铱 (Ir)

发现史

发现: 1803 · Smithson Tennant

The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. The first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a description of an unknown noble metal found between Darién and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy". From their first encounters with platinum, the Spanish generally saw the metal as a kind of impurity in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding the adulteration of gold with platinum impurities.

用途与应用

Iridium forms trihydride at very high pressures. Only one binary oxide is well-characterized: iridium dioxide, IrO2. It is a blue-black solid that adopts the fluorite structure. A sesquioxide, Ir2O3, has been described as a blue-black powder, which is oxidized to IrO2 by HNO3. Other iridium chalcogenides include the sulfides IrS2, Ir2S3 and IrS3, and the selenides IrSe2 and Ir2Se3. Binary trihalides, IrX3, are known for all of the halogens. For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known. Iridium hexafluoride, IrF6, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to IrF4. Iridium pentafluoride is also a strong oxidant, but it is a tetramer, Ir4F20, formed by four corner-sharing octahedra.

同位素

Iridium has two naturally occurring stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3 % and 62.7 %, respectively. There are also 40 known radioisotopes, ranging in mass number from 164 to 205. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, has a half-life of 73.82 days, and finds application in brachytherapy and in industrial radiography, particularly for nondestructive testing of welds in steel in the oil and gas industries; iridium-192 sources have been involved in a number of radiological accidents. Three other isotopes have half-lives of at least a day—188Ir, 189Ir, and 190Ir. Isotopes with masses below 191 decay by some combination of β+ decay, α decay, and (rare) proton emission. Synthetic isotopes heavier than 193 decay by β− decay, and 192Ir has both, but β+ decay (95.24 %) dominates over electron capture (4.76 %). At least 32