Didymium Facts and Uses

Sometimes you hear words that sound like element names, like didymium, coronium, or dilithium. Yet, when you search the periodic table, you don't find these elements.

Didymium Definition

Didymium is a mixture of the rare earth elements praseodymium and neodymium and sometimes other rare earths. The term comes from the Greek word didumus, meaning twin, with the -ium ending. The word sounds like an element name because at one time didymium was considered to be an element. In fact, it appears on Mendeleev's original periodic table.

Didymium History and Properties

Swedish chemistry Carl Mosander (1797-1858) discovered didymium in 1843 from a sample of ceria (cerite) supplied by Jons Jakob Berzelius. Mosander believed didymium was an element, which is understandable because the rare earths were notoriously difficult to separate at that time. The element didymium had atomic number 95, the symbol Di, and an atomic weight based on the belief that the element was divalent. In fact, these rare earth elements are trivalent, so Mendeleev's values were only about 67% of the true atomic weight. Didymium was known to be responsible for a pink color in ceria salts.

Per Teodor Cleve determined didymium must be made of at least two elements in 1874. In 1879, Lecoq de Boisbaudran isolated samarium from a sample containing didymium, leaving Carl Auer von Welsbach to separate the two remaining elements in 1885. Welsbach named these two elements praseodidymium (green didymium) and neodidymium (new didymium). The "di" part of the names was dropped and these elements came to be known as praseodymium and neodymium.

As the mineral was already in use for glassblower's goggles, the name didymium remains. The chemical composition of didymium is not fixed, plus the mixture may contain other rare earths besides just praseodymium and neodymium. In the United States, "didymium" is the material remaining after cerium is removed from the mineral monazite. This composition contains about 46% lanthanum, 34% neodymium, and 11% gadolinium, with a smaller amount of samarium and gadolinium. While the ratio of neodymium and praseodymium varies, didymium usually contains about three times more neodymium than praseodymium. This is why element 60 is the one named neodymium.

Didymium Uses

Although you may have never heard of didymium, you may have encountered it:

• Didymium and its rare earth oxides are used to color glass. The glass is important for blacksmithing and glassblowing safety glasses. Unlike dark welder glasses, didymium glass selectively filters out yellow light, around 589 nm, reducing the risk of Glassblower's cataract and other damage while preserving visibility.
• Didymium is also used in photographic filters as an optical band-stop filter. It removes the orange portion of the spectrum, which makes it useful for enhancing photos of autumn scenery.
• A 1:1 ratio of neodymium and praseodymium may be used to make "Heliolite" glass, a color of glass devised by Leo Moser in the 1920s that changes color from amber to red to green depending on the light. An "Alexandrit" color is also based on rare earth elements, exhibiting color changes similar to the alexandrite gemstone.
• Didymium is also used as a spectroscopy calibration material and for use manufacturing petroleum cracking catalysts.

Didymium Fun Fact

There are reports that didymium glass was used to transmit Morse Code messages across battlefields in World War I. The glass made it so the brightness of lamp light would not appear to be noticeably changing to most viewers, but would enable a receiver using filtered binoculars to see the on/off code in the light absorption bands.

References

• Welsbach, Carl Auer (1885), "Die Zerlegung des Didyms in seine Elemente", Monatshefte für Chemie, 6 (1): 477–491.
• Venable, W. H.; Eckerle, K. L. "Didymium Glass Filters for Calibrating the Wavelength Scale of Spectrophotometers SRMs 2009, 2010, 2013 and 2014", NBS Special Publication 260-66.