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Holmium
Holmium (Ho, atomic number 67) is a heavy lanthanide with HCP structure, melting point of 1,472 °C, and the highest magnetic moment of any naturally occurring element in its trivalent ion form (Ho³⁺, ~10.6 µB, J = 8). Ho is monoisotopic — ¹⁶⁵Ho is the sole stable isotope (100% natural abundance, I = 7/2, NMR-active) — making it especially useful for precise spectroscopic calibration. It is moderately reactive, tarnishing slowly in dry air and reacting with water and acids; it is extracted from monazite and xenotime ores alongside other heavy lanthanides. Global production is only a few tonnes per year, reflecting its specialized applications.
The Ho:YAG laser (2,090 nm) is the dominant mid-infrared surgical laser for urology (kidney stone lithotripsy, BPH treatment), arthroscopy, and soft tissue surgery — replacing Nd:YAG in many procedures because the 2.09 µm wavelength is strongly absorbed by water (~80 cm⁻¹) and hydroxyapatite, giving precise, shallow tissue ablation with minimal thermal damage to surrounding structures. Ho:YAG is typically pumped by a Tm fiber laser or a thulium-doped fiber pump at 1,908 nm, which efficiently populates the ⁵I₇ → ⁵I₈ Ho³⁺ laser transition at 2.09 µm. The Ho:YSGG variant (2.09 µm) and fiber-delivered Ho:YAG (via zirconium fluoride or silver halide fibers) are standard tools in endoscopic urology. Eye-safe Ho lasers are also used for atmospheric CO₂ and H₂O LIDAR and wind profiling.
Holmium's extreme magnetic moment makes Ho metal and Ho-doped materials valuable for generating ultra-high magnetic field concentrators: Ho poles in hybrid permanent magnet / superconducting magnet designs focus flux to >20 T in small volumes, and Ho nuclear spin (I = 7/2) has been investigated for single-atom qubit storage with sub-angstrom magnetic bit footprints. ¹⁶⁵Ho(n,γ)¹⁶⁶Ho produces ¹⁶⁶Ho (t½ = 26.8 hr, β⁻ + γ), a theranostic radioisotope used for radioembolization of liver tumors (HepaSphere-¹⁶⁶Ho microspheres) and for bone metastasis palliation, combining SPECT imaging capability with β⁻ tissue dose delivery. Ho₂O₃ is also the standard wavelength calibration reference for NIR spectrophotometers (sharp 4f absorption bands at 444, 537, 640, 1,153, 1,461 nm).
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 67 | 4f¹¹6s²; +3 is the only stable oxidation state. Ho³⁺ (4f¹⁰, ⁵I₈, J = 8) has the highest Hund's rule magnetic moment of any tripositive lanthanide ion (~10.6 µB). ¹⁶⁵Ho (I = 7/2) is NMR-active; the only stable Ho isotope, making Ho monoisotopic. |
| Atomic Mass | 164.930 u | Monoisotopic element — ¹⁶⁵Ho is the sole stable isotope (100% natural abundance). This simplifies ICP-MS analysis (no multi-isotope corrections) and makes Ho a useful internal standard in REE analytical work. ¹⁶⁵Ho(n,γ)¹⁶⁶Ho (σ = 64 barn) produces ¹⁶⁶Ho (t½ = 26.8 hr), the clinically used radiotherapeutic isotope. |
| Density (20 °C) | 8.795 g/cm³ | Moderate-high density consistent with the heavy lanthanide series. Relevant to Ho:YAG crystal mass calculations in laser cavity design and to Ho microsphere loading calculations in ¹⁶⁶Ho radioembolization treatment planning. |
| Melting Point | 1,472 °C (1,745 K) | High melting point for a heavy lanthanide. Ho oxidizes above ~200 °C in air; vacuum arc or Ar-atmosphere induction melting is required for bulk Ho metal processing. |
| Boiling Point | 2,700 °C | Moderately high boiling point. Relevant to Ho evaporation losses during vacuum arc melting of Ho-containing alloys and to HoF₃ or Ho₂O₃ sputtering target fabrication for thin-film deposition. |
| Thermal Conductivity | 16.2 W/m·K | Low-moderate conductivity typical of heavy lanthanides. Relevant to thermal management of Ho:YAG laser crystals under high-repetition-rate pulsed operation, where thermal lensing limits average output power. |
