Saturday, 21 January 2012

In a squeeze

Elements under pressure reveal secrets of extreme chemistry.

Bruce Banner isn’t the only scientist who could crush you with one mighty squeeze. These days, the Hulk’s superhuman strength is matched by researchers who squish all kinds of stuff in superscience experiments.

The goal isn’t to save the world from baddies, but to explore new frontiers in the nature of matter. After all, most material in the universe exists at bone-crushing pressures. Think massive stars and planetary cores — realms no comic book fan or other Earth dweller has ever seen.

Deep within the planet, rock experiences pressures more than 1 million times as great as the “1 atmosphere” that ordinary humans live under at sea level. Pressures at the centers of ultradense neutron stars are some trillion quadrillion times greater. Under such extreme conditions, atoms themselves begin to buckle.

To mimic these hellish realms, scientists are ramping up pressure in the lab, like the Hulk getting ever stronger as he gets madder. In the process, they’re squeezing out some surprising insights.

One team has found a new kind of iron oxide, a compound that somehow had never been seen before, even though it contains two of the most common elements in Earth’s crust. Another group argues that hydrogen’s odd behavior at high pressures means that the cores of giant gas planets, such as Jupiter, are eroding in a slow hydrogen drip. Meanwhile scientists at the National Ignition Facility in Livermore, Calif., have squeezed diamond to record pressures, uncovering unexpected and exotic behaviors.

Chemistry, it seems, is a different beast under high pressure. “We’re developing a whole new paradigm for understanding the nature of matter,” says Russell Hemley, a chemist at the Carnegie Institution for Science in Washington, D.C.

Hydrogen crush

The idea of squeezing materials to see what happens dates back at least to the 17th century, when British chemist Robert Boyle discovered that doubling the pressure on an ideal gas halved its volume. Around the same time, researchers at the Accademia del Cimento, a scientific society in Florence, Italy, were exploring whether liquids, too, could be compressed. The scientists filled a metal sphere with water and banged it with a hammer. Perhaps not surprisingly, the sphere leaked. But that experiment, Hemley says, set the stage for far more technologically adept investigations.

By the 20th century scientists knew that ordinary matter — be it solid, liquid or gas — behaves according to chemical rules laid out by its electrons, those negatively charged particles that buzz around atomic nuclei in well-defined regions known as orbitals. It turns out that squishing matter doesn’t just compress its atoms so that they stack closer together, like a pile of well-arranged oranges at a farmers market. Compression also radically alters electron orbitals, in different ways depending on their original shapes.

Suddenly electrons can zip around in places they haven’t been before, and the rules typically governing the periodic table of the elements go out the window.

Perhaps the poster child for odd behavior at high pressures is hydrogen, the most common element in the universe. As the simplest element, with just one proton in its nucleus and one orbiting electron, hydrogen seems like it should behave in a straightforward way. But recent experiments have shown that, like Bruce Banner, it suffers from multiple personalities.

Most intriguing, scientists say, is the fact that if you squeeze hydrogen hard enough, this flighty gas transforms into a dense fluid whose electrons move in an ill-defined sea, allowing it to conduct electricity and behave as a metal. Understanding how two atoms linked as a gaseous H2 molecule split and form single atoms flowing as a liquid could illuminate what happens to more complicated molecules under pressure, says physicist Alexander Goncharov of Carnegie. “Once we understand that simple system, others may become simpler,” he says.

Goncharov, Hemley and many other scientists probe hydrogen and other materials by crushing them between two small diamonds in a machine known as a diamond anvil cell. The pointy ends of the cut diamonds narrow to a tiny tip where, when squeezed together, the pressure soars. In a small dent where the diamonds meet, an injected sample can be compressed to unfathomably high pressures.

Using such a device, scientists at the Max Planck Institute for Chemistry in Mainz, Germany, announced in Nature Materials in November that they had created metallic hydrogen at room temperature and pressures around 2.6 million times Earth’s atmosphere (SN: 12/17/11, p. 9). If confirmed, the discovery would fulfill a long-sought goal; scientists first predicted the existence of metallic hydrogen in 1935.

But some experts are withholding judgment on the new work. It’s one thing to squeeze materials at high pressures and see something unusual; it’s another to establish conclusively what that unusual observation means. Several researchers say they have data that contradict the metallic hydrogen claim, but they do not want to discuss their work in detail until it appears in peer-reviewed journals.

The Max Planck group, led by Mikhail Eremets, is involved in another high-pressure disagreement. In a paper appearing in Science in 2008, Eremets’ team, along with colleagues from the University of Saskatchewan in Canada, reported that a mix of silicon and hydrogen became superconducting at high pressures. This compound, known as silane, is made of one silicon atom bonded with four hydrogen atoms. As an industrial compound, silane is used as a coating agent, a water repellent and in other applications. But mash it in a diamond anvil cell, and at around 960,000 atmospheres it starts allowing electrons to flow freely, the researchers reported.

Not so fast, other scientists said. One challenge with studying hydrogen is that at high enough pressures and temperatures, it starts reacting with just about everything around it — even elements that are usually chemically inert. Theorists led by Duck Young Kim, now at Carnegie, have reported that hydrogen may hook up with famously unreactive platinum at pressures around 210,000 atmospheres. At higher pressures, 700,000 atmospheres or above, this newborn platinum hydride may even start to superconduct, shuttling electrons without resistance, the scientists wrote in September in Physical Review Letters.

