Plutonium (Pu)

Metallurgists who have worked with plutonium tend to describe it in the language of a difficult animal rather than a material. It is the only element with six distinct crystal structures at ordinary pressure, it changes volume violently between several of them, one of those forms contracts as it gets hotter, and the solid metal floats on its own liquid. Nothing else in the periodic table does more than one of those things.

Six ways of being a solid

Most metals have one crystal structure, or occasionally two. Heat plutonium from room temperature to its melting point and it passes through six: alpha, beta, gamma, delta, delta-prime and epsilon, in that order, with a seventh appearing under pressure.

The alpha form, the one stable at room temperature, is not metallic in any of the ways metals usually are. Its unit cell is monoclinic and contains sixteen atoms in a low-symmetry arrangement that looks more like a mineral than a metal. It is hard, brittle, and a very poor conductor of heat — the worst of any metal — with an electrical resistivity closer to a semiconductor's than to copper's.

The delta form is the interesting one. It is face-centred cubic, soft, ductile, and roughly a quarter less dense than alpha. That figure is the source of most of plutonium's engineering difficulty: a piece of metal that shifts between those two phases changes volume by an amount no structure can absorb. Delta is not stable at room temperature on its own, but small additions of gallium or aluminum hold it there, which is why plutonium is almost always encountered as an alloy rather than as the pure element.

Delta also does something almost unique. Over part of its range it exhibits negative thermal expansion — it shrinks as it is warmed — which is a consequence of the 5f electrons sitting right at the boundary between being bound to their own atoms and joining the conduction band. That knife-edge is the accepted explanation for the whole zoo of phases: the 5f shell in plutonium cannot decide what it is, and small changes in temperature tip it one way or the other.

One more oddity follows from the same instability. Plutonium contracts when it melts, so the solid is less dense than the liquid and floats on it, a property shared with water, gallium and bismuth and with almost nothing else in the metals.

Pu, not Pl

The element was made in December 1940 at the Berkeley 60-inch cyclotron by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl, who produced plutonium-238 by deuteron bombardment of uranium and identified the far more consequential plutonium-239 the following spring. The work was classified almost at once and did not appear in the open literature until 1946.

Pluto had been discovered in 1930 and was firmly a planet, so the naming sequence that ran Uranus–Neptune–Pluto carried straight through. The symbol did not follow the obvious route. Seaborg proposed Pu rather than Pl, by his own account as a joke on the noise children make at a bad smell, and fully expected the committee to correct him. Nobody did, and the joke is now on the periodic table.

Four colours in one solution

Plutonium's aqueous chemistry is as unsettled as its metallurgy. It can occupy four oxidation states — +3, +4, +5 and +6 — that are so close in energy that all four can be present in the same solution at once, each with its own colour: blue-violet, tan, pink and orange. A single sample can therefore be a muddy composite whose shade depends on acidity and on how long it has been sitting. No other element in the table maintains four oxidation states in equilibrium like this, and it is the central problem in separating plutonium from anything else, because a separation designed around one oxidation state has to contend with three others.

Still transmitting from interstellar space

Plutonium's most visible legacy outside weapons is plutonium-238, an isotope with an 87.7-year half-life that releases, from alpha decay alone, a little over half a watt for every gram present. Encased as an oxide and surrounded by thermocouples, it makes a power supply with no moving parts that runs for decades.

• Both Voyager spacecraft, launched in 1977, are still returning data from beyond the heliopause on plutonium-238 heat, with output slowly falling as the isotope decays and the thermocouples degrade.
• Cassini ran on it for thirteen years at Saturn, where sunlight is a hundredth of Earth's.
• New Horizons crossed to Pluto — the element's namesake — on it, arriving in 2015.
• The Curiosity and Perseverance rovers use it, which is why they operate through Martian winters and dust storms that ended the solar-powered Spirit and Opportunity.
• In the 1970s, several thousand cardiac pacemakers were built around tiny plutonium-238 sources, chosen because a battery that never needs replacing avoids a second operation.

Plutonium-239, by contrast, has a 24,100-year half-life and is a fissile material; plutonium-240, which accumulates alongside it, undergoes spontaneous fission at a far higher rate, and the ratio between those two isotopes is what distinguishes one grade of the metal from another.

A layer in the sediment, and a signal from a dead star

Roughly five tonnes of plutonium were dispersed into the atmosphere by nuclear testing before the 1963 Partial Test Ban Treaty, and it settled worldwide. Geologists now use that fallout as a stratigraphic marker: a thin plutonium-bearing horizon appears in lake sediments, ice cores and peat bogs across the planet, rising from nothing in the late 1940s to a sharp peak in 1963. It is one of the leading candidate markers for the base of a proposed Anthropocene epoch, for the straightforward reason that it is globally synchronous and unambiguously anthropogenic.

The longest-lived isotope, plutonium-244, has an 80-million-year half-life — far too short to survive from the formation of the solar system in any detectable quantity, but long enough to travel. In 2021 a team led by Anton Wallner reported live plutonium-244 alongside iron-60 in a deep-sea ferromanganese crust, laid down over the last few million years. Neither isotope can be made on Earth in those quantities. Both arrived, which points to debris from nearby stellar explosions drifting through the solar system recently enough to still be decaying in the seabed.