Thulium (Tm)

Thulium is routinely described as the rarest of the stable rare earths, which is true, and almost always taken to mean there is very little of it, which is not. Thulium's crustal abundance is around half a part per million — several times that of silver, and more than a hundred times that of gold.

The difficulty is not quantity. It is that thulium has never been concentrated into an ore of its own anywhere on Earth. It occurs as a minor constituent of monazite and the ion-adsorption clays, chemically almost indistinguishable from thirteen close relatives, and the only way to get it is to separate it from all of them.

Why the odd-numbered elements lose

There is a real reason thulium sits in an abundance trough, and it is nuclear rather than geological. Plot the abundance of the elements against atomic number and the line zigzags: even-numbered elements are consistently more abundant than their odd-numbered neighbours, sometimes by a factor of ten.

The cause is nuclear stability. Protons and neutrons pair up, and a nucleus with an even number of protons has more ways to arrange itself into a tightly bound configuration. Even-Z elements therefore have more stable isotopes and are produced more efficiently by stellar nucleosynthesis. Thulium's atomic number is 69, sitting between erbium at 68 and ytterbium at 70, and it has exactly one stable isotope where each of its neighbours has six or seven. Being odd costs it roughly an order of magnitude.

Excluding promethium, which has no stable isotope at all, thulium is the least abundant lanthanide — and the reason is that it drew an odd number.

Cleve's leftovers, and fifteen thousand recrystallisations

Per Teodor Cleve found thulium in 1879 by continuing to take erbia apart in Uppsala, obtaining it alongside the substance that became holmium. He named it after Thule, the far-northern land at the edge of the classical world, standing in for Scandinavia.

What Cleve had was an impure oxide, and it stayed impure for another thirty years. Pure thulium was finally obtained by Charles James at the University of New Hampshire around 1911, and the method was brute repetition: fractional crystallisation of thulium bromate, carried out an estimated fifteen thousand times.

That number deserves a moment. Each cycle involved dissolving the salt, allowing partial crystallisation, separating the crystals from the liquor and repeating, with each pass enriching the sample by a fraction of a per cent. James ran the sequence for years. It stands as one of the most extreme demonstrations of persistence in the history of preparative chemistry, and it is the reason the rare earths only became tractable when ion-exchange chromatography arrived four decades later.

An X-ray machine with no electricity

Thulium's oldest practical use exploits what happens when its single stable isotope absorbs a neutron. Thulium-170, half-life 129 days, emits beta particles, and the deceleration of those betas plus the decay itself produces X-rays in the vicinity of 84 keV.

That gives a compact, entirely passive X-ray source. Sealed in a small capsule, thulium-170 will radiograph thin-walled castings, welds in small-bore pipework and machined components with no power supply, no high-voltage generator and no cooling — which makes it the tool of choice where the work is somewhere an X-ray set cannot go. It is used in field inspection, in aircraft maintenance and in medical and dental radiography in settings without reliable electricity. Its relatively low photon energy is well suited to light alloys and thin sections, where a cobalt-60 or iridium-192 source would simply pass straight through and record nothing.

Two microns, and a fight in the operating theatre

Thulium's modern significance comes from lasers, and specifically from thulium-doped silica fibre, which lases in a band around 1.9 to 2.0 micrometres and is pumped by inexpensive diodes at 793 nanometres.

Two things follow from the fibre format. The beam quality is excellent and stays excellent at high power, because the light is guided by the fibre rather than bouncing between mirrors. And the laser can be switched electronically at very high rates, unlike a flashlamp-pumped crystal.

In urology, that combination has produced a serious challenge to a well-established incumbent. Thulium fibre laser lithotripsy fragments kidney stones at pulse rates in the hundreds or thousands per second, where a holmium laser manages tens, and it does so through a thinner fibre that leaves more room for irrigation in a narrow ureteroscope. The result is finer fragmentation — dust rather than chips — and randomised comparisons against holmium are running across a range of procedures, including prostate enucleation. Two lanthanides two squares apart, competing head-to-head for the same clinical job, is close to unheard of.

Away from medicine, the same 2-micrometre output is used for welding plastics that are transparent at the near-infrared wavelengths ordinary fibre lasers produce, and for atmospheric sensing and rangefinding, where the wavelength is comparatively eye-safe and propagates well through air.

Beyond the C-band

Thulium may also end up extending the capacity of the world's optical fibre. The erbium amplifier covers roughly 1530 to 1565 nanometres, and long-haul traffic has been pressing against the edges of that band for years. The obvious response is to use more of the fibre's transparent window.

Thulium-doped fibre amplifiers cover the S-band, roughly 1460 to 1530 nanometres, sitting just below erbium's territory. Combined with amplifiers for the longer-wavelength L-band, they offer a route to two or three times the usable spectrum on cable that is already in the ground and on the seabed — which is considerably cheaper than laying more of it. Multi-band systems using thulium amplification are moving from demonstration into deployment.

A metal you purify by boiling it

Thulium metal has one convenient physical quirk. Its boiling point is low for a lanthanide, and the gap between melting and boiling is unusually narrow, so the metal has a substantial vapour pressure not far above its melting point.

That makes distillation a practical purification step: thulium can be reduced from its fluoride and then boiled away from the less volatile residue, which is how high-purity thulium metal is actually made. Europium and ytterbium share the trait, and among the fifteen lanthanides those three are the ones a metallurgist can purify by evaporation rather than by chemistry.

Thulium ions also fluoresce blue, at around 450 nanometres, which puts thulium into phosphor blends and security inks where a sharp blue emission line is wanted. It is a small use for an element the world produces only a few dozen tonnes of annually — but it is one of the few places outside a laboratory where thulium can be said to be visible.