| Electrical Resistivity | 79 nΩ·m (20 °C) | Moderate resistivity consistent with heavy lanthanides. Shows anomalous increases near the magnetic ordering temperatures (Néel: ~132 K, ferromagnetic below ~20 K) due to spin-disorder scattering — studied in the context of rare-earth magnetism. |
| Crystal Structure | HCP (α-Ho), a = 3.578 Å, c = 5.618 Å | HCP stable at RT; BCC above ~1,400 °C. Ho is helical antiferromagnetic below 132 K (Néel) and has a more complex conical ferromagnetic structure below ~20 K. Ho metal poles in superconducting magnets concentrate magnetic flux, exploiting Ho's large magnetization and very large magnetic susceptibility at low temperatures. |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Tensile Strength | ~225 MPa (approximate) | Approximate value for annealed Ho; Ho is not used structurally. Relevant to Ho foil and rod processing for sputtering targets and neutron flux monitor applications. |
| Young's Modulus | 64 GPa | Low-moderate modulus typical of heavy lanthanides. Used in thermal stress modeling of Ho:YAG laser crystals and in Ho₂O₃ ceramic compressive strength calculations. |
| Hardness | ~60–65 HB (annealed) | Moderate hardness for an annealed heavy lanthanide. Ho can be machined under inert atmosphere and rolled into foil for sputtering targets and neutron flux monitor applications. |
| Elongation at Break | ~15% | Moderate ductility in high-purity annealed form. Ho foil is used as a sputtering target for Ho-doped film deposition and as a neutron flux monitor (¹⁶⁵Ho activation analysis). |
| Poisson's Ratio | 0.23 | Typical for an HCP lanthanide. Used in FEA modeling of Ho:YAG crystal thermal gradients under pulsed pump loading. |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Oxidation States | +3 only (Ho³⁺: HoCl₃, Ho₂O₃, Ho(NO₃)₃) | Ho³⁺ (4f¹⁰, ⁵I₈) follows standard trivalent lanthanide chemistry — stable salts, precipitates as Ho(OH)₃ above pH ~7, chelates with EDTA and macrocyclic ligands. The sharp 4f absorption bands of Ho³⁺ in solution and in Ho₂O₃ glass are used as wavelength calibration references for NIR spectrophotometers (certified reference material, 444–1,461 nm). |
| Corrosion Resistance | Moderate in dry air; tarnishes slowly; reacts with water and dilute acids | Ho is more air-stable than the light lanthanides (La, Ce, Pr, Nd) but should be stored under inert atmosphere for prolonged periods. Ho powder is potentially flammable. Dissolves readily in dilute HCl and HNO₃ to give pale yellow Ho³⁺ solutions. |
| Surface Oxide | Ho₂O₃ (cubic C-type, pale yellow) forms in air | Ho₂O₃ is the primary precursor for Ho:YAG crystal growth (Ho₂O₃ + Y₂O₃ + Al₂O₃ melt by Czochralski), for ¹⁶⁶Ho microsphere fabrication (Ho₂O₃ neutron activation), and as a NIR wavelength calibration standard. Ho₂O₃ is also studied as a high-κ gate dielectric (κ ~ 12–15) for next-generation CMOS transistors. |
| Identifier | Value |
|---|---|
| Symbol | Ho |
| Atomic Number | 67 |
| CAS Number | 7440-60-0 |
| UN Number | Not classified |
| EINECS Number | 231-169-0 |
| Isotope | Type | Notes |
|---|---|---|
| ¹⁶⁵Ho | Stable | 100% natural abundance; I = 7/2, NMR-active. Ho is monoisotopic — ¹⁶⁵Ho is the only stable holmium isotope. ¹⁶⁵Ho NMR (I = 7/2; large quadrupole moment; broad lines in low-symmetry environments) is used to characterize Ho³⁺ coordination in optical host crystals and Ho-doped glasses relevant to laser and spectroscopic applications. Thermal neutron absorption σ = 64 barn; ¹⁶⁵Ho(n,γ)¹⁶⁶Ho produces ¹⁶⁶Ho (t½ = 26.8 hr, β⁻ 1.855 MeV max + 80.6 keV γ), which is activated by neutron irradiation of Ho₂O₃ microspheres or poly(L-lactic acid) microspheres for hepatic radioembolization