Understanding Thulium from Xenotime — The Rarest Stable Lanthanide

Per Teodor Cleve discovered thulium in 1879 while working at Uppsala University in Sweden. He separated erbia into three components: holmia, erbia proper, and thulia — a greenish-tinged oxide. He named it thulium after Thule, the ancient name for Scandinavia or the most northerly land known to the Greeks and Romans. Thulium was among the last lanthanides discovered, requiring exceptionally tedious fractional crystallization to isolate.

Thulium is the rarest stable lanthanide at 0.52 ppm crustal abundance — rarer than silver. Only promethium (which has no stable isotopes) is less abundant among the lanthanides. Thulium concentrates in xenotime at 0.5-1% of the rare earth fraction and in ionic adsorption clays. Annual production is approximately 50 tonnes of thulium oxide, making it one of the smallest rare earth markets. It is priced at $50-200 per kilogram of oxide.

Thulium-170 (half-life 128.6 days) emits soft gamma rays at 84 keV — ideal for portable X-ray devices. When thulium-169 is irradiated in a nuclear reactor, it produces Tm-170, which can be sealed in a small capsule and used as an X-ray source in remote locations without electricity. These portable radiographic units are used for non-destructive testing of welds in pipelines, bridges, and aircraft structures in the field.

The thulium-doped fiber laser emits at 1,940-2,013 nm in the 'eye-safe' infrared band. This wavelength is strongly absorbed by water, making it effective for soft-tissue surgery while being safe for retinal exposure at low powers. Tm:fiber lasers are used for lithotripsy (kidney stone fragmentation), prostate surgery (ThuLEP), and as pump sources for mid-infrared optical parametric oscillators used in chemical sensing and environmental monitoring.

Tm³⁺ emits blue light at 450-475 nm, used in the blue phosphor component of euro banknote security features. Under UV illumination, genuine euros show terbium green, europium red, and thulium blue fluorescence in specific patterns. Thulium-doped upconversion phosphors also produce blue emission when pumped with 980 nm infrared — used in anti-counterfeiting labels and security inks that are invisible until excited by specific wavelengths.

Thulium barium copper oxide (TmBCO or TmBa₂Cu₃O₇) is a high-temperature superconductor with a critical temperature of 88 K — part of the REBCO (rare earth barium copper oxide) family. TmBCO thin films are grown on buffered metal substrates to create superconducting tapes for MRI magnets, particle accelerators, and fusion reactor magnets. The small ionic radius of Tm³⁺ creates favorable strain effects in epitaxial film growth.

Thulium is a bright silvery metal that is soft and malleable. Melting point is 1545°C, density 9.32 g/cm³. It oxidizes slowly in air and reacts slowly with water. Thulium has thirteen stable and radioactive isotopes. The metal is ferromagnetic below 32 K and exhibits an interesting sinusoidal magnetic structure between 32 K and 56 K. Thulium is obtained by calcium reduction of thulium fluoride or by metallothermic reduction.

Thulium-doped yttrium lithium fluoride (Tm:YLF) crystals produce laser output at 1,880-1,910 nm for atmospheric LIDAR applications — detecting water vapor concentration profiles in the atmosphere for weather forecasting. Thulium-doped silicon is being researched for photonic integrated circuits operating at 1,800-2,000 nm. Tm-171 is used in Mössbauer spectroscopy studies of electronic structure in compounds.

Thulium's rarity creates a paradox: it is produced as a byproduct of heavy rare earth separation but has limited commercial demand. Most thulium oxide ends up in stockpiles or is sold to researchers at relatively modest prices despite its scarcity. Growing interest in thulium fiber lasers for surgery and defense applications may increase demand. Recycling is negligible due to the tiny quantities used in each application.

Record thulium's key data: atomic number 69, density 9.32 g/cm³, melting point 1545°C, bright silvery metal. Thulium is the least used lanthanide commercially, yet it has unique properties that are irreplaceable in niche applications. The development of high-power thulium fiber lasers is the most promising growth area — their 2 μm wavelength offers advantages in surgery, remote sensing, and defense that no other laser medium can match.