Lanthanum

• 1. The Hidden Element

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  Lanthanum carries a name that is a confession. It comes from the ancient Greek lanthanein, meaning 'to lie hidden'. The Swedish chemist Carl Gustaf Mosander chose that name in 1839 after finding the element where nobody expected it. It was lurking as an impurity inside cerium nitrate, disguised by another element entirely. Mosander was studying a substance called ceria, which everyone assumed was a single oxide. Instead he pulled two new oxides out of it, naming them lanthana and didymia. How does an element stay invisible for so long, then turn out to be the 28th most abundant in the Earth's crust? Why does a metal almost three times as common as lead still get filed under 'rare earths'? And how did something that hides in plain sight end up powering hybrid cars, treating kidney patients, and lighting movie sets? This is the story of lanthanum, the element with the atomic number 57 and the symbol La.

• 2. First Of A Kind

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  Lanthanum sits at the very front of a group of 15 similar elements stretching from lanthanum to lutetium, the lanthanide series. It is the prototype, the first of the f-block elements by the reckoning of authors who write on the subject. In the periodic table it stands to the right of barium and to the left of cerium. Its 57 electrons settle into the configuration Xe5d6s, leaving three valence electrons outside a noble gas core. Those three electrons define lanthanum's chemistry. In reactions it almost always surrenders them from the 5d and 6s subshells, forming the +3 oxidation state and matching the stable arrangement of xenon. A few lanthanum(II) compounds exist, but they are much less stable. Among the lanthanides, lanthanum is the odd one out: as a single gas-phase atom it has no 4f electrons at all. That absence makes it only very weakly paramagnetic, unlike the strongly paramagnetic lanthanides that follow. Yet the 4f shell does not stay empty forever. In chemical environments it can become partially occupied and join in bonding. That availability is why lanthanum earns its place in the f-block despite its anomalous ground state. Lanthanum monoxide leaves another fingerprint far from any laboratory, producing strong absorption bands in some stellar spectra.

• 3. A Metal That Rusts Away

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  A centimeter-sized sample of lanthanum will corrode completely within a year. Its oxide spalls off like iron rust rather than forming the protective coating that shields aluminium, scandium, yttrium, and lutetium. Lanthanum has the largest atomic radius of all the lanthanides, which makes it the most reactive of them. Exposed to air it tarnishes quickly, turning completely dark after several hours, and it can readily burn to form lanthanum(III) oxide, an oxide almost as basic as calcium oxide. It is a soft metal, fitting for the start of a series that grows harder as it is traversed. Lanthanum carries a relatively high resistivity of 615 nanoohm-meters at room temperature; the good conductor aluminium sits far below at 26.50. It is the least volatile of the lanthanides and has the second-lowest melting point among the trivalent ones, at 920 degrees Celsius. Its crystal structure shifts as it heats. At room temperature it is hexagonal, at 310 degrees Celsius it becomes face-centered cubic, and at 865 degrees Celsius it turns body-centered cubic. Lanthanum reacts with the halogens at room temperature to form trihalides, and on warming it binds with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon, and arsenic. It is the strongest and hardest base among the rare earth elements, exactly what one would expect from the largest of them.

• 4. The Cerite Trail

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  In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnas, later named cerite. Thirty years on, the fifteen-year-old Wilhelm Hisinger, whose family owned the mine, sent a sample to Carl Scheele, who found no new elements in it. Hisinger did not give up. In 1803, after he had become an ironmaster, he returned to the mineral with Jons Jacob Berzelius and isolated a new oxide. They named it ceria after the dwarf planet Ceres, discovered two years before. Martin Heinrich Klaproth isolated the same oxide independently in Germany. Then Mosander, a surgeon and chemist who lived in the same house as Berzelius and studied under him, showed between 1839 and 1843 that ceria was no single substance but a mixture. He partially decomposed cerium nitrate by roasting it in air, then treated the resulting oxide with dilute nitric acid. That same year Axel Erdmann, a student at the Karolinska Institute, found lanthanum in a new mineral from Laven island in a Norwegian fjord. Mosander later explained his delay by admitting he had pulled yet another element from cerium, which he called didymium. Didymium too was a disguise. In 1885 it was split into praseodymium and neodymium. Relatively pure lanthanum metal was not isolated until 1923.