of liver tumors (HepaSphere-¹⁶⁶Ho, QuiremSpheres). The 80.6 keV γ enables simultaneous SPECT imaging for dosimetry, combining therapy and imaging (theranostic approach) in a single administration. Ho is also a monoisotopic internal standard for ICP-MS REE analysis (no isotope overlap corrections needed). |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Quantum Bit (Single-Atom Qubit) Research | Single Ho atoms deposited on MgO/Ag(100) surfaces by STM; Ho atoms in Ho₂Pc₂ double-decker phthalocyanine complexes | Single Ho atoms on MgO(100) surfaces exhibit bistable magnetic states with hyperfine-split levels from the I = 7/2 nuclear spin, giving 8 nuclear spin states resolvable by spin-polarized STM at 0.6 K. Demonstrated as a single-atom bit with sub-angstrom footprint (Natterer et al., Nature 2017). Ho₂Pc₂ single-molecule magnets show high blocking temperatures and nuclear spin coherence, pursued as molecular qubits for quantum computing. |
| Ho:YAG & Ho:YSGG Laser Research | Ho:YAG single crystals (0.5–2 at% Ho, Czochralski-grown); Ho:YLF; fiber-delivered via ZrF₄ or hollow-core fibers | Ho:YAG (2,090 nm, ⁵I₇→⁵I₈ transition) research focuses on high-average-power operation (cryogenic cooling), efficient Tm-fiber pumping at 1,908 nm, ultrashort pulse generation (mode-locked Ho:YAG for sub-picosecond mid-IR pulses), and fiber delivery for endoscopic applications. The 2.09 µm output is strongly absorbed by water and tissue, making it the primary wavelength for precision urological and orthopedic laser research. |
| Magnetic Pole & Flux Concentrator Research | Ho metal poles machined to shape (99.9%+); Ho-doped permanent magnet composites | Ho's extremely large saturation magnetization at low temperatures and high magnetic permeability make Ho metal poles effective flux concentrators in hybrid superconducting magnets (NbTi/Nb₃Sn + Ho pole inserts), enabling fields above 20 T in small bore volumes. Used in high-field NMR magnet shimming and in compact high-field research magnets for condensed matter experiments. |
| NIR Wavelength Calibration | Ho₂O₃ in glass reference standard; aqueous Ho³⁺ solution (certified reference material) | Ho₂O₃ glass and Ho³⁺ solution are NIST-traceable wavelength calibration standards for NIR spectrophotometers — Ho³⁺ 4f absorption bands at 444, 537, 640, 1,153, and 1,461 nm are sharp, temperature-stable references used to verify and correct wavelength accuracy in analytical instruments per ASTM E275 and USP ‹857›. |
| ¹⁶⁶Ho Radiotherapy Research | Ho₂O₃ powder neutron-activated to ¹⁶⁶Ho; ¹⁶⁶Ho-DOTMP chelate for bone metastasis | ¹⁶⁶Ho (t½ = 26.8 hr, β⁻ + 80.6 keV γ) combines a high-energy β⁻ for tissue dose delivery with a γ suitable for SPECT imaging, enabling dose verification in the same patient. Research focuses on optimized Ho₂O₃ microsphere size distribution for lobar vs. segmental liver radioembolization, on ¹⁶⁶Ho-DOTMP for bone metastasis palliation, and on dosimetry algorithms using SPECT/CT ¹⁶⁶Ho imaging. |
Industrial & Commercial Applications
| Sector | Form / Grade Used | Description |
|---|---|---|
| Medical & Surgical Lasers (Ho:YAG) | Ho:YAG laser crystals (1–2 at% Ho, 99.9%+ Ho₂O₃ precursor); pulsed systems 0.2–100 J/pulse | Ho:YAG (2,090 nm) is the standard laser for endoscopic urology — laser lithotripsy of kidney and ureteric stones, BPH (benign prostatic hyperplasia) vaporization (HoLEP), and bladder tumor resection. The 2.09 µm wavelength is strongly absorbed by both water and stone mineral phases; pulse shaping controls fragmentation vs. dust generation. Ho:YAG systems are also used for arthroscopic cartilage ablation and soft tissue surgical procedures across general, ENT, and gynecological surgery. |
