Lanthanum (La)

Lanthanum was not found in a mineral. It was found inside another element.

Cerium had been announced in 1803 and accepted for thirty-six years as a perfectly respectable substance with a known oxide and known salts. In 1839 Carl Gustaf Mosander, working in Berzelius's laboratory in Stockholm, partially decomposed cerium nitrate by heating it, treated the product with dilute acid, and dissolved out something that was not cerium. He had taken an element apart.

Berzelius suggested the name. From Greek lanthanein, to lie hidden or to escape notice, it is a comment on the circumstances of the discovery rather than on any property of the metal — an admission, in the name itself, that chemists had been handling a mixture and calling it an element. The admission turned out to be premature in its own way, because two years later Mosander pulled a further substance, didymium, out of the same cerium, and didymium was itself two elements that took another forty-four years to separate.

Why the rare earths took a century to sort out

The problem is that lanthanum and its neighbours are chemically almost the same. Each additional electron across the series goes into a 4f orbital buried beneath the filled 5s and 5p shells, so it barely participates in bonding and barely alters how the atom behaves. All of them form trivalent ions; all of those ions are similar in size and shrink only gradually across the row.

Separating them therefore could not rely on any difference in reaction, only on tiny differences in solubility, repeated. Nineteenth-century chemists performed fractional crystallisations hundreds and sometimes thousands of times over, each cycle enriching one fraction slightly, and the discovery of a rare earth was typically an announcement that someone had finally run enough cycles. Industrial separation only became straightforward with solvent extraction in the middle of the twentieth century, and it is still the reason rare-earth refining is capital-intensive and concentrated in few places.

Every barrel of crude passes over it

The single largest use of lanthanum is invisible to everybody outside the oil industry. Fluid catalytic cracking breaks heavy petroleum fractions into petrol-range molecules over a zeolite catalyst, and the catalyst circulates continuously between the reactor and a regenerator where coke is burned off it at around 700 °C in steam.

Bare zeolite Y does not survive that treatment; its aluminum is stripped out and the framework collapses. Exchanging lanthanum ions into the zeolite's cages stabilises the structure against steam at those temperatures and increases the activity per site. Lanthanum-exchanged catalyst is used in essentially every refinery in the world, consuming thousands of tonnes of lanthanum oxide a year, and no other element does the job as cheaply.

The hybrid car that ran on lanthanum

The other historically large use was energy storage. A nickel-metal hydride cell stores hydrogen in the metal lattice of its negative electrode, and the alloy that made that practical was LaNi₅ and its mischmetal-substituted variants — an AB₅ intermetallic that absorbs and releases hydrogen reversibly through tens of thousands of cycles at ordinary temperatures.

Every first-generation hybrid vehicle carried a NiMH pack, and each pack contained on the order of ten kilograms of lanthanum-bearing alloy. For roughly fifteen years, hybrid production was one of the largest single draws on world lanthanum supply. Lithium-ion has since taken almost all of that market on energy density, and NiMH survives mainly in consumer cells and in a few vehicles chosen for the chemistry's tolerance of abuse.

Lanthanum's third significant use is optical. Lanthanum oxide raises the refractive index of glass while keeping dispersion low, which is exactly the combination a lens designer wants and cannot otherwise get. Kodak's rare-earth glasses in the 1930s were the first, and modern camera, projector and microscope objectives depend on lanthanum-bearing glass types to correct aberrations without stacking up elements. Lanthanum carbonate also has a genuine medical role, binding phosphate in the gut of dialysis patients whose kidneys can no longer excrete it.

The balance problem

Rare-earth mining has an economic pathology, and lanthanum is the clearest illustration of it.

The elements come out of the ground in fixed proportions set by the deposit, not by demand. A typical light rare-earth ore is roughly a quarter to a third lanthanum and rather more cerium, with the neodymium and praseodymium that actually command high prices making up a much smaller fraction. A producer who mines enough ore to satisfy the world's appetite for magnet metals necessarily produces several times more lanthanum than anyone wants.

The result is a persistent glut. Lanthanum trades at a small fraction of neodymium's price, stockpiles accumulate at separation plants, and a good part of the economics of a rare-earth project turns on whether the operator can find any use at all for the lanthanum and cerium fractions. This is what the industry calls the balance problem, and it is why proposals for new applications of lanthanum — in cast irons, in fuel-cell electrolytes, in water treatment — get attention out of proportion to their volume. Solving it would lower the cost of every other rare earth.

Lanthanum-138, a nuclide nobody can quite explain

Lanthanum is almost entirely lanthanum-139. The remaining 0.09% is lanthanum-138, and it is one of the strangest nuclides in nature.

It is radioactive, decaying both by electron capture to barium-138 and by beta emission to cerium-138, with a half-life around 10^11 years. More interestingly, it cannot be made by either of the two processes that build most heavy elements. Slow and rapid neutron capture both proceed along paths that bypass it entirely, which makes lanthanum-138 a p-nuclide — one of about thirty-five proton-rich isotopes that the standard neutron-capture picture cannot account for.

The leading explanation is the neutrino process: during a core-collapse supernova the enormous neutrino flux passing through the overlying shells knocks neutrons and protons out of existing nuclei, and lanthanum-138 is one of the few nuclides thought to be produced predominantly this way. Its abundance is therefore used as a constraint on supernova neutrino physics, which makes one of the rarest primordial isotopes on Earth a probe of what happens inside an exploding star.