Such a mix of platinum and hydrogen could explain the superconductivity reported in silane, an international team argued in August in Physical Review B. The team’s calculations suggest that platinum hydride could form as the silane breaks apart into silicon and hydrogen — and that hydrogen reacts with platinum electrodes used in the experiment. One particular crystal form of platinum hydride, the scientists say, could explain the superconductivity supposedly observed.

Eremets’ team stands by its work, but the experience underscores how complicated high-pressure science can be.

Core compression

Despite the difficulties involved in working under extreme conditions in the lab, it is still the only way to figure out what’s happening in many places in the universe, including the ground under people’s feet. For geologists, high-pressure experimentation is about as close as they will ever get to a journey to the center of the Earth. And the latest high-pressure studies show how many surprises still lurk there.

Iron, for instance, is the fourth most abundant element in the Earth’s crust and makes up nearly all of the planet’s core. Yet researchers have only now discovered an entirely new iron compound; it contains four atoms of iron and five of oxygen and exists only at high pressure.

Barbara Lavina of the University of Nevada, Las Vegas and colleagues synthesized this compound in a diamond anvil cell by smooshing a different compound made of iron, carbon and oxygen. The compound began to break apart, and at about 100,000 atmospheres and 1,800 kelvins (1,500˚ Celsius) a new type of crystal appeared.

Other iron oxides are common in nature, but this was the first time this particular chemical combination had been seen. “It was thrilling for me just to write the formula Fe4O5,” says Lavina, whose report appeared October 18 in the Proceedings of the National Academy of Sciences.

Understanding the details of how iron and oxygen atoms bond with one another may also reveal key properties of the Earth’s innards, such as how heat flows within the planet. One mineral crucial to revealing these details is wüstite, or FeO. Independent teams at the University of Chicago and Osaka University in Japan recently squeezed wüstite and found that it conducts electricity at pressures and temperatures similar to those found in the planet’s outer core and lower mantle, the layer just above the core.

Pockets rich in wüstite may exist at the core-mantle boundary, where the mineral may transfer heat from the core into more shallow depths, says Chicago’s Rebecca Fischer. Metallic FeO could also help explain why oxygen dissolves in metal more readily at high pressures, such as in the planet’s core, Fischer’s team reports in an upcoming Geophysical Research Letters.

The world of high-pressure discovery also extends well beyond Earth — to other planets in the solar system, and on to other planetary systems. In particular, the cores of gas giant planets are “the least accessible but in many ways the most important objects in the solar system,” says Hugh Wilson, a planetary chemist at the University of California, Berkeley. The very existence of the cores allowed Jupiter and Saturn to coalesce around them; the gravitational pull of the completed gas giants then helped dictate how the rest of the solar system grew.

Yet scientists don’t know much about how the giant planet cores formed. In principle, they were born as bits of rock and ice swirling around the newborn sun began to glom together, becoming big enough to start attracting hydrogen and helium gas to make up the rest of the planet. Today researchers don’t agree on how big the cores are, much less the conditions that exist inside them.

One new idea, born from some high-pressure theoretical calculations involving hydrogen, even suggests that giant planet cores are slowly dissolving away. Over time, watery ice in Jupiter’s core dissolves in the hydrogen-rich material swirling above so that the core shrinks, Wilson and Burkhard Militzer, also of Berkeley, write in an upcoming Astrophysical Journal. “What’s going on inside Jupiter is more complicated and less homogeneous than had been taken into account in previous models,” Wilson says.

The work may even shed light on planets in other solar systems, which astronomers have glimpsed only indirectly so far. Many known exoplanets are more massive than Jupiter, and so they are also hotter inside. Cores of these exoplanets would have started eroding away even faster than Jupiter’s, Wilson says. As a result, elements may have leached from the core and become well-mixed in the gassy atmosphere. One day, if astronomers on Earth can get a detailed picture of an exoplanet’s atmosphere from afar, they may need to account for such internal chemical mixing in order to properly understand what they’re seeing.

The squeeze machine

In perhaps the ultimate test of high-pressure science, researchers are gearing up to squeeze things at the world’s most powerful laser machine. The three-football-field-long National Ignition Facility will focus 192 laser beams on a single tiny target. The eventual goal is to fuse the nuclei of hydrogen atoms, thus harnessing in the lab what the sun and billions of other stars do daily.

But for now, as NIF ramps up toward full power, other scientists are taking advantage of early, prefusion tests to see what happens to materials put in the path of the beams. “NIF is uniquely capable of trying to explore this realm of compression science,” says Jon Eggert, a material scientist at the lab.

This spring, NIF scientists squished diamond in the facility’s laser beams to pressures up to a crushing 50 million atmospheres — more than double the previous record set using a different compression technique by the OMEGA laser at the University of Rochester. So far, Eggert and his colleagues have seen stress patterns appear in diamond crystals that no one has seen before. The researchers have also put tantalum into the machine and spotted what may be a new transition between crystal forms at around 3.4 million atmospheres.

Later this year, the NIF team plans to squeeze materials at 100 million atmospheres. In theory, NIF could approach or exceed the pressure required to seriously disrupt the shell structure of atoms, called the atomic unit of pressure. That would come at around 300 million atmospheres, Eggert says, and would bring with it breakthrough insights into how matter behaves under such conditions. But it’s still not clear, he says, whether the geometry of how the laser beams come together will allow the team to achieve the atomic unit of pressure.

No matter how high the NIF manages to go, Hemley says, it and the other experiments will continue to uncover new surprises. “Exploring these simple planetary materials under extreme conditions is really deepening our understanding of chemistry,” he says. “A lot of what we thought we knew is wrong.”

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