Terbium is an element with the symbol Tb and atomic number 65.

It is a silvery-white rare earth metal that is both malleable and ductile.

As the ninth member of the lanthanide series, terbium is a fairly electropositive metal that reacts with water, releasing hydrogen gas in the process.

It never occurs in nature as a free element, but it is present in a range of minerals including cerite, gadolinite, monazite, xenotime, and euxenite.

Swedish chemist Carl Gustaf Mosander identified terbium as a distinct element in 1843, detecting it as an impurity within yttrium oxide. Terbium shares its naming origin with yttrium, erbium, and ytterbium, all four being named after the Swedish village of Ytterby. Isolating terbium in pure form required the development of ion exchange techniques, which came later.

Terbium is used as a dopant in calcium fluoride, calcium tungstate, and strontium molybdate in solid-state devices, and it serves as a crystal stabilizer in fuel cells operating at high temperatures, typically alongside zirconium dioxide. As a component of an alloy called Terfenol-D, it is used in actuators, naval sonar systems, sensors, and other magnetomechanical applications. Terfenol-D expands and contracts in response to magnetic fields more dramatically than any other known alloy, a property called magnetostriction. Terbium is also used to increase the Verdet constant in long-distance fiber optic communication and in terbium-doped garnets used in optical isolators, which prevent reflected light from traveling back along the fiber. Terbium is often considered non-hazardous, though its biological role and toxicity have not been studied in depth.

One of the most notable uses of terbium in the world is in green phosphors for trichromatic lighting. Terbium oxide appears in fluorescent lamps, color television tubes, and flat-screen monitors. In trichromatic lighting systems, terbium green phosphors, which produce a brilliant lemon-yellow fluorescence, are combined with divalent europium blue phosphors and trivalent europium red phosphors to generate high-efficiency white light. This approach delivers substantially greater light output per unit of electrical energy than conventional incandescent lighting. Along with most other lanthanides, terbium is luminescent in its trivalent oxidation state, and that vivid fluorescence has also made it useful as a probe in biochemistry, where its behavior bears some resemblance to calcium.

Extracting terbium from ore involves crushing terbium-bearing minerals and treating them with hot concentrated sulfuric acid to dissolve the rare earth elements as water-soluble sulfates. The resulting solution is partially neutralized to cause thorium to precipitate out as a hydroxide, which is then removed. Ammonium oxalate is added to convert the remaining rare earths into insoluble oxalates, which are then decomposed into oxides through heating. Dissolving the oxides in nitric acid excludes cerium, whose oxide does not dissolve in that acid. Terbium is then isolated as a double salt with ammonium nitrate through crystallization.

Despite its classification as a rare earth, terbium is not exceptionally scarce. Its concentration in Earth’s crust runs to approximately 1.1 parts per million.