Holmium is a chemical element with the symbol Ho and atomic number 67 and is a rare earth element. It was discovered by Swedish chemist Per Theodor Cleve. Its oxide was first isolated from rare earth ores in 1878 and the element was named after the city of Stockholm.
It is a relatively soft and malleable silvery-white metal. It is too reactive to be found uncombined in nature, but when isolated, is relatively stable in dry air at room temperature. However, it reacts with water and rusts readily, and will also burn in air when heated.
Holmium has the highest magnetic strength of any element and therefore is used for the polepieces of the strongest static magnets.
Holmium oxide appears to have different colours depending on changes in ambient lighting. Under natural light, it’s yellow, but under fluorescent lighting, it’s pink.
|Ho2O3, left: natural light, right: fluorescent lamp light|
Holmium is used in yttrium-iron-garnet (YIG)- and yttrium-lanthanum-fluoride (YLF) solid-state lasers found in microwave equipment (which are in turn found in a variety of medical and dental settings). Holmium lasers emit at 2.08 micrometres, and therefore are safe to eyes. They are used in medical, dental, and fibre-optical applications.
Holmium is one of the colorants used for cubic zirconia and glass, providing yellow or red colouring. Glass containing holmium oxide and holmium oxide solutions (usually in perchloric acid) have sharp optical absorption peaks in the spectral range 200–900 nm. They are therefore used as a calibration standard for optical spectrophotometers and are available commercially.
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