• 5. Common By Any Other Name

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  Lanthanum makes up 39 milligrams per kilogram of the Earth's crust, sitting behind neodymium at 41.5 and cerium at 66.5. The 'rare-earth' label is a historical accident. Lanthanum is rarer than common earths like lime and magnesia, and when it was first recognized only a few deposits were known. It also earns the name because mining it is difficult, time-consuming, and expensive. Lanthanum rarely dominates the minerals that hold it; in chemical formulas it usually trails behind cerium, and La-dominant minerals like monazite-(La) and lanthanite-(La) are rare examples. Because the lanthanum ion is sized like the early lanthanides of the cerium group, it tends to occur alongside them in phosphate, silicate, and carbonate minerals such as monazite and bastnasite. Bastnasite is the kinder ore. It usually lacks thorium and the heavy lanthanides, so purifying the light lanthanides from it is less involved. Workers crush and grind the ore, then treat it with hot concentrated sulfuric acid, which evolves carbon dioxide, hydrogen fluoride, and silicon tetrafluoride. The dried product is leached with water, leaving lanthanum and the other early lanthanide ions in solution. Monazite demands far more. It holds all the rare earths plus thorium, and can be separated by repeated electromagnetic separation thanks to its magnetic properties. After acid treatment, the filtrates are partially neutralized with sodium hydroxide to a pH of 3 to 4, dropping thorium out as a hydroxide. Ammonium oxalate then converts the rare earths to insoluble oxalates, which annealing turns to oxides. Handlers must take care with some residues, which contain a strong gamma emitter. Lanthanum itself is relatively easy to extract, since its only neighbouring lanthanide, cerium, can be oxidised to the +4 state and removed.

• 6. From Gas Mantles To Hybrids

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  The first historical use of lanthanum was in gas lantern mantles. Carl Auer von Welsbach mixed lanthanum oxide with zirconium oxide into a material he called Actinophor, patented in 1886. The original mantles gave a green-tinted light and never caught on. His first company opened a factory in Atzgersdorf in 1887 and failed in 1889. The modern uses run far deeper. Lanthanum feeds the anodic material of nickel-metal hydride batteries, the kind found in many Toyota Prius models sold in the US. Because extracting the other lanthanides is costly, a mischmetal with more than 50 percent lanthanum stands in for the pure metal. The 2008 Toyota Prius battery alone requires 10 to 15 kilograms of lanthanum, and pushing the technology for better fuel efficiency could demand twice that per vehicle. Hydrogen sponge alloys containing lanthanum can store up to 400 times their own volume of hydrogen gas in a reversible process that releases heat each time. Mischmetal used in lighter flints carries 25 to 45 percent lanthanum. Lanthanum oxide and the boride serve as hot cathode materials in electronic vacuum tubes, with crystals used in high-brightness electron sources for electron microscopes and Hall-effect thrusters. Lanthanum trifluoride is essential to a heavy fluoride glass named ZBLAN, prized for its infrared transmittance in fiber-optical communication. Cerium-doped lanthanum bromide and lanthanum chloride are inorganic scintillators with high light yield and fast response, already widely used in detectors of neutrons or gamma rays. Carbon arc lamps once consumed about 25 percent of all rare-earth compounds produced, especially for studio lighting and projection in the motion picture industry, until those lamps were phased out. Lanthanum oxide additive to tungsten now substitutes for radioactive thorium in gas tungsten arc welding electrodes.

• 7. Medicine And Microbes

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  Lanthanum carbonate, sold as Fosrenol by Shire Pharmaceuticals, was approved as a medication to absorb excess phosphate in cases of hyperphosphatemia seen in end-stage kidney disease. The element is very poorly absorbed after oral administration, and when injected it is eliminated very slowly. Lanthanum has no known biological role in humans. It is not particularly toxic, though it does show some antimicrobial activity. Its effects on the body are oddly precise. Lanthanum acts on several receptors and ion channels, but its specificity for the GABA receptor is unique among trivalent cations. It binds at the same modulatory site as zinc, a known negative allosteric modulator, yet the lanthanum cation acts as a positive one. It increases open channel time and decreases desensitization, depending on the subunit configuration. Some bacteria put the element to work directly. Lanthanum is a cofactor for the methanol dehydrogenase of the methanotrophic bacterium Methylacidiphilum fumariolicum SolV. The lanthanides are so chemically alike that cerium, praseodymium, or neodymium can substitute without ill effects, while the smaller samarium, europium, or gadolinium cause only slower growth. In molecular biology, lanthanum serves as an electron-dense tracer, much like horseradish peroxidase. In two cases, exposure to carbon arc light left lanthanum and other rare-earth elements in lung tissue.

• 8. The Price Of Hiding

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  For most of the stretch from early 2001 to September 2010, a metric ton of 99 percent lanthanum oxide cost below 2,000 US dollars, with a brief jump to 10,000 dollars in 2008. Then the market lurched. The price climbed steeply to 140,000 dollars in mid-2011 before falling just as fast to 38,000 dollars by early 2012. These figures come from the Institute of Rare Earths Elements and Strategic Metals. By the six months from April to September 2022, the same institute placed 99.9 percent lanthanum oxide from China at 1,308 euros per metric ton, and lanthanum metal at 99 percent purity at 3,706 euros per metric ton. The element that lay hidden in cerium for so long now finds steadier ground in newer roles. Lanthanum telluride is being considered for radioisotope power systems, where its conversion capabilities are significant and its transmuted elements will not react with the material itself, leaving the power plant unharmed.