeting vector at different dose modalities. |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Terfenol-D Magnetostrictive Research | Terfenol-D (Tb₀.₃Dy₀.₇Fe₂) single crystals and polycrystalline rods; Tb metal (99%+) as alloy input | Terfenol-D (λ_s ≈ 1,600 ppm) is the benchmark giant magnetostrictive material. Research addresses <110>-textured directional solidification for maximum strain output, nanocomposite Terfenol-D/epoxy composites for broadband energy harvesting, and high-frequency sonar transducer optimisation at 1–50 kHz. |
| TbIG Magneto-Optical Isolator Research | Tb₃Fe₅O₁₂ (TbIG) and (TbBi)₃Fe₅O₁₂ single crystals; sputtered TbIG films | TbIG has the highest Faraday rotation of any oxide (~−200 deg/cm at 1,064 nm). Bi-substituted TbIG thin films (Faraday rotation > −1,000 deg/cm) are investigated for integrated photonic isolators on Si and InP platforms, eliminating the need for bulk optical isolators in on-chip laser protection. |
| Tb³⁺ TRFIA / Time-Gated Fluorescence | Tb-DOTA, Tb-DTPA, and proprietary Tb³⁺ chelate labels; Tb-cryptate complexes | Tb³⁺ ms-lifetime green emission (⁵D₄→⁷F₅, 543 nm) enables time-gated detection with background rejection of ns-lifetime autofluorescence. Applied in TRFIA immunoassays (DELFIA platform), HTRF homogeneous assays, and live-cell time-gated FLIM microscopy for protein–protein interaction mapping. |
| Tb Radioisotope Research (CERN-MEDICIS) | Isotope-separated ¹⁴⁹Tb, ¹⁵²Tb, ¹⁵⁵Tb, ¹⁶⁰Tb produced at ISOLDE/MEDICIS | The four Tb radioisotopes provide matched α (¹⁴⁹Tb), β⁺-PET (¹⁵²Tb), Auger/SPECT (¹⁵⁵Tb), and β⁻ (¹⁶⁰Tb) emissions with identical coordination chemistry — enabling pre-clinical dose-escalation studies, imaging-guided therapy planning, and theranostic treatment using the same targeting vector across multiple modalities. |
Industrial & Commercial Applications
| Sector | Form / Grade Used | Description |
|---|---|---|
| NdFeB Magnet Coercivity Enhancement (GBD) | TbFe alloy or TbH₂ for grain boundary diffusion (GBD); Tb metal (99%+) | Grain boundary diffusion of Tb into pre-sintered NdFeB raises coercivity at 150 °C from ~0.8 T (undoped) to ~2.5–3 T, enabling use in EV traction motors and wind turbine generators without full bulk Tb substitution. GBD uses ~3–5 kg Tb per tonne of magnet, minimising Tb cost while maximising coercivity gain at grain boundaries where magnetisation reversal nucleates. |
| Terfenol-D Transducers | Terfenol-D rods (polycrystalline or directionally solidified); Tb metal (93–99%) as alloy input | Terfenol-D transducers are used in naval sonar systems (low-frequency, high-power), ultrasonic cleaning and welding equipment, active vibration control, and precision linear actuators. The material's large magnetostrictive strain and high energy density make it superior to PZT for high-force, moderate-displacement actuation. |
| Green Phosphors (Fluorescent Lamps & Displays) | Tb₄O₇ or Tb₂O₃ (99%+) calcined into LaPO₄:Ce,Tb or GdMgB₅O₁₀:Ce,Tb host lattices | Tb³⁺ ⁵D₄→⁷F₅ at 543 nm is the standard green emission for tri-band fluorescent lamps. Although being displaced by LED in many markets, Tb-based phosphors remain in use in high-CRI tri-band lamps for retail and art lighting. Tb:YAG (Y₃Al₅O₁₂:Tb) provides green emission in X-ray scintillator screens. |
| TbIG Magneto-Optical Isolators | TbIG or Bi-TbIG crystals (99.9%+ Tb₂O₃ precursor); bulk and thin-film formats | Bulk TbIG Faraday rotators are integrated into commercial optical isolators protecting Nd:YAG and fibre laser sources from back-reflected light. Each isolator contains a few grams of TbIG crystal. High-Bi TbIG films are produced by LPE or sputtering for next-generation integrated photonic isolator research. |
| Purity | Applications | Notes |
|---|---|---|
| 93% (1N3) | Bulk magnetostrictive alloys and phosphor base materials. | Industrial-grade purity for high-volume functional materials. |
| 99% (2N) | Phosphor compounds and optical ceramics. | Intermediate grade offering balance of cost and performance. |
| 99.9% (3N) | Advanced luminescent materials and R&D. | High-purity grade for electronics, photonics, and analytical research. |
| Synonym / Alternative Name | Context |
|---|---|
| Tb | Chemical symbol; from Ytterby (Sweden), the village that also gave its name to Er, Yb, and Y. Used in TbFe GBD alloy specs, Terfenol-D datasheets, and ICP-MS REE databases (m/z 159, no isobaric interference). |
| Tb metal | Commercial form designation for ingot, rod, foil, or powder. Used in GBD alloy procurement, Terfenol-D alloy preparation, and TbIG sputtering target specs. |
| Tb element | Scientific designation used in crystal structure, magnetism (helimagnetic ordering), and NMR spectroscopy literature. |
| Terbium metal | Full commercial designation in REACH/RoHS documentation and ASTM REE metal standards. |
| Terbium element | Used in academic databases, CERN-MEDICIS radioisotope platform publications, and phosphor chemistry texts. |
| Terbium rare earth metal | Trade designation; Tb is classified as highly critical (HREE) on EU and US critical materials lists for its non-substitutable role in NdFeB GBD coercivity enhancement and Terfenol-D, with >90% supply from China. |
| Terbium rare earth element | Geochemical designation in HREE deposit assessments (ion-adsorption clays in southern China) and chondrite-normalised REE pattern databases. |
| Element 65 | Periodic table designation used in XRF/ICP-MS software, nuclear data libraries (¹⁵⁹Tb neutron cross-section, ¹⁶⁰Tb activation data), and reactor physics codes. |