Elemental Metamorphosis

PHYSICS & ASTRONOMY_High-Pressure Research

The inside of planets, stellar shells and numerous other uncomfortable spots in space have one thing in common: matter there is under extreme pressure of several million atmospheres. Mikhail Eremets and his colleagues produce such cosmic pressures in their lab at the Max Planck Institute for Chemistry in Mainz – and they do so in surprisingly simple experiments. They are researching which unique transformations gases, but also metals, undergo under these conditions.

TEXT ROLAND WENGENMAYR

he cosmos is full of places Since this world-record-setting mea- where the extreme is nor- surement, many chemists, materials mal. “In the core of Jupiter, scientists and physicists are contem- for example, the pressure is plating new, fascinating questions: more than 30 million atmo- Does hydrogen even become super- T spheres,” explains Mikhail Eremets at conducting at room temperature at the Max Planck Institute for Chemistry even higher pressure – in other words,

Ich weiß nicht genau, was zusammengehört, in Mainz: “And planetary researchers does it conduct electricity entirely have long suspected that hydrogen, without electrical resistance? That is which is the primary component of precisely what theoreticians predict. these gas planets, is metallic there.” Only residents of Jupiter – if there American physicists Eugene Wigner were any – would be able to use super- and Hillard Bell Huntington predicted conductive extreme-pressure hydro- this strange, solid state of hydrogen as gen in electrical cables. However, the far back as 1935. Until recently, though, discovery of room-temperature super- it remained just speculation. It wasn’t conductivity in hydrogen could help until 2011 that the small Max Planck in solving the mystery of high-tem- team led by Mikhail Eremets, a Russian, perature superconductors and in tai- provided certainty with its experiments. loring these materials for technical In fact, the Mainz-based scientists were applications. Ceramics lose their resis- the first to observe how hydrogen be- tance at higher temperatures than comes a metal – at an enormous pres- conventional superconductors, but sure of more than 2.7 million bar, or still too far below zero to really be 2.7 million atmospheres. technologically useful. >

46 MaxPlanckResearch 4 | 12 The researchers in Mainz store their diamonds, not in a pocket-type safe, but in an anvil cell. They produce up to 4.4 million atmospheres of pressure between the flattened tips of the precious stones. Photo: Thomas Hartmann Creating pressure through a gentle turn: An Allen wrench and four screws are sufficient to produce pressures of several million atmospheres inside the anvil cell. Only diamonds can withstand this load. However, the stones turn brown where they crash into each other, as can be seen by looking through one of the diamonds in the cell (right).

Regardless of potential new findings for lions of kilometers of cosmic travel sep- small, explains Eremets, “the largest energy technology, the experiments arating us from the answer to our ques- one is just 0.1 carat.” He picks up a lit- with hydrogen are of interest to physi- tion. The solution is located right in tle box with small, glittering stones, all cists and chemists for a very basic rea- Eremets’ cramped study, where an old of which shattered in experiments. son: hydrogen is the simplest atom and clock placidly ticks away the time. It’s Even the hardest material can’t with- the most common of all chemical ele- lying on a workbench with a stereomi- stand every pressure, and that’s what ments in the cosmos. This makes it the croscope, and at first glance, it looks al- makes this field of research so difficult. model organism, the Drosophila, of most disappointingly unspectacular. Eremets and his colleague Ivan quantum physics. In fact, it was the at- Troyan, who is also Russian, even had tempt at a physically precise descrip- EXPERIMENTS AT A PRESSURE to become experts in precision grind- tion of the hydrogen atom that first LIKE THAT INSIDE THE EARTH ing of diamonds. The shape they re- gave rise to modern quantum mechan- quire has nothing in common with ics. At the provisional end of this quan- Smiling, Eremets hands his visitor a the grinding that gives diamonds their tum revolution stands the semiconduc- compact metal tube. The brass-colored fire. The Max Planck scientists first tor electronics that catapulted our object has the diameter of a large coin give the small diamonds a basic coni- culture into the Internet age. and vaguely resembles a specialty part cal shape. Then they grind a tiny piece As a matter of fact, at more than 2.2 of a water fitting, but the massive caps of the tip off. This creates a flat surface million atmospheres, hydrogen, too, that seal the two ends of the metal there measuring just around 20 mi- first becomes a semiconductor before tube dispel this impression. Through crometers in diameter. Then the re- it mutates into a metal. This is anoth- small, thick windows, they permit a searchers put their diamonds – flat- er thing the group in Mainz observed glimpse of the inside. And precisely tened tips pointed toward each other for the first time – even imaginative there, in a volume of about five mi- – into a tiny, ring-shaped mount that theoreticians didn’t expect this behav- crometers in diameter – one microme- is likewise highly stable. ior. It is precisely this transformability ter is one-thousandth of a millimeter Before an experiment, the Mainz- under increasing pressure that makes – pressure conditions like those inside based researchers fill the tiny volume hydrogen so fascinating for science, the earth prevail. At least when the de- between the diamond tips in a closed which aims to use such experiments to vice is in operation. box with ultra-pure hydrogen – or with draw basic conclusions about the mat- Of course this raises the question of another material: sodium and nitrogen ter in our cosmos. But how do the Max which materials can withstand such have also been used in the cells in Planck scientists in Mainz manage to enormous pressure. In answer, one Mainz. Now the enormous pressure is study their samples under such ex- might borrow Marilyn Monroe’s im- needed. Mikhail Eremets demonstrates treme conditions? mortal song “... but diamonds are a re- how amazingly easily this is done. He In any case, they don’t need, for ex- searcher’s best friend.” Diamonds are, doesn’t need a gigantic press, as a lay- ample, expensive space probes that in fact, the high-pressure researcher’s person might expect. All the scientist crash into the bottomless abyss of the best friends. The diamond anvil cell, as needs is a screwdriver and the ring of gas planet Jupiter. So, at the institute in the small tube is called, contains two precision screws that enclose each of

