We continue to review some of the most important materials in heat treatment and metallurgy.
Holmium (chemical symbol: Ho)
Holmium (Fig. 1) is a bright, soft and silvery-white rare-earth metal that is relatively soft and malleable. While it is stable in dry air at room temperature, it will react rapidly with water or when heated to form a yellow oxide (Ho2O3).
In nature, holmium is found in monazite and gadolinite mineral deposits, where it is almost always in a trivalent state, containing Ho(III) ions. These trivalent holmium ions have similar fluorescent properties as other rare-earth ions and are often used in similar applications. Holmium is unique in that it has unusually high magnetic properties and is recognized as having the highest magnetic permeability of all known elements.
Holmium’s existence was first observed as an unknown spectroscopic line during the spectrographic analysis of other rare-earth metals in 1878 by Swiss chemists Marc Delafontaine and Jacques-Louis Soret. It wasn’t until 1879 that a Swedish chemist, Per Theodor Cleve, had discovered two new materials – one brown and one green – while extracting the known impurities of erbium oxide, Er2O3 (Fig. 2). Upon analyzing the brown material, he determined that it was an oxide of a previously undiscovered element that he later named holmium, after his hometown of Stockholm. The green material was later found to be an oxide of the element thulium.
Currently, holmium only has very specific uses in commercial applications. Due to its high magnetic strength, however, holmium does see some use as an alloying element for the production of special types of magnets. It also is used as a magnetic pole piece placed within other high-strength magnets to create the strongest artificially generated magnetic fields, called magnetic-flux concentrators. Furthermore, since holmium isotopes are good neutron absorbers, they can be used in nuclear-reactor control rods. Holmium oxide also can be used as a colorant for cubic zirconia and certain types of glass, which lends it a yellow or red coloring.
Here are a few important facts about holmium.[2,5]
- Atomic number: 67
- Atomic weight: 164.93033
- Melting point: 1747 K (1474°C or 2685°F)
- Boiling point: 2973 K (2700°C or 4892°F)
- Density: 8.80 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 6
- Group number: none
- Group name: Lanthanide
- Electron configuration: [Xe] 4f116s2