Terbium is a heavy rare earth used in very small quantities but playing a key role, mainly in two classes of high-tech materials: phosphors and magnetostrictive alloys. Under cathode-ray and ultraviolet excitation, trivalent terbium emits an extremely pure green light, which is why it is the activator of the green component in trichromatic phosphors, energy-saving lamp tubes, and white LEDs; without it, high-color-rendering white light cannot be achieved. The Tb-Dy-Fe alloy Terfenol-D is the commercial material with the strongest room-temperature magnetostrictive effect: under an applied magnetic field it elongates or contracts with precise control, and is used to make sonar transducers, ultrasonic machining vibrators, precision displacement actuators, and fuel injectors. In magneto-optical disks (TbFeCo amorphous alloy), terbium provides the perpendicular magnetization required by the magneto-optical Kerr effect, and was once the recording medium of rewritable optical disks. In NdFeB magnets it likewise raises high-temperature coercivity and is one of the essential additive elements in the high-end magnets used in new-energy vehicle motors.
Tb4O7 (usually metered as terbium(III) oxide, Tb2O3) is the principal industrial oxide of terbium. It is a brownish powder and the starting point for preparing green phosphors, magnetostrictive alloys, and terbium metal. TbCl3 (terbium trichloride) is an intermediate in the extraction and separation flowsheet, used for the molten-salt electrolytic production of terbium metal and the synthesis of other terbium compounds. Tb-Dy alloys are commonly used in the form of Tb-Dy-Fe (TbxDy1-xFe2), the magnetostrictive material sold under the trade name Terfenol-D; in a magnetic field it produces strain several times larger than that of ordinary ferromagnetic materials, and is the core of sonar, ultrasonic transducers, and precision actuators. Together with the terbium activation center in the Gd2O2S:Tb scintillator screen, it embodies the unique dual capability of the terbium ion: it both luminesces and deforms under a magnetic field.
