Yttrium

• 1. Introduction

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  Yttrium sits on the periodic table with symbol Y and atomic number 39, but its story begins not in a laboratory, but in a Swedish quarry. In 1787, a part-time chemist named Carl Axel Arrhenius picked up a heavy black rock near the village of Ytterby, outside Stockholm, and set in motion a chain of discoveries that would eventually touch your television, your hospital, and even the light-emitting diode overhead.

  Arrhenius thought he had found an unknown mineral containing tungsten. He sent samples to chemists across Europe. What they found instead was something stranger: a new earth, then a new element, then several new elements, then still more hiding inside those. Ytterby, a village so small it sits within the Stockholm Archipelago, would end up lending its name to four chemical elements.

  Yttrium itself turned out to be a silvery, lustrous metal that never appears free in nature. It is always locked inside rare-earth minerals alongside the lanthanides, a family of elements it so closely resembles that chemists routinely classify it as one of their own. That resemblance is not superficial. It shapes where yttrium is found, how it is extracted, and, crucially, what it can do. The questions worth following are these: how did one rock from one quarry rewrite chemistry? And why does an element no one has heard of end up inside cancer treatments, jet engines, and the brightest blue pigment discovered in two hundred years?

• 2. One Village, Four Elements

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  Johan Gadolin at the Royal Academy of Åbo, in Turku, was the first to identify a new oxide inside Arrhenius's sample in 1789. He published his completed analysis in 1794. Anders Gustaf Ekeberg confirmed the finding in 1797 and named the oxide yttria. Because this was the era just after Antoine Lavoisier established the first modern definition of chemical elements, the discovery of a new earth was widely taken to mean the discovery of a new element hiding within it.

  Friedrich Wohler is credited with first isolating the metal in 1828, by reacting a volatile chloride he believed to be yttrium chloride with potassium. Yet even what Wohler isolated was not pure. In 1843, Carl Gustaf Mosander examined samples of yttria and found three distinct oxides inside: white yttrium oxide, yellow terbium oxide, and rose-colored erbium oxide. Crucially, the names were initially reversed, so what was called erbia turned out to be terbia, and vice versa. A fourth oxide, ytterbium oxide, was isolated in 1878 by Jean Charles Galissard de Marignac. New elements were later extracted from each, and all of them were named, in some fashion, after Ytterby.

  The scope of that one quarry's contribution kept expanding. In the following decades, seven other new metals were discovered in what had been called Gadolin's yttria. Since yttria itself proved to be a mineral rather than a simple oxide, Martin Heinrich Klaproth renamed it gadolinite in honor of Gadolin. Until the early 1920s, yttrium was represented in chemistry by the symbol Yt, after which Y came into common use.

• 3. Closer to Lanthanides Than Neighbors

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  Yttrium belongs to group 3 and sits as the first d-block element in the fifth period. On paper it is a neighbor of scandium and zirconium. In practice, its chemical personality is entirely different. If you plotted its physical properties against atomic number, yttrium would appear to have an atomic number of 64.5 to 67.5, placing it squarely between the lanthanides gadolinium and erbium.

  This is not a quirk. It is a consequence of the lanthanide contraction, a phenomenon by which the addition of electrons to the f-shell across the lanthanide series shrinks their atomic radii. The result is that lanthanide ions in the lower half of that series end up almost exactly the same size as yttrium ions. In solution, yttrium behaves as if it were a member of what chemists call the yttrium group of heavy lanthanides. It mimics terbium and dysprosium in its chemical reactivity.

  One place where yttrium does diverge is valence. Yttrium is almost exclusively trivalent, giving up all three of its valence electrons in compounds. About half the lanthanides can adopt valences other than three, though for only four of the fifteen lanthanides do those other valences matter in water-based solutions: cerium, samarium, europium, and ytterbium. Yttrium's near-exclusive +3 oxidation state makes its chemistry more predictable and, in many industrial processes, more manageable.

• 4. From Quarry to Modern Mine

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  About 31 parts per million of the Earth's crust is yttrium, making it the 43rd most abundant element. It is found in soil at concentrations between 10 and 150 ppm, with a dry weight average of 23 ppm, and in seawater at 9 parts per trillion. Lunar rock samples collected during the American Apollo Project contained a relatively high yttrium content.

  Rare-earth ore comes mainly from four types of deposits. Bastnäsite, a carbonate and fluoride ore, contains on average only 0.1% yttrium. The Mountain Pass mine in California was the main source of bastnäsite from the 1960s to the 1990s, making the United States the largest producer of rare-earth elements during that period. Monazite, a phosphate mineral formed by the erosion and gravitational sorting of granite, contains around 2 to 3% yttrium. The largest monazite deposits were found in India and Brazil in the early twentieth century, making those two countries the leading producers in the first half of that century. Xenotime, a heavy rare-earth ore, can contain as much as 60% yttrium by weight. Since the closure of the Mountain Pass mine in the 1990s, the Bayan Obo deposit in China has made China the dominant exporter of heavy rare-earth elements.

