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Holmium is a chemical element with the symbol Ho and atomic number 67. Part of the lanthanide series, holmium is a relatively soft and malleable silvery-white metallic element, which is stable in dry air at room temperature. A rare earth metal, it is found in the minerals monazite and gadolinite. Holmium has the highest magnetic strength of any element and therefore is used for the polepieces of the strongest static magnets. Because holmium strongly absorbs nuclear fission-bred neutrons, it is also used in nuclear control rods.
Holmium is a relatively soft and malleable element that is fairly corrosion-resistant and stable in dry air at standard temperature and pressure. In moist air and at higher temperatures, however, it quickly oxidizes, forming a yellowish oxide. In pure form, holmium possesses a metallic, bright silvery luster.
Holmium oxide has some fairly dramatic color changes depending on the lighting conditions. In daylight, it is a tannish yellow color. Under trichromatic light, it is a fiery orange red, almost indistinguishable from the way erbium oxide looks under this same lighting. This is related to the sharp emission bands of the phosphors.
A trivalent metallic rare earth element, holmium has the highest magnetic moment (10.6 µB) of any naturally-occurring element and possesses other unusual magnetic properties. When combined with yttrium, it forms highly magnetic compounds. Holmium is paramagnetic at ambient conditions, but is ferromagnetic at temperatures below 19 K.
Holmium metal tarnishes slowly in air and burns readily to form holmium(III) oxide:
4 Ho + 3 O2 → 2 Ho2O3
Holmium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form holmium hydroxide:
2 Ho (s) + 6 H2O (l) → 2 Ho(OH)3 (aq) + 3 H2 (g)
Holmium metal reacts with all the halogens:
Holmium dissolves readily in dilute sulfuric acid to form solutions containing the yellow Ho(III) ions, which exist as a [Ho(OH2)9]3+ complexes:
2 Ho (s) + 3 H2SO4 (aq) → 2 Ho3+ (aq) + 3 SO2−4 (aq) + 3 H2 (g)
Natural holmium contains one stable isotope, holmium-165. Some synthetic radioactive isotopes are known; the most stable one is holmium-163, with a half life of 4570 years. All other radioisotopes have ground-state half lives not greater than 1.117 days, and most have half lives under 3 hours. However, the metastable 166m1Ho has a half life of around 1200 years because of its high spin. This fact, combined with a high excitation energy resulting in a particularly rich spectrum of decay gamma rays produced when the metastable state de-excites, makes this isotope useful in nuclear physics experiments as a means for calibrating energy responses and intrinsic efficiencies of gamma ray spectrometers.
Holmium (Holmia, Latin name for Stockholm) was discovered by Marc Delafontaine and Jacques-Louis Soret in 1878 who noticed the aberrant spectrographic absorption bands of the then-unknown element (they called it "Element X"). Later in 1878, Per Teodor Cleve independently discovered the element while he was working on erbia earth (erbium oxide).
Using the method developed by Carl Gustaf Mosander, Cleve first removed all of the known contaminants from erbia. The result of that effort was two new materials, one brown and one green. He named the brown substance holmia (after the Latin name for Cleve's home town, Stockholm) and the green one thulia. Holmia was later found to be the holmium oxide and thulia was thulium oxide.
Occurrence and production
Like all other rare earths, holmium is not naturally found as a free element. It does occur combined with other elements in the minerals gadolinite, monazite, and in other rare-earth minerals. The main mining areas are China, United States, Brazil, India, Sri Lanka and Australia with reserves of holmium estimated as 400,000 tonnes.
It is commercially extracted via ion-exchange from monazite sand (0.05% holmium) but is still difficult to separate from other rare earths. The element has been isolated through the reduction of its anhydrous chloride or fluoride with metallic calcium. Its estimated abundance in the Earth's crust is 1.3 mg/kg. Holmium obeys the Oddo-Harkins rule: as an odd-numbered element, it is less abundant than its immediate even-numbered neighbors, dysprosium and erbium. However, it is the most abundant of the odd-numbered heavy lanthanides. The principal current source are some of the ion-adsorption clays of southern China. Some of these have a rare-earth composition similar to that found in xenotime or gadolinite. Yttrium makes up about two-thirds of the total by weight; holmium is around 1.5%. The original ores themselves are very lean, maybe only 0.1% total lanthanide, but are easily extracted. Holmium is relatively inexpensive for a rare-earth metal with the price about US$ 1000 per kg.
Holmium has the highest magnetic strength of any element, and therefore is used to create the strongest artificially-generated magnetic fields, when placed within high-strength magnets as a magnetic pole piece (also called a magnetic flux concentrator). Since it can absorb nuclear fission-bred neutrons, it is also used in nuclear control rods.
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 microns, and therefore are safe to eyes. They are used in medical, dental, and fiber-optical applications.
Holmium is one of the colorants used for cubic zirconia and glass, providing yellow or red coloring. 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.
The radioactive but long-lived Ho-166m1 (see "Isotopes" above) is used in calibration of gamma ray spectrometers.
The element, as with other rare earths, appears to have a low degree of acute toxicity. Holmium plays no biological role in humans, but may be able to stimulate metabolism.
* Holmium compounds
1. ^ Yiguo Su et al. (2008). "Hydrothermal Synthesis of GdVO4:Ho3+ Nanorods with a Novel White-light Emission". Chemistry Letters 37: 762. doi:10.1246/cl.2008.762.
* Guide to the Elements – Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
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