yttrium

yttrium (Y), chemical element, a rare-earth metal of Group 3 of the periodic table.

Yttrium is a silvery white, moderately soft, ductile metal. It is quite stable in air; rapid oxidation begins above approximately 450 °C (840 °F), resulting in Y₂O₃. The metal readily reacts with diluted acids—except hydrofluoric acid (HF), in which the insoluble protective layer of YF₃ that forms on the surface of the metal prevents further reaction. Yttrium turnings ignite readily in air, burning white-hot. The metal is paramagnetic with a temperature-independent magnetic susceptibility between 10 and 300 K (−263 and 27 °C, or −442 and 80 °F). It becomes superconducting at 1.3 K (−271.9 °C, or −457 °F) at pressures exceeding 110 kilobars.

In 1794 Finnish chemist Johan Gadolin isolated yttria, a new earth or metallic oxide, from a mineral found at Ytterby, Sweden. Yttria, the first rare earth to be discovered, turned out to be a mixture of oxides from which, over a span of more than a century, nine elements—yttrium, scandium (atomic number 21), and the heavy lanthanide metals from terbium (atomic number 65) to lutetium (atomic number 71)—were separated. Yttrium occurs especially in the heavy rare-earth ores, of which laterite clays, gadolinite, euxenite, and xenotime are the most important. In the igneous rocks of Earth’s crust, this element is more plentiful than any of the other rare-earth elements except cerium and is twice as abundant as lead. Yttrium also occurs in products of nuclear fission.

Stable yttrium-89 is the only naturally occurring isotope. A total of 33 (excluding nuclear isomers) radioactive isotopes of yttrium ranging in mass from 77 to 109 and half-life from 41 milliseconds (yttrium-108) to 106.63 days (yttrium-88) have been reported.

Commercially, yttrium is separated from the other rare earths by liquid-liquid or ion-exchange extraction, and the metal is produced by metallothermic reduction of the anhydrous fluoride with calcium. Yttrium exists in two allotropic (structural) forms. The α-phase is close-packed hexagonal with a = 3.6482 Å and c = 5.7318 Å at room temperature. The β-phase is body-centered cubic with a = 4.10 Å at 1,478 °C (2,692 °F).

Yttrium and its compounds have numerous uses. Major applications include hosts for red phosphors for fluorescent lamps, colour displays, and TV screens that use cathode-ray tubes. Yttrium aluminum garnet (YAG) doped with other rare earths is used in lasers; yttrium iron garnet (YIG) is used for microwave filters, radars, communications, and synthetic gems; and yttrium oxide-stabilized cubic zirconia is used in oxygen sensors, structural ceramics, thermal barrier coatings, and synthetic diamonds. A major use of yttrium is in high-temperature superconducting ceramics, such as YBa₂Cu₃O₇, which has a superconducting transition temperature of 93 K (−180 °C, or −292 °F) for electrical power transmission lines and superconducting magnets. The metal is used as an alloying addition to ferrous and nonferrous alloys for improved corrosion resistance and oxidation resistance. Yttrium compounds are used in optical glasses and as catalysts.

Yttrium behaves chemically as a typical rare-earth element having an oxidation state of +3. Its ionic radius is near the radii of dysprosium and holmium, making separation from those elements difficult. Besides the white sesquioxide, yttrium forms a series of nearly white salts including the sulfate, the trichloride, and the carbonate.