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Thulium
Thulium (Tm, atomic number 69) is the second-rarest naturally occurring lanthanide (~0.5 ppm crustal), a silvery-gray HCP metal with a melting point of 1,545 °C and a 4f¹³6s² configuration that leaves a single hole in the 4f shell — giving Tm³⁺ (4f¹²) a modest magnetic moment and Tm²⁺ (4f¹³) a half-filled-shell-like electronic structure accessible under strongly reducing conditions. ¹⁶⁹Tm is the only natural isotope (monoisotopic, 100%, stable, I = 1/2, NMR-active). Tm is a heavy rare-earth element extracted primarily from ion-adsorption clays and xenotime, with China the dominant producer.
Tm:YAG and Tm:YLF lasers operating at ~2,013–2,090 nm (³H₄→³F₄ transition) are the dominant surgical lasers for urological soft-tissue procedures, including holmium-sparing applications where the shorter wavelength of Tm relative to Ho:YAG gives superior soft-tissue coagulation and vaporisation with reduced retropulsion of kidney stones in ureteroscopy. Tm fibre lasers (Tm³⁺-doped silica or ZBLAN fibre, 1,900–2,050 nm, pumped at 793 nm by AlGaAs diodes) have reached >1 kW CW output and are used for polymer welding, metal cutting, and as pumps for Ho:YAG and Ho:YLF surgical lasers. The 2 µm wavelength is eye-safer than 1 µm Nd:YAG at equivalent power.
¹⁷⁰Tm (t½ = 128.6 days, β⁻ + soft X-ray, produced by neutron irradiation of ¹⁶⁹Tm) was historically the primary portable X-ray source for field radiography (weld inspection, dental imaging) before being largely displaced by miniaturised X-ray tubes, and remains in use in security screening and small-bore pipe weld inspection where electrical power is unavailable. Tm³⁺ emission at 800 nm and 1,470 nm (in fluoride glasses) is used for NIR upconversion phosphors and for optical amplification in the S-band (1,460–1,530 nm) of optical communications, extending fibre amplifier coverage beyond the EDFA C-band.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 69 | 4f¹³6s²; +3 dominant in all practical applications (Tm³⁺: TmCl₃, Tm₂O₃). Tm²⁺ is accessible under strongly reducing conditions (e.g. TmI₂ in THF) but less stable than Sm²⁺ or Yb²⁺. ¹⁶⁹Tm (I = 1/2) is NMR-active; the only lanthanide with I = 1/2, giving narrow ¹⁶⁹Tm NMR lines analogous to ¹H or ¹³C NMR. |
| Atomic Mass | 168.934 u | Monoisotopic: ¹⁶⁹Tm (100%, stable). Single ICP-MS signal at m/z 169. ¹⁷⁰Tm (t½ = 128.6 days, β⁻ 968 keV max + 84 keV X-ray; produced by ¹⁶⁹Tm(n,γ), σ = 105 barn) is used as a portable radiographic source. |
| Density (20 °C) | 9.321 g/cm³ | High density consistent with position near the end of the lanthanide series and the full lanthanide contraction. Relevant to Tm fibre and crystal laser mass calculations. |
| Melting Point | 1,545 °C (1,818 K) | High melting point; Tm metal requires Ar or vacuum processing. Produced by Ca-reduction of TmF₃. Relatively stable in dry air compared to light lanthanides. |
| Boiling Point | 1,950 °C | Relatively low boiling point for a heavy lanthanide (analogous to Sm's low boiling point). Tm evaporation must be controlled during Tm:YAG Czochralski crystal growth. |
| Thermal Conductivity | 17.0 W/m·K | Moderate conductivity for a heavy lanthanide. Relevant to thermal management of Tm:YAG laser crystal rods under high-average-power diode pumping. |
| Electrical Resistivity | 142 nΩ·m (20 °C) | High resistivity. Tm is antiferromagnetic below 56 K and has a ferrimagnetic intermediate phase. Magnetic ordering temperatures are low, relevant only in cryogenic fundamental research. |