  A fourth source, ion absorption clays in southern China, contain only 1% rare-earth elements but can yield ore concentrates with as much as 8% yttrium after processing. Annual world production of yttrium oxide reached 600 tonnes by 2001; by 2014 it had risen to 7,000 short tons. Global reserves were estimated in 2014 to exceed 500,000 short tons. Research has also identified very large potential deposits in seafloor mud several hundred kilometers from the Japanese island of Minami-Torishima. A study published in Scientific Reports estimated that more than 16 million short tons of rare-earth elements, including yttrium, might be exploitable from that site.

• 5. Inside Every Screen and Spark Plug

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  The red color in color television cathode ray tubes came from europium, but yttrium made it possible. The yttrium oxide or yttrium oxysulfide host lattice absorbed energy from the electron gun and passed it to the europium phosphor, which then emitted the red light viewers saw. For decades, this was yttrium's most prominent commercial role.

  White LEDs replaced that application with a different but equally yttrium-dependent chemistry. Cerium-doped yttrium aluminium garnet, known as YAG:Ce, is a phosphor critical to producing white light from LEDs. Yttrium aluminium garnet itself has a hardness of 8.5 and doubles as a gemstone used to simulate diamond in jewelry. Yttrium iron garnet, or YIG, functions as an effective microwave filter and an efficient transmitter and transducer of acoustic energy. Research has shown that YIG has magnetic interactions more complex and longer-ranged than understood over the previous four decades.

  YAG lasers can operate at high power and are used for drilling and cutting metal. YAG, yttria, yttrium lithium fluoride, and yttrium orthovanadate are also used as host materials in near-infrared lasers when doped with neodymium, erbium, or ytterbium. Single crystals for these lasers are normally grown by the Czochralski process.

  Small additions of yttrium, between 0.1 and 0.2%, reduce grain sizes in chromium, molybdenum, titanium, and zirconium. Added to aluminium and magnesium alloys, it improves workability, resists high-temperature recrystallization, and significantly enhances resistance to oxidation at elevated temperatures. In 2009, Professor Mas Subramanian and colleagues at Oregon State University discovered that combining yttrium with indium and manganese produces YInMn blue, an intensely blue, non-toxic, inert pigment. It was the first new blue pigment discovered in two hundred years.

• 6. Yttrium in Medicine and Superconductors

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  The radioisotope yttrium-90 has a half-life of 64 hours and decays by emitting intense beta radiation. That property makes it useful for killing cancer cells. Yttrium-90 is used to label drugs including edotreotide and ibritumomab tiuxetan for the treatment of lymphoma, leukemia, and liver, ovarian, colorectal, pancreatic, and bone cancers. It adheres to monoclonal antibodies that in turn bind to cancer cells and deliver the radiation dose directly.

  A technique called radioembolization applies this principle to hepatocellular carcinoma and liver metastasis. Millions of tiny glass or resin microspheres containing yttrium-90 are delivered directly into the blood vessels feeding specific liver tumors. The procedure is minimally invasive, and patients are usually discharged within a few hours, though multiple procedures may be required to address different segments or lobes. Needles made of yttrium-90 have also been used to sever pain-transmitting nerves in the spinal cord, and yttrium-90 carries out radionuclide synovectomy for inflamed joints, especially knees, in patients with conditions such as rheumatoid arthritis.

  In 1987, researchers at the University of Alabama in Huntsville and the University of Houston discovered that yttrium barium copper oxide, known as YBCO or 1-2-3, achieves superconductivity at temperatures above 77.1 K, the boiling point of liquid nitrogen. This was only the second material known to achieve high-temperature superconductivity, and the first to cross that economically important threshold. Liquid nitrogen is far less expensive than the liquid helium required by conventional metallic superconductors, which matters for the operating cost of any practical application. The precise chemical formula is often written as YBa2Cu3O7-d, where d must remain below 0.7 for superconductivity to occur. Why the oxygen vacancies in the copper oxide planes produce this behavior remains an open question, unexplained by the BCS theory of 1957 that governs conventional superconductors.

• 7. Hazards and Safe Limits

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  Yttrium has no known biological role in humans. As little as 0.5 mg is found in the entire human body under normal circumstances, and human breast milk contains around 4 ppm. Cabbage contains the highest yttrium concentrations of edible plants, while seeds of woody plants can hold as much as 700 ppm.

  Water-soluble yttrium compounds are considered mildly toxic; insoluble compounds are non-toxic. In animal experiments, yttrium and its compounds caused lung and liver damage, with results varying by compound. Inhalation of yttrium citrate in rats caused pulmonary edema and dyspnea, while yttrium chloride caused liver edema, pleural effusions, and pulmonary congestion. In human workers, exposure to airborne yttrium europium vanadate dust caused mild eye, skin, and upper respiratory tract irritation, though the vanadium content may have been the cause rather than the yttrium itself.

  The Occupational Safety and Health Administration limits workplace yttrium exposure to 1 mg/m3 over an eight-hour workday, a limit matched by the National Institute for Occupational Safety and Health's recommended exposure limit. At concentrations of 500 mg/m3, yttrium becomes immediately dangerous to life and health. Yttrium dust is also highly flammable. These hazards are worth keeping in mind as seafloor mining proposals move forward, given the study's estimate of more than 16 million short tons of rare-earth material near Minami-Torishima waiting to be extracted.