Mainz, there aren’t millions and mil- diamonds. However, they are quite the two windows in the metal caps. Photos: Thomas Hartmann (left, 2); MPI for Chemistry (right)

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» At a pressure above 2.2 million atmospheres, hydrogen initially becomes a semiconductor, before mutating into a metal. The researchers in Mainz were the first to observe this phenomenon.

The rest is pure and simple mechan- a couple of delicate turns of the screws mittedly yet more extreme – states of ics. By slowly tightening the screws to ultimately result in one, two, three matter. And still there are only a few around the coin-sized surface of the or more million atmospheres of pres- research groups worldwide that can cap, the researcher applies growing sure in the core of the cell. The abso- play in the top extreme-pressure league pressure to the inside of the cell. This lute record for the group in Mainz is with the Max Planck scientists in pressure, which can reach up to one currently 4.4 million atmospheres. Mainz. This is by no means due to a ton, focuses the conical internal work- lack of interest in such experiments. ings on the tiny diamond tip. Its sur- THE PRINCIPLE IS SIMPLE – The reason is the many difficulties in face is just a few square micrometers THE DEVIL IS IN THE DETAIL the details, which cause even experi- in size, while the cap surface, on the enced researchers to fail. The story of other hand, measuring a few square So the principle is surprisingly simple, this years-long battle is also told in centimeters, is huge in comparison. especially considering how much tech- precisely those shattered diamonds in This extreme area ratio acts like an nical effort other disciplines, such as Eremets’ little box. Persistence is what enormous transmission gear, allowing particle physics, expend to reach – ad- helped his group succeed with its ex-

At the trigger: Ivan Troyan (left) and Mikhail Eremets prepare an experiment in which they compress hydrogen to an extreme extent. In doing so, they discovered the previously unknown semiconducting state of the gas, and turned the lightest of all the elements into a metal for the first time. Photo: Thomas Hartmann PHYSICS & ASTRONOMY_High-Pressure Research

» The European Research Council has been supporting the group since 2011, to the tune of 1.9 million euros – a great acknowledgement of the research field and the team’s discoveries.

periments. “We also owe it to the good polished cutting surface always exhib- chamber. Despite this, it protects the conditions at the Max Planck Society, its microcracks. Under pressure, the cracked diamonds against the aggres- which allows us to work on long-term small hydrogen molecules penetrate sive hydrogen. This trick is what ulti- projects,” stresses Mikhail Eremets. into these cracks and shatter the stones. mately helped the group in Mainz to For years, this was the cause of failure become the first to successfully put hy- A METAL FILM PROTECTS THE for every experimenter worldwide. Ere- drogen under a pressure of three mil- DIAMONDS AGAINST HYDROGEN mets’ group was no exception – until lion atmospheres at room temperature they came up with a crucial idea. They without their diamonds shattering. Particularly hydrogen presented chal- vapor-deposited a metal film, for exam- The question of whether the wear lenges that caused the diamonds of all ple of gold, copper or aluminum, on on the diamonds is costly brings a smile high-pressure researchers to date to the diamond anvil surfaces. The film is to Eremets’ lips. “We end up with break- shatter. This is due to the fact that no so thin that light can still penetrate it, age of maybe 20 small diamonds a diamond is truly perfect. Even a finely so it is still possible to look into the year,” he calculates, “which costs about