| Crystal Structure | HCP, a = 3.538 Å, c = 5.554 Å | HCP structure; small lattice parameters reflecting near-complete lanthanide contraction (only Lu is smaller). Transforms to BCC above ~1,400 °C. |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Tensile Strength | 210 MPa | Moderate for a heavy lanthanide. Tm can be machined and rolled under inert atmosphere; foil is used as sputtering target for Tm-doped oxide film deposition. |
| Young's Modulus | 56 GPa | Low-moderate modulus. Used in thermal stress modelling of Tm:YAG laser crystal rods. |
| Hardness | ~52–60 HB (annealed) | Moderate hardness; harder than light lanthanides. Tm can be machined under brief dry-air exposure. |
| Elongation at Break | ~20% | Good ductility. Tm foil (99.7%+) is used as sputtering target and alloy precursor. |
| Poisson's Ratio | 0.23 | Slightly lower than typical HCP lanthanides. |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Oxidation States | +3 dominant (TmCl₃, Tm₂O₃, Tm(NO₃)₃); +2 rare (TmI₂) | Tm³⁺ (4f¹², ³H₆ ground state) follows standard trivalent lanthanide chemistry. Tm²⁺ is less stable than Sm²⁺ or Yb²⁺ and is mainly of academic interest. Tm³⁺ upconversion emission (green/blue via ³H₄→³H₆) is exploited in Tm-doped NaYF₄ upconversion nanoparticles for bioimaging. |
| Corrosion Resistance | Moderate; more air-stable than light lanthanides; reacts slowly with water; dissolves in dilute acids | Tm can be handled briefly in dry air. Fine powder should be stored under Ar. More stable than Nd or Pr in equivalent conditions. |
| Surface Oxide | Tm₂O₃ (cubic C-type) forms in air | Tm₂O₃ is the precursor for Tm:YAG crystal growth, Tm fibre laser preform doping, and ¹⁷⁰Tm production target preparation (Tm₂O₃ pellets irradiated in high-flux reactor). |
| Identifier | Value |
|---|---|
| Symbol | Tm |
| Atomic Number | 69 |
| CAS Number | 7440-30-4 |
| UN Number | UN3089 (powder) |
| EINECS Number | 231-140-2 |
| Isotope | Type | Notes |
|---|---|---|
| ¹⁶⁹Tm | Stable | 100%; I = 1/2, NMR-active. Monoisotopic. The only lanthanide with I = 1/2 gives narrow, well-resolved ¹⁶⁹Tm NMR signals used to characterise Tm³⁺ coordination in laser crystal precursors, upconversion nanoparticle shells, and Tm-doped fibre preform glass. σ(thermal) = 105 barn; ¹⁶⁹Tm(n,γ)¹⁷⁰Tm produces the portable X-ray source isotope ¹⁷⁰Tm (t½ = 128.6 days, β⁻ 968 keV max + characteristic 84 keV X-rays) used in field radiography of welds and castings where mains power is unavailable. Enriched ¹⁶⁹Tm₂O₃ targets are irradiated in high-flux reactors to produce ¹⁷⁰Tm sources at ~1–5 TBq activity. |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Tm Fibre Laser Research | Tm³⁺-doped silica fibre (0.5–3 wt% Tm₂O₃ in core glass); Tm-doped ZBLAN fibre for mid-IR extension | Tm fibre lasers (1,850–2,050 nm) are pumped by 793 nm AlGaAs diodes with ~80% quantum efficiency via the ³H₆→³H₄ cross-relaxation two-for-one photon process. Research addresses power scaling beyond 1 kW CW, ultra-short pulse generation (mode-locked at <100 fs), and mid-IR supercontinuum generation in Tm-doped ZBLAN for spectroscopy and medical applications. |
| Tm:YAG & Tm:YLF Laser Research | Tm:YAG (3–5 at% Tm, Czochralski); Tm:YLF rods and slabs | Tm:YAG (2,013 nm) and Tm:YLF (1,908 nm and 1,991 nm) are studied for intracavity-pumped Ho:YAG surgical lasers (1,908 nm Tm → 2,090 nm Ho), for direct urological soft-tissue laser surgery, and for coherent LIDAR wind-velocity measurement at 2 µm where CO₂ absorption lines are accessible. |
| Tm³⁺ Upconversion Nanoparticles | Tm-doped NaYF₄ core/shell nanoparticles (0.5–2 mol% Tm); sensitised by Yb³⁺ co-dopant | Tm³⁺/Yb³⁺ co-doped NaYF₄ nanoparticles convert 980 nm NIR excitation to blue (450 nm, ¹D₂→³F₄), red (660 nm), and NIR (800 nm) emissions, enabling deep-tissue bioimaging without autofluorescence. Used in multiplex immunoassays, photoactivation of photosensitisers, and super-resolution STED-like microscopy using upconversion saturation. |
