Rhenium (Re)

Rhenium was the last hole in the periodic table that could be filled with a naturally occurring element having a stable isotope. Everything discovered afterwards — technetium, promethium, astatine, francium and all the transuranics — is radioactive throughout.

That it took until 1925 is a function of rarity and of camouflage. Rhenium's crustal abundance is around 0.7 parts per billion, which puts it among the scarcest of the non-radioactive elements, and it has no ore of its own to speak of. It substitutes quietly for molybdenum in molybdenite, at concentrations of a few hundred parts per million, and is invisible in the chemistry of everything around it.

Nipponium, and a plate that survived

The conventional credit belongs to Walter Noddack, Ida Tacke and Otto Berg, who reported the element in 1925 after concentrating it from gadolinite and molybdenite and identifying it by its X-ray spectrum. They named it after the Rhine. Tacke — later Ida Noddack — has an additional and mostly unacknowledged claim on history: in 1934 she was the first person in print to suggest that a uranium nucleus struck by a neutron might break into large fragments, and she was ignored for four years.

The data card above names Masataka Ogawa instead, and the reason is a genuine and unresolved historical argument.

In 1908 Ogawa, working in Japan, announced a new element extracted from thorianite and named it nipponium. He placed it at atomic number 43. That assignment was wrong, and the claim was discarded. But in 2004 Kenji Yoshihara re-examined an X-ray photographic plate of Ogawa's sample that had been preserved by his family, read the spectrum, and found no element 43 and clear evidence of element 75. On that reading Ogawa had separated rhenium seventeen years before the Noddacks, and simply put it in the wrong box.

Not everyone accepts it. Eric Scerri, who examined the case in detail, concluded that Ogawa's original claim does not hold up as a discovery. The dispute turns on what a discovery requires: Ogawa had the material and demonstrably had the element in it, but he did not know what he had. The symbol Np, incidentally, was briefly in use for nipponium; it later went to neptunium.

A volcano that breathes rhenium

Because rhenium has no ore, essentially all of it is a byproduct of a byproduct. Porphyry copper deposits are mined for copper; molybdenite is recovered as a secondary product; roasting the molybdenite drives off rhenium as its volatile heptoxide, which is scrubbed from the flue gas and worked up into ammonium perrhenate. Chile, the United States, Poland and Kazakhstan supply most of it, and the world makes only a few dozen tonnes each year.

There is exactly one place where rhenium behaves like an ordinary mineral. On the Kudriavy volcano, on Iturup in the Kuril Islands, high-temperature fumaroles deposit rheniite, a rhenium sulfide, in visible crystals — the only known rhenium mineral occurring in quantity anywhere. Estimates suggest that single volcano vents something like twenty kilograms of rhenium a year into the atmosphere, which is a startling number for an element this scarce.

Three per cent of a turbine blade

Around three-quarters of all rhenium goes into one application, and understanding it requires one idea. A jet engine's efficiency rises with the temperature at the turbine inlet, and the limit on that temperature is what the first-stage blades can survive. Modern blades are grown as single crystals, with no grain boundaries for creep to exploit, from nickel-based superalloys.

Rhenium added to those alloys at a few per cent slows atomic diffusion within the nickel matrix. Creep at high temperature is diffusion-driven, so slowing diffusion raises the temperature and stress the blade can take for a given service life. Second-generation single-crystal alloys carry around 3% rhenium; third-generation alloys roughly double that. The gain per blade is tens of degrees, which in engine terms is worth a great deal of fuel.

The dependence became visible in 2006-2008, when rhenium prices spiked above $10,000 a kilogram as aerospace demand and Chinese industrial growth collided. Engine makers responded by developing rhenium-lean alloy variants, building closed-loop recycling of scrapped blades, and in one case buying into a mine. Rhenium is on critical-materials lists in both the United States and the European Union for precisely this reason.

The second real use is chemical. Platinum-rhenium bimetallic catalysts, introduced at the end of the 1960s, transformed catalytic reforming — the refinery process that turns straight-run naphtha into high-octane petrol components and, incidentally, supplies most of the world's hydrogen and aromatics. Rhenium keeps the platinum from sintering and makes the catalyst tolerant of coke, which allows the reformer to run at lower pressure and higher yield.

Ductile where its neighbours are not

Rhenium's melting point is second only to tungsten's among metals, and its boiling point is higher still — high enough that most compilations, including the figures above, place rhenium rather than tungsten at the top of the entire periodic table for boiling point. The two are within about forty kelvin of each other and different data sets order them differently, so the honest statement is that rhenium and tungsten contend for the title and rhenium usually wins by a nose.

More useful than either number is a property rhenium has that tungsten and molybdenum do not: it stays ductile. Tungsten and molybdenum have a ductile-to-brittle transition and shatter when cold-worked; rhenium can be bent, drawn and rolled from cryogenic temperatures upward without one. Alloying rhenium into tungsten transfers some of that behaviour — the "rhenium effect" — which is why tungsten-rhenium alloys are used for the highest-temperature thermocouples, for filaments in mass spectrometers, and for the rotating anode targets in X-ray tubes, where a pure tungsten surface crazes under thermal cycling and a tungsten-rhenium one does not.

The bond that started a field

In 1964 F. Albert Cotton determined the structure of the octachlorodirhenate ion and found the two rhenium atoms held together by a quadruple bond — a sigma bond, two pi bonds and a delta bond formed from overlapping d orbitals. Nothing in the existing theory of chemical bonding had a place for a fourth bond between two atoms.

The result opened the whole field of metal-metal multiple bonding, which now runs from chromium quintuple bonds through the cluster chemistry underpinning much of modern organometallics. Rhenium was where it started, because its d-electron count and its size make the arrangement unusually stable.

A half-life that depends on the electrons

Rhenium-187, which makes up nearly two-thirds of natural rhenium, beta-decays to osmium-187 with a half-life of about 41.6 billion years. The pairing supports the rhenium-osmium chronometer used to date sulfide ores directly.

The strangeness is what happens when the atom is stripped of its electrons. The decay releases only about 2.5 keV, one of the lowest energy releases of any beta decay, and that leaves an option normally unavailable: instead of ejecting the electron into free space, the nucleus can place it directly into a vacant atomic orbital. In 1996 an experiment at GSI in Darmstadt circulated fully ionised rhenium-187 nuclei in a storage ring and measured the half-life of this bound-state decay at roughly 33 years — a billion-fold acceleration.

The consequence is astrophysical. Inside a hot star, rhenium is substantially ionised, so the rhenium-osmium clock does not run at its laboratory rate. Any attempt to use the pair to date the galaxy has to model the ionisation history of the material, which is one of the few situations in which a nuclear half-life is not a constant of nature but a property of the environment.