The researchers working with Mikhail Eremets used different spectroscopic methods to track what happens in their high-pressure experiments. Here, they shine laser light into the cell. At the same time, this setup allows them to study the electrical properties of their samples at different temperatures. 10,000 dollars, making it cheap com- pared with many other experiments.” The fact that the Mainz-based researchers can replace their losses again and again owes to the 1.9 million euros in funding that Eremets’ group receives from the European Research Council. This support, which began in 2011 and is set to continue for several years, is a great acknowledgement of the research field and the team’s discoveries. It allows the small group of a half dozen scientists to exert a good deal of pressure, as it were. And to travel with their experiments to the world’s great research facilities – another advantage of the ex- ceptionally handy cell. “I can simply put it in my pocket and take it with me,” says Eremets enthusiastically. The transparent diamonds allow different kinds of radiation into the samples, and each one provides different information about the behavior of The Mainz-based researchers use light in the near – that is, relatively short-wave – the molecules, atoms and electrons infrared range (NIR) to conduct Raman spectroscopy, which tells them something about under pressure. In their lab in Mainz, the structural changes under high pressure. the researchers shoot a strong laser light in the visible spectrum into the samples. From the light that the sam- same group as the alkali metals lithium, semiconducting state that the group ple then reflects back, they obtain de- sodium and potassium. At slightly discovered. In a semiconductor, certain tailed information about the behavior above two million atmospheres of pres- electrons move nearly freely, but not of the molecules. Additional informa- sure, its diatomic molecules are pressed entirely. They need a small energy kick tion is provided by a particularly per- together into a kind of warm hydrogen to hop over an energy barrier onto the fect kind of infrared radiation from ice. “This substance can then be further electron highway. This energy can be synchrotrons, large accelerator rings compressed to a great degree,” says Ere- provided by light or an electric current. for which Eremets and his colleagues mets, “to a twentieth of its volume.” travel, for instance, to Grenoble, France, The hydrogen molecules squeeze ever THE CONDUCTIVITY PROVES the Swiss town of Villigen, or Chicago more tightly together under the in- THE METALLIC STATE or Hamburg. creasing pressure. “In the process, something happens One can just picture the tension rising THE MOLECULES BECOME with their electrons,” explains the re- right along with the pressure in the lab WARM HYDROGEN ICE searcher. The hydrogen molecule con- in Mainz. Finally, at 2.7 million atmo- sists of two hydrogen atoms, and nor- spheres, the hydrogen made the desired X-ray radiation provides important mally its two electrons are bound to jump. It switched from the unexpected data as well. This electromagnetic radi- this molecule. But now they sense the semiconductor state to the hoped-for ation is so short-waved that it can neighboring molecules closing in on metallic state. The freely moving elec- sharply image the positions of individ- them and essentially expand their radi- trons generated the typical shimmering ual atoms in crystals. This also helps in us of action. This interaction between light reflection of a metal. However, examining metals, which, like many the molecules eventually creates quan- this high reflectivity is not yet proof of solids, have a crystalline structure. For tum highways on which electrons can a metallic state. The electrical conduc- all of these highly specialized radiation fly throughout the entire material un- tivity must be measured, and Eremets’ sources, the researchers need only bring hindered. This is typical for electrically group succeeded in doing so. their little cells – there is no elaborate conductive metals. But an old scientific dream also died equipment to set up. The group in Mainz observed this in Mainz. There had been a hope that, Incidentally, it comes as no surprise transition to nearly freely moving elec- once the hydrogen had been com- to chemists or physicists that hydrogen trons: starting at a pressure of 2.2 mil- pressed into a metal, it would remain could possess metal-like properties. In lion atmospheres, the hydrogen be- stable when the pressure subsided. In

Photos: Thomas Hartmann the periodic system, it belongs to the came black and opaque. This is the the lab in Mainz, they saw for the first

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» It is still not known whether metallic hydrogen is really solid. It could also be liquid like mercury.

time that it relaxes into a gas again as pressure even further. At some point, ductors are solid. Helium, on the oth- the pressure is reduced. Too bad, as it the hydrogen molecules should be er hand, for example, becomes super- might have resulted in the possibility completely crushed. “Theoretically, liquid at very low temperatures. of room-temperature superconductors they should dissolve at about five mil- Like superconductivity, superli- from a press, as it were. But it is still lion atmospheres,” the scientist ex- quidity is a collective quantum state undetermined whether the metallic plains. Theoreticians predict that hy- of many quantum particles that still hydrogen is really solid. It could also drogen could then not only become isn’t completely understood. What be liquid like mercury, suggests Mikhail superconducting, but at the same disappears is not the electrical, but Eremets. His team is currently working time, also superfluid. It would then rather the mechanical resistance: such on clarifying this issue. transform into an exotic quantum liq- a liquid no longer has any internal The researchers also want to know uid that combines two extraordinary friction. As a result, it takes on what, what happens when they increase the quantum effects. All known supercon- for our everyday understanding,

