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Didymium ( el, , twin) is a mixture of the elements
praseodymium Praseodymium is a chemical element with the symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for i ...
and neodymium. It is used in
safety glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or t ...
for glassblowing and blacksmithing, especially with a gas ( propane)-powered forge, where it provides a filter that selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass without having a detrimental effect on general vision, unlike dark welder's glasses. The usefulness of didymium glass for eye protection of this sort was discovered by Sir
William Crookes Sir William Crookes (; 17 June 1832 – 4 April 1919) was a British chemist and physicist who attended the Royal College of Chemistry, now part of Imperial College London, and worked on spectroscopy. He was a pioneer of vacuum tubes, inventing ...
. Didymium
photographic filter In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a ...
s are often used to enhance autumn scenery by making leaves appear more vibrant. It does this by removing part of the orange region of the color spectrum, acting as an optical
band-stop filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a b ...
. Unfiltered, this group of colors tends to make certain elements of a picture appear "muddy". These photographic filters are also used by nightscape photographers, as they absorb part of the light pollution caused by sodium street lights. Didymium was also used in the
sodium vapor process The sodium vapor process (occasionally referred to as yellowscreen) is a photochemical film technique for combining actors and background footage. It originated in the British film industry in the late 1950s and was used extensively by Walt Disne ...
for matte work due to its ability to absorb the yellow color produced by its eponymous sodium lighting. Didymium is also used in calibration materials for spectroscopy.


Discovery

Didymium was discovered by
Carl Mosander Carl Gustaf Mosander (10 September 1797 – 15 October 1858) was a Swedish chemist. He discovered the rare earth elements lanthanum, erbium and terbium. Early life and education Born in Kalmar, Mosander attended school there until he moved ...
in 1841. It was named after the Greek word ("twin") because it is very similar to lanthanum and cerium, with which it was found. Mosander wrongly believed didymium to be an element, under the impression that "ceria" (sometimes called cerite) isolated by Jöns Jakob Berzelius in 1803 was really a mixture of
cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...
, lanthanum, and didymium. He was right about lanthanum's being an element, but not about didymium. Since spectroscopy had not yet been invented, Mosander did as well as could be expected at the time. His three "elements" accounted for at least 95% of the rare earths in the original cerite from
Bastnäs Bastnäs ( sv, Bastnäs or ) is an ore field near Riddarhyttan, Västmanland, Sweden. The mines in Bastnäs were earliest mentioned in 1692. Iron, copper and rare-earth elements were extracted from the mines and 4,500 tons of cerium was produced b ...
, Sweden. Didymium had not been difficult to find, since in trivalent form it tinged the salts of ceria pink. During the period when didymium was believed to be an element, the symbol Di was used for it. In the illustration of Mendeleev's first attempt at a periodic table, shown on the right, the atomic weights assigned to the various lanthanides, including didymium, reflect the original belief that they were divalent. Their actual
oxidation number In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Co ...
of 3 implies that Mendeleev underestimated atomic weights for them by . In 1874, Per Teodor Cleve deduced that didymium was made up of at least two elements. In 1879, Lecoq de Boisbaudran succeeded in isolating samarium, from didymium contained in North Carolinian
samarskite Samarskite is a radioactive rare earth mineral series which includes samarskite-(Y), with the chemical formula and samarskite-(Yb), with the chemical formula . The formula for samarskite-(Y) is also given as . Samarskite crystallizes in the orth ...
. Then in 1885,
Carl Auer von Welsbach Carl Auer von Welsbach (1 September 1858 – 4 August 1929), who received the Austrian noble title of Freiherr Auer von Welsbach in 1901, was an Austrian scientist and inventor, who separated didymium into the elements neodymium and praseody ...
succeeded in separating salts of the last two component elements,
praseodymium Praseodymium is a chemical element with the symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for i ...
and neodymium. The method used was a fractional crystallization of the double
ammonium nitrate Ammonium nitrate is a chemical compound with the chemical formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is p ...
s from a solution of nitric acid. Welsbach had decided to name his two new elements " praseodidymium" ("green didymium") and " neodidymium" ("new didymium"), but one syllable was soon dropped from each name. Despite being abbreviated in the new elements' names, the untruncated name "didymium" persisted, partly due to its use as an ingredient in glassblowers' goggles, and colored glass. The name "didymium" also was retained in mineralogical texts.


Glassmaking

During World War I, didymium mirrors were reportedly used to transmit
Morse Code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one ...
across battlefields. Didymium does not absorb enough light to make the variation in lamp's light output obvious, but someone with binoculars attached to a prism in the correct fashion could see the absorption bands flash on and off. In the late 1920s, Leo Moser (Moser glass-works Director General, 1916 to 1932) recombined praseodymium and neodymium in a 1:1 ratio to create his "Heliolite" glass ("Heliolit" in
Czech Czech may refer to: * Anything from or related to the Czech Republic, a country in Europe ** Czech language ** Czechs, the people of the area ** Czech culture ** Czech cuisine * One of three mythical brothers, Lech, Czech, and Rus' Places * Czech ...
), which has color-changing properties between amber, reddish, and green depending on the light source. This was one of a number of decorative glasses using rare earth colorants, with "Heliolit" and "Alexandrit" being the first two, introduced by Moser in 1929. Leo Moser's papers in the Corning Glass Museum make it clear that the first experimental glass melts done by Moser involving any of the rare earths occurred in November 1927. After a year of further development, the rare earth glasses were introduced to great acclaim at the Spring 1929 trade show in Leipzig. The Alexandrit and Heliolit names were registered as trademarks in June 1929. The earlier date of 1925 sometimes given for rare earth glass refers to an award for glass design, not glass composition.


Industrial use

The name "didymium" continued to be used in the rare earth metal industry. In the US, commercial "didymium" salts were what remained after cerium had been removed from the natural products obtained from monazite, and thus it contained lanthanum, as well as Mosander's "didymium". A typical composition might have been 46% lanthanum, 34% neodymium, and 11% praseodymium, with the remainder mostly being samarium and gadolinium, for material extracted from South African "rock monazite" from the
Steenkampskraal mine The Steenkampskraal mine is a rare-earth elements (REEs) mine north of Vanrhynsdorp in the Western Cape province of South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in ...
. Typically, in ores, neodymium is higher in relative abundance in monazite, as compared to the
bastnäsite The mineral bastnäsite (or bastnaesite) is one of a family of three carbonate-fluoride minerals, which includes bastnäsite-( Ce) with a formula of (Ce, La)CO3F, bastnäsite-( La) with a formula of (La, Ce)CO3F, and bastnäsite-( Y) with a formul ...
compositions, and the difference is noticeable when unseparated mixtures derived from each are examined side-by-side: the monazite-derived products are more pinkish, and the bastnäsite-derived products are more brownish in tinge, due to the latter's increased relative praseodymium content. (The original cerite from Bastnäs has a rare earth composition highly similar to that of monazite sand.) The European use was closer to Mosander's concept. Such cerium-depleted light lanthanide mixtures have been widely used to make petroleum-cracking catalysts. The actual ratio of praseodymium to neodymium varies somewhat depending on the source of the mineral, but it is often around 1:3. Neodymium always dominates, which is why it got the "neo" appellation, being responsible for most of the color of the old didymium in its salts.


References

{{Authority control Rare earth alloys Praseodymium compounds Neodymium compounds Misidentified chemical elements