| ¹⁶⁹Tm NMR Spectroscopy | TmCl₃ or Tm(NO₃)₃ solutions; Tm-doped solid powders | ¹⁶⁹Tm NMR (I = 1/2; relatively narrow lines) is used to characterise Tm³⁺ coordination in laser crystal precursor solutions, ZBLAN fibre glass formulations, and Tm-doped upconversion nanoparticle surface chemistry. The I = 1/2 spin gives simpler spectra than the I = 7/2 or I = 5/2 NMR of other lanthanides. |
Industrial & Commercial Applications
| Sector | Form / Grade Used | Description |
|---|---|---|
| Tm Fibre Lasers (Industrial & Medical) | Tm-doped double-clad silica fibre; 99.7%+ Tm₂O₃ precursor for MCVD/OVD fibre preform doping | Commercial Tm fibre lasers (>100 W to >1 kW CW) are used for polymer and thermoplastic welding, metal surface modification, and as high-brightness pump sources for Ho:YAG surgical lasers. The 2 µm wavelength is strongly absorbed by water and biological tissue, giving good haemostasis in soft-tissue surgical applications. |
| Tm:YAG Urological Surgical Lasers | Tm:YAG crystal rods (3–5 at% Tm, 99.7%+ Tm₂O₃ precursor); pulsed and CW configurations | Tm:YAG lasers at 2,013 nm are used for laser enucleation of the prostate (ThuLEP), bladder tumour ablation, and ureteral stricture treatment. The shorter wavelength compared to Ho:YAG gives reduced stone retropulsion and finer tissue ablation at the fibre tip, advantageous in ureteroscopic procedures. |
| ¹⁷⁰Tm Portable X-Ray Sources | ¹⁷⁰Tm₂O₃ ceramic discs or pellets (~1–5 TBq activity); shielded portable enclosures | ¹⁷⁰Tm sources emit characteristic 84 keV X-rays suitable for radiographic inspection of steel welds up to ~12 mm thickness and aluminium up to ~40 mm. Used in pipeline inspection, offshore weld QC, and developing-world field radiography where electrical power for X-ray tubes is unavailable. Being displaced by battery-operated X-ray tubes in many markets but retains niche use in remote environments. |
| Purity | Applications | Notes |
|---|---|---|
| 99% (2N) | Industrial laser materials and portable X-ray sources. | Mid-grade purity; balances cost and functional performance. |
| 99.7% (2N7) | High-end photonics, analytical research, and medical devices. | High-purity grade for sensitive applications. |
| Synonym / Alternative Name | Context |
|---|---|
| Tm | Chemical symbol; from Thule (ancient name for Scandinavia), named by Per Teodor Cleve in 1879. Used in Tm fibre laser datasheets, Tm:YAG surgical laser specs, and ICP-MS REE databases (m/z 169, no isobaric interference). |
| Tm metal | Commercial form designation for ingot, rod, foil, or powder. Used in Tm:YAG crystal growth procurement and sputtering target specifications. |
| Tm element | Scientific designation used in ¹⁶⁹Tm NMR spectroscopy and magnetic ordering (antiferromagnetic, ferrimagnetic phase) literature. |
| Thulium metal | Full commercial designation in REACH/RoHS documentation and ASTM REE metal standards. |
| Thulium element | Used in academic databases, Tm fibre laser physics publications, and nuclear data libraries for ¹⁷⁰Tm production and dosimetry. |
| Thulium rare earth metal | Trade designation; Tm is classified as critical on EU and US materials lists. Its rarity (~0.5 ppm crustal) and China-dominated production make supply security a concern for medical laser and nuclear applications. |
| Thulium rare earth element | Geochemical designation in HREE deposit assessments and chondrite-normalised REE pattern databases. |
| Element 69 | Periodic table designation used in XRF/ICP-MS software, nuclear data libraries (¹⁶⁹Tm neutron cross-section, ¹⁷⁰Tm β⁻ decay data), and reactor physics codes tracking ¹⁷⁰Tm as an activation product in irradiated targets. |