Ivan Troyan (in the foreground) and Mikhail Eremets follow the images and measurement data on their computers during the experiments. The shiny metallic hydrogen can be seen on the screen in the background. Photo: Thomas Hartmann

52 MaxPlanckResearch 4 | 12 Inconspicuous spots with spectacular importance: When looking through a diamond into the hydrogen-filled cell, its interior remains transparent as long as the hydrogen is gaseous, at less than two million atmospheres (left). At 2.2 million atmospheres, the element turns a dark color when it changes to the semiconducting state (center). Finally, at 2.7 million atmospheres, the hydrogen takes on the metallic sheen (right).

which knows only normal liquids, are become a single bond. With their oth- is particularly fascinated by this, be- strange properties: in the form of an er, now free, hands, they reached for cause cg nitrogen theoretically con- ultrathin film, it can crawl up the their neighboring molecules, forming a tains more chemical energy than any sides of a container and run over the crystal with atoms nicely arranged in a other material. “Then we would have, edge by itself. It’s a good thing our cube shape. “That’s something like a ni- for example, a fuel that is superior to beverages aren’t superfluid. trogen diamond,” says Eremets. all fuels in existence today,” says the It may even be possible to stably researcher enthusiastically. So, we may HIGH PRESSURE MAKES SODIUM bring this cg nitrogen into normal one day drive with a tank full of nitro- AS TRANSPARENT AS GLASS pressure conditions. This exotic mate- gen cubes. That would then be a fur- rial did, after all, remain stable down ther completely unexpected applica- In the high-pressure experiments in to 420,000 atmospheres in the Mainz tion like those that basic research turns Mainz, however, it isn’t restricted to experiments in 2004. Mikhail Eremets out again and again. gases turning into metals. Conversely, a metal lost its metallic properties. In 2009, the researchers observed that sodium turns black at about one million TO THE POINT atmospheres, and at twice that pressure, it becomes as transparent as yel- ● Extreme pressures of several million bar – one bar is one atmosphere – prevail lowish glass. In the process, its volume inside planets and stars. Researchers learn a lot about the basic properties of matter from experiments under such conditions. shrank to a fifth of the space it takes up at normal pressure. As it turned out, the ● Researchers at the Max Planck Institute for Chemistry produce extreme pressures between two small diamonds in a handy metal device; their current record is sodium transformed into a kind of salt. 4.4 million atmospheres. Nitrogen, too, behaves strangely ● Matter takes on exotic characteristics under such conditions: hydrogen becomes under pressure. The nitrogen mole- a metal, sodium becomes a salt, and nitrogen forms a diamond-like structure. cules that constitute 78 percent of our atmosphere are chemically extremely stable: its two atoms cling tightly to each other in a triple chemical bond. It GLOSSARY takes a lot of energy to break this bond. Superliquid: Substances whose atoms take on a collective quantum state below a In 2004, the team working with Ere- certain temperature, losing their internal friction in the process. So far, only the helium mets and Troyan approached this tena- isotopes helium-3 and helium-4, and the lithium isotope lithium-6, are known to cious gas with a mix of high pressure become superfluid at temperatures near absolute zero, or minus 273.15 degrees Celsius. and heat. They shot a strong laser into So far, no substances have been discovered that are simultaneously superconducting and superfluid. the diamond cell, where there was a tiny black surface that heated up the Superconductor: Metals, ceramics and some ferrous compounds that lose their electrical resistance below a certain temperature because their electrons form compressed gas. Cooper pairs and, as such, no longer interact with the crystal lattice. At a temperature of more than 1,700 A distinction is made between conventional metallic superconductors that lose degrees Celsius and a pressure of more their resistance only at temperatures of less than minus 250 degrees Celsius, and than 1.1 million atmospheres, the ni- unconventional superconductors, which also include the ceramic high-temperature superconductors. These lose their resistance at the temperature of liquid nitrogen. trogen molecule triple bond burst. Each The current record is minus 135 degrees Celsius. pair was now holding on with just one

Photos: MPI for Chemistry (3) chemical hand – the triple bond had

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