thulium

Thulium is a silvery-gray, soft, malleable metal and a member of the lanthanide series. It has the chemical symbol Tm and atomic number 69. In nature it occurs in extremely small amounts, primarily in rare earth minerals such as monazite, gadolinite, and euxenite, and is typically recovered as a byproduct of refining other rare earths. It is one of the rarest stable elements in the Earth's crust.

Thulium is relatively soft and can be cut with a knife. It forms a protective oxide layer in air and is stable in oxidizing environments at room temperature. It has a melting point of about 1545°C and a boiling point near 1950°C. The metal is trivalent in most compounds (Tm3+), and its chemistry is dominated by the +3 oxidation state, though other oxidation states are known in some compounds.

Isotopes and occurrence: Thulium has one stable isotope, 169Tm. Dozens of radioisotopes have been synthesized, with a range of half-lives; these are mainly of interest to research. The element is produced commercially as a byproduct of processing other rare earths.

Applications: Thulium is used as a dopant in solid-state lasers and in thulium-doped fiber lasers and amplifiers that emit light near 2 micrometers, useful in medical procedures, remote sensing, and materials processing. Thulium-doped crystals (for example Tm:YAG) are used in solid-state laser systems. Thulium oxide and other compounds find use in glass, ceramics, and phosphors. Some radioisotopes of thulium have been investigated for radiopharmaceuticals and cancer therapy.

History: Thulium was discovered in 1879 by Per Teodor Cleve in Sweden, in a mineral containing erbia and ytterbia. It was named after Thule, a classical reference to a distant northern land.