Terbium (Tb)

Terbium's name is a fragment. It comes from Ytterby, a village of a few hundred people on Resarö, an island east of Stockholm, and it is one of four element names carved out of those three syllables.

A village of a hundred people, and four elements

The quarry at Ytterby supplied feldspar and quartz to the porcelain industry, which is an entirely unremarkable thing for a Swedish quarry to have done. In 1787 an artillery lieutenant and amateur mineralogist named Carl Axel Arrhenius picked up a black stone there that was heavier than it had any right to be, and passed it around.

Johan Gadolin analysed it in 1794 and found in it a new earth — an oxide of something nobody had described. Anders Gustaf Ekeberg named the oxide yttria in 1797, after the village; the mineral itself was later renamed gadolinite, after Gadolin.

That single specimen turned out to be the entrance to an entire branch of the periodic table. Yttria was not one substance but a mixture, and pulling it apart occupied the best analytical chemists in Europe for the next hundred and twenty years. Four elements ended up named directly after the village that supplied the rock:

• Yttrium, from Ytterby, the first out.
• Terbium, from the middle syllables.
• Erbium, from the same middle syllables, differently sliced.
• Ytterbium, from the whole name.

Several more were first isolated from Ytterby minerals without taking the name: scandium, holmium, thulium and gadolinium among them, and Ekeberg found tantalum in a mineral from the same quarry. Depending on how strictly you count, seven to nine elements trace their discovery to one small hole in the ground outside Stockholm. It is the highest concentration of element discoveries at any single locality on Earth, and the quarry is now a recognised historic site with a plaque rather than a working pit.

The other three Ytterby elements have their own pages — yttrium, erbium and ytterbium — and this is where the naming story lives for all four.

Mosander splits yttria three ways, and gets it backwards

Carl Gustaf Mosander took Gadolin's yttria apart in 1843 and found three fractions: a colourless one he kept calling yttria, a yellow one he named erbia, and a rose-coloured one he named terbia. Terbium is his rose fraction.

Except that it is not. Between 1860 and 1877 the two names were swapped. Marc Delafontaine, working with better spectroscopy, applied the name erbium to the substance giving pink salts and terbium to the one giving a yellow oxide — the reverse of Mosander's assignment — and by 1877 the chemical community had settled on Delafontaine's usage. Modern terbium is Mosander's erbium. Modern erbium is Mosander's terbium. The labels changed places and nobody has ever changed them back.

The green third of white light

A fluorescent lamp does not emit white light. It emits three narrow bands — blue, green and red — which the eye adds together into something acceptable, and the arrangement works only because each band is efficient and pure. Terbium supplies the green.

Tb³⁺ emits strongly at about 542 nanometres, close to the peak of human visual sensitivity, which means a watt of terbium green looks brighter than a watt of anything else. In lamp phosphors it is usually hosted in cerium-doped lanthanum phosphate, where the cerium absorbs the ultraviolet and hands the energy to the terbium. The blue and red come from europium.

Terbium's larger contribution to public health is invisible in a different way. Gadolinium oxysulfide activated with terbium is the standard scintillator in X-ray intensifying screens and in the detectors of CT scanners and digital radiography panels. It converts X-rays to visible light far more efficiently than the calcium tungstate screens it replaced, which means a diagnostic image of the same quality can be obtained with a substantially lower dose to the patient. Terbium is in the detector of essentially every medical X-ray machine built in the last forty years.

A metal that changes shape in a magnetic field

Magnetostriction — a material changing dimensions when magnetised — is a real but tiny effect in ordinary metals, a few parts per million in iron. Terbium's is enormous, and in the 1970s the US Naval Ordnance Laboratory turned that into a material.

Terfenol-D is an alloy of terbium, dysprosium and iron; the name packs in terbium, fe, the lab's initials and the D for the dysprosium added later at Ames. It strains by around a tenth to two tenths of a per cent under an applied field — a thousand times iron's response — and it does so quickly, forcefully and reversibly.

The original purpose was sonar. A transducer that converts electrical drive into physical motion with that much authority can push a large volume of water at low frequency, which is what long-range sonar requires. The material has since spread into precision positioning actuators, active vibration cancellation, fuel injectors, and compact transducers that turn an ordinary panel — a window, a tabletop — into a loudspeaker by vibrating it directly.

Terbium-iron-cobalt alloys did a different job in the 1990s: as the recording layer in magneto-optical discs, including the MiniDisc, where a laser heated a spot past its Curie point so that a weak magnetic field could flip it, and the data was read back by the rotation the magnetised spot imposed on reflected light.

Painted onto the grain boundaries

The application driving terbium demand now is one where the element is barely present.

Neodymium magnets in an electric vehicle's traction motor run hot, and heat erodes their resistance to demagnetisation. Adding a heavy rare earth fixes this, and terbium is the most effective per atom of any candidate — better than dysprosium, and considerably scarcer and dearer.

Alloying terbium uniformly through a magnet therefore wastes most of it, because demagnetisation does not begin uniformly. It begins at the surfaces of individual crystal grains. Grain boundary diffusion exploits that: a terbium-rich compound is applied to the outside of a finished magnet and heated, and the terbium migrates along the grain boundaries into the outer shell of each grain, leaving the interiors untouched. The magnet gains the coercivity of a heavily alloyed one while containing a fraction — often a quarter or less — of the heavy rare earth. It is now standard practice in automotive magnet manufacture, and it is one of the few genuine successes in reducing dependence on the scarcest part of the rare earth supply chain.

Four isotopes for four different scans

Natural terbium is a single isotope, terbium-159, but the artificial ones have an unusual collective virtue. Terbium-149 emits alpha particles, terbium-152 emits positrons for PET imaging, terbium-155 emits gamma rays suited to SPECT, and terbium-161 emits beta particles and Auger electrons for therapy.

Four different nuclear behaviours, one chemistry. A targeting molecule labelled with any of the four behaves identically in the body, so a patient can be imaged with one isotope and treated with another using exactly the same drug, with no need to validate a second compound. No other element offers a matched set covering all four modes, and the production work at CERN and the Paul Scherrer Institute exists largely to exploit it.