The big squeeze

What form do molecules of different elements take when they are put under extreme pressure? Is there more to this finding or is it yet another simple science trick?, asks Kenneth Chang...

In a recurring comic bit, David Letterman used to place household items – a plate of jelly doughnuts, a six-pack of beer, in an 80-ton hydraulic press, gleefully watching as the items squirted, exploded and disintegrated.

That was but a light touch compared with the pressures Russell J Hemley and his colleagues exert on molecules at the Carnegie Institution for Science here.

When substances are pressed between two diamonds, they achieve a sort of alchemy. No, iron does not change to gold, but familiar atoms and molecules behave differently. Oxygen turns blue, then scarlet, and finally into a shiny metal. Peanut butter, as an early pioneer in the field demonstrated at General Electric in the 1950s, turns to diamond.

New tests on the blockThese are not just stupid science tricks. At the Carnegie Institution’s Geophysical Laboratory, the interest in high-pressure science grew out of the laboratory’s mission to study Earth’s interior. The research over the decades has broadened.

Scientists use the high-pressure transformations to explore permutations of matter that do not exist in most of the universe, casting insight on what is going on near Earth’s core or within Jupiter.

“It’s a new kind of chemistry,” Hemley said. It certainly gives new meaning to the term “high pressure.” At sea level, the air pressure is 14.7 pounds per square inch, or 1.03 kilograms per square centimeter. At the bottom of the Mariana Trench in the western Pacific, the deepest slice of the ocean, seven miles of water impose a pressure of about 8 tons, or about 16,000 pounds, per square inch.

With the diamond anvils at the Carnegie Institution, the pressure reaches 50 million psi. In Germany, researchers have devised versions – essentially an anvil within an anvil, that can more than double that.

Even so, certain places in the universe are far more crushing. The pressure at the center of Jupiter is more than 1 billion psi. And then there are neutron stars, the collapsed remnants of burned-out suns, where gravity pulls atoms so closely together that the pressures are thought to reach a billion trillion times that of Jupiter’s core.

In appearance, the anvils used at Carnegie and other laboratories around the world are rather unremarkable. Designs vary, but the housing is often a pancake-shaped metal cylinder about 2 inches wide and less than 1 inch high. To exert pressure, the scientists sometimes turn the screws on the top of the cylinder, pulling the top and bottom plates closer together.

Inside, the bending of the cylinder plates presses together tips of two small diamonds, each a quarter to half a carat, typically no bigger than a quarter of an inch. On one diamond tip, a notch shaped like the crater of a volcano has been carved to trap the material that is to be squeezed. The other tip presses down, like a stiletto heel crushing a bug.

The screws apply only a few pounds of force. But those translate into tremendous pressure, because the diamond tips are so small. “Pressure is just force divided by area,” Hemley said.Sunlight into electricity?

One of the Carnegie scientists, Timothy A Strobel, has been using these techniques to create a new form of silicon that could more easily turn sunlight into electricity. The usual form of silicon cannot directly absorb the photons of sunlight. “The atoms in the lattice need to shake a little bit to sort of kick the electron in the right position,” Strobel said.

By squeezing a mixture of silicon and sodium, he has created a new tubelike structure. After chemically extracting the sodium, the tubes of silicon possess the desired electronic property of absorbing photons without the shaking.

Created under pressure, the silicon structure is metastable. That is, it does not snap back into its original form when the pressure is removed.

“We’re not playing with the same set of materials everybody else is playing with,” Strobel said. One of the surprises is the changes that atoms undergo under pressure.
 At modest pressures, atoms stack neatly, like cannonballs, and scientists expected that the atoms would remain in this efficiently packed pattern as they were squeezed closer together.But while the distance between atoms does indeed narrow, they no longer remain in neat piles. As the atoms converge, the electrons are squirted into different locations, reconfiguring the molecules they are part of. The process turns some metals, which readily conduct electricity, into insulators, which do not. Some insulators turn into superconductors, ferrying current without resistance.

Effects of researchEven noble gases like xenon, which rarely interact with other atoms, intertwine with hydrogen to form novel, complex structures. Malcolm McMahon, a physicist at the University of Edinburgh in Scotland, was curious about the red oxygen. Within an anvil, he and his collaborators managed to create a single ruby crystal of oxygen, off which they bounced X-rays to determine the structure.

The oxygen atoms, usually bonded in pairs, had been pushed together into clusters of eight, and that structure absorbed the shorter blue wavelengths of light. The remaining wavelengths passed through.

Carbon is another element of particular interest. The Carnegie Geophysical Laboratory has a leading role in the Deep Carbon Observatory, a 10-year effort to better understand what happens to carbon in the high-pressure, high-temperature conditions within the planet. The unknowns include just how much carbon exists in Earth, Hemley said.

Perhaps the biggest puzzle in high-pressure physics involves the simplest and most abundant atom, hydrogen. At the extreme pressures at the center of Jupiter, hydrogen is believed to turn into a fluid metallic state, with the churning flows generating the planet’s powerful magnetic fields. In the laboratory, that has been elusive.

Theoretical calculations had predicted that the transition to metal would occur at pressures achievable in the anvils. Hemley’s and other groups found signs of the transformation at about 45 million psi, but the evidence was not definitive.