| Nuclear Reactor Control & Flux Monitoring | Ho₂O₃ or Ho metal foils as neutron flux monitors; Ho-containing control rod composites | ¹⁶⁵Ho foil activation (¹⁶⁵Ho(n,γ)¹⁶⁶Ho, σ = 64 barn) is used for thermal neutron flux measurement in research reactors. Ho₂O₃ is also investigated as a neutron absorber component in advanced reactor control rod composites, valued for its moderate cross-section and the favorable decay properties of ¹⁶⁶Ho for post-irradiation dosimetry. |
| Glass Colorant & Optical Calibration Standards | Ho₂O₃ powder additions (0.1–2 wt%) to specialty glass; Ho₂O₃ in borosilicate reference glass | Ho₂O₃ additions to glass produce characteristic yellow-orange coloration from Ho³⁺ 4f absorption bands, used in decorative specialty glass and optical filter fabrication. Ho-doped borosilicate glass (Schott HO) is an ASTM / USP certified wavelength calibration reference for UV-Vis-NIR spectrophotometers, present in analytical laboratories worldwide. |
| Magneto-Optical Devices | Ho:YIG (Ho-substituted yttrium iron garnet) films and crystals; Ho-doped optical fiber | Ho substitution in YIG (Y₃₋ₓHoₓFe₅O₁₂) increases the Faraday rotation at 1.3–1.5 µm telecom wavelengths, enabling more compact Faraday rotators and optical isolators for fiber communications. Ho-doped fiber is studied for 2 µm fiber amplifiers (for Tm/Ho co-doped fiber laser systems) and for mid-IR supercontinuum generation in Ho:ZBLAN fluoride fibers. |
| Purity | Main Uses | Notes |
|---|---|---|
| 99% (2N) | Glass coloring, nuclear shielding, alloy development. | Standard grade for bulk industrial applications; lower cost. |
| 99.9% (3N) | Scientific research, magneto-optical devices, laser systems. | Preferred for research and high-performance technologies. |
| Synonym / Alternative Name | Context |
|---|---|
| Ho | Chemical symbol; from Holmia (Latin for Stockholm), named by Per Teodor Cleve in 1879. Used in ICP-MS REE analysis (Ho as monoisotopic internal standard), Ho:YAG laser crystal specifications, and ¹⁶⁶Ho radioembolization microsphere product documentation (QuiremSpheres, HepaSphere). |
| Ho metal | Commercial designation for elemental Ho in ingot, rod, foil, or powder form. Used in superconducting magnet pole specifications, sputtering target datasheets, and STM single-atom deposition research publications specifying Ho metal evaporation source purity. |
| Ho element | Scientific designation distinguishing elemental Ho from Ho compounds. Used in condensed matter physics literature on Ho magnetic ordering, single-atom qubit papers, and spectroscopy publications characterizing Ho³⁺ optical transitions in laser host crystals. |
| Holmium metal | Full commercial designation used in REACH/RoHS compliance documentation, ASTM REE metal standards, and procurement specifications for Ho:YAG crystal growth precursor materials and Ho flux concentrator pole fabrication. |
| Holmium element | Used in academic databases (WebElements, NIST), educational resources, and nuclear engineering texts specifying ¹⁶⁵Ho neutron activation cross-section (64 barn) and ¹⁶⁶Ho radiotherapy properties. |
| Holmium rare earth metal | Trade and regulatory designation classifying Ho among the heavy rare-earth elements (HREEs); used in critical materials supply chain assessments and policy documents for Ho-dependent medical laser and radiotherapy applications. |
| Holmium rare earth element | Geochemical and mineralogical designation used in REE deposit assessments (xenotime, monazite), IUPAC nomenclature for Ho-bearing mineral phases, and in environmental REE monitoring studies. |
| Element 67 | Periodic table designation used in XRF/ICP-MS software, nuclear data libraries (ENDF/B-VIII for ¹⁶⁵Ho cross-section data), and reactor physics codes tracking ¹⁶⁶Ho activation and decay in neutron-irradiated Ho-containing materials. |