Thulium

Thulium is a rare earth metal belonging to the lanthanide series, symbolised by Tm and bearing the atomic number 69. It is one of the least abundant of the lanthanides, occurring in trace amounts within minerals such as monazite and xenotime. Despite its relative scarcity, thulium has gained scientific and industrial attention due to its unique optical, magnetic, and radioactive properties, which make it valuable in several niche yet impactful technological and medical applications.

Background and Discovery

Thulium was discovered in 1879 by the Swedish chemist Per Teodor Cleve, who isolated it from erbia (erbium oxide) while studying rare earth oxides. He named the new element after Thule, an ancient Greek and Roman name for the northernmost region of the known world—believed to refer to Scandinavia. Early research into thulium was hindered by its rarity and the difficulty of separating it from other lanthanides. The development of ion-exchange and solvent extraction methods in the mid-20th century allowed for purer isolation of thulium and, consequently, the exploration of its potential uses.
Thulium is a silvery-grey metal, soft and malleable, which tarnishes slowly in air. It has a melting point of 1545°C and a density of 9.32 g/cm³. The element is relatively stable when compared with other lanthanides and is easily machined or rolled into thin foils.

Chemical and Physical Properties

Thulium exhibits a typical +3 oxidation state, forming compounds such as thulium oxide (Tm₂O₃), thulium chloride (TmCl₃), and thulium fluoride (TmF₃). Its electronic configuration is [Xe] 4f¹³ 6s², giving rise to distinctive optical and magnetic characteristics.Key properties include:

• Paramagnetism: Thulium shows strong paramagnetic behaviour due to unpaired 4f electrons.
• Optical emission: It emits blue and green fluorescence under certain excitation wavelengths, useful in laser and lighting technologies.
• Radioactivity: Natural thulium is stable, but isotopes such as Thulium-170 (Tm-170) are radioactive and used in specialised applications.

Everyday and Medical Applications

Although not encountered directly in everyday consumer goods, thulium contributes indirectly to several modern conveniences and healthcare technologies. One of its most notable uses is in portable X-ray devices, where the isotope Tm-170 serves as a compact and efficient radiation source. Unlike traditional bulky X-ray machines, thulium-based sources allow for lightweight, portable imaging systems, particularly valuable in remote medical facilities and field hospitals.
In laser surgery, thulium-doped lasers are employed for precise cutting and coagulation. These thulium lasers operate typically at wavelengths around 2 µm, where water absorption is high, allowing controlled tissue ablation with minimal collateral damage. They are extensively used in urology, especially for treating benign prostatic hyperplasia (BPH), and in ENT (ear, nose, and throat) procedures.
Thulium is also used in fibre laser technology, where thulium-doped fibre amplifiers (TDFAs) serve in telecommunications and optical data transmission. These systems enable efficient signal amplification in S-band optical networks, contributing to the infrastructure behind modern internet and communication systems.

Industrial and Technological Uses

Industrial applications of thulium extend to both traditional and advanced technologies:

• Nuclear Reactors: Thulium’s isotopic form, Tm-170, is used as a radiation source for calibration and radiography. Its relatively soft gamma radiation makes it suitable for industrial inspection without requiring extensive shielding.
• Lighting: Thulium-doped phosphors are employed in high-intensity discharge lamps and fluorescent lights to generate blue hues, improving colour rendering.
• Magnets and Alloys: Although not commonly used in bulk magnet production, thulium can enhance the magnetic performance of specific samarium–cobalt and neodymium–iron–boron alloys for scientific instruments.
• Electronics: Thulium oxide is used in certain semiconductor and thin-film coating processes, providing optical filtering or luminescent effects in precision devices.

Economic Significance and Supply

Thulium is one of the rarest and most expensive rare earth elements, often extracted as a by-product of heavy rare earth processing. Global production is estimated to be only a few tens of tonnes annually. Major sources include deposits in China, which dominates the world’s rare earth market, as well as smaller contributions from Australia, India, and the United States.
Due to its scarcity, thulium’s market value remains high, making its usage limited to situations where its unique properties justify the cost. The refinement process is complex and energy-intensive, relying on solvent extraction from mixed lanthanide ores. As demand grows for rare earth elements in green technologies, thulium’s role in lasers, medical devices, and fibre optics adds strategic importance to its economic value.
The sustainability of thulium supply is closely tied to the global rare earth supply chain, which is vulnerable to geopolitical and environmental factors. Research into recycling electronic waste and developing alternative extraction technologies is ongoing to ensure long-term availability.

Research and Emerging Applications

Contemporary research continues to reveal new possibilities for thulium-based materials. Advances in solid-state physics and quantum electronics have explored thulium’s potential as a quantum memory medium due to its narrow optical transitions and long coherence times. In addition, thulium-doped upconversion nanoparticles are being investigated for use in biomedical imaging, drug delivery, and photodynamic therapy, where they convert infrared light to visible emissions suitable for deep-tissue imaging.
Another growing area is environmental sensing, where thulium’s optical properties enable the creation of infrared sensors for monitoring gases and pollutants. Its high stability under thermal stress makes it valuable in high-temperature sensor applications in industrial furnaces and combustion systems.

Environmental and Safety Considerations

Thulium and its compounds are generally considered to have low toxicity, although standard precautions are necessary during handling, especially in powdered or dust form. Radioactive thulium isotopes require specialised containment and disposal protocols. Environmental impact concerns are primarily linked to mining and refining processes, which can produce toxic waste and radiation exposure if not properly managed. The rare earth extraction industry has increasingly adopted cleaner separation technologies to mitigate such impacts.

Broader Context and Relevance

In the broader landscape of materials science, thulium represents a case of niche high-value utilisation—a metal whose applications are limited in scale but essential in precision industries. Its contribution to medical imaging, laser surgery, telecommunications, and nuclear safety underscores its importance in both civil and technological progress. As research continues, thulium’s integration into next-generation optical systems and quantum devices may further enhance its industrial and scientific relevance.