Diamond vise turns hydrogen into a metal potentially ending 80-year quest
Diamond vise turns hydrogen into a metal, potentially ending
Using two diamonds, scientists squeezed hydrogen to pressures above those in Earth's core.
Sang-Heon Shim, Arizona State University
By Robert F. ServiceJan. 26, 2017 , 2:00 PM
Last October, Harvard University physicist Isaac Silvera invited a few colleagues to stop by his lab to glimpse
something that may not exist anywhere else in the universe. Word got around, and the next morning there was a
line. Throughout the day, hundreds ﬁled in to peer through a benchtop microscope at a reddish silver dot trapped
between two diamond tips. Silvera ﬁnally closed shop at 6 p.m. to go home. "It took weeks for the excitement to
die down," Silvera says.
That excitement swirled because by squeezing hydrogen to pressures well beyond those in the center of Earth,
Silvera and his postdoc Ranga Dias had seen a hint that it had morphed into a solid metal, capable of
conducting electricity. "If it's true it would be fantastic," says Reinhard Boehler, a physicist at the Carnegie
Institution for Science in Washington, D.C. "This is something we as a community have been pushing to see for
The feat, reported online this week in Science, is more than an oddity. Solid metallic hydrogen is thought to be a
superconductor, able to conduct electricity without resistance. It may even be metastable, meaning that like
diamond, also formed at high pressures, the metallic hydrogen would maintain its state—and even its
superconductivity—once brought back to room temperatures and pressures.
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Still, claims of solid metallic hydrogen have come and gone before, and some experts want more proof. "From
our point of view it's not convincing," says Mikhail Eremets, who is pursuing solid metallic hydrogen at the Max
Planck Institute for Chemistry in Mainz, Germany. Others in the contentious ﬁeld are downright hostile to the
result. "The word garbage cannot really describe it," says Eugene Gregoryanz, a high-pressure physicist at the
University of Edinburgh, who objects to several of the experiment's procedures.
The dispute arises because high-pressure hydrogen experiments are hard to pull oﬀ, and even harder to
interpret. First, scientists place a thin metal gasket between two ﬂat-tipped diamonds. The gasket holds the
hydrogen in place between the tips as the diamonds are cranked together. The intense pressure can force
hydrogen into defects on the surface of the diamonds, causing them to become brittle and crack. So researchers
have learned to apply transparent protective coatings to their diamonds. But the additional material makes it
tricky to interpret laser measurements of what's going on in the center. Furthermore, past pressures of about 400
gigapascals (GPa)—about 4 million times atmospheric pressure—the hydrogen turns black, preventing laser
light from getting in.
A pressing matter
Squeezing hydrogen at ultracold temperatures, scientists may have found the boundary where it
becomes solid metal. (Hydrogen is a gas at low pressures, in a region too small to be seen in the
Ranga Dias and Isaac
Scientists have already made liquid
metal hydrogen—the substance
thought to form the interior of giant
planets like Jupiter—by ramping up
pressure at higher temperatures.
Silvera wanted to work at low
temperatures and transform hydrogen
into something still more exotic: solid
metal. At cryogenic temperatures,
hydrogen is a liquid. As the pressure
rises, the liquid quickly becomes a
nonmetallic solid (see diagram, left). In
1935, Princeton University physicists
Eugene Wigner and Hillard Bell Huntington predicted that beyond 25 GPa, the nonconductive solid hydrogen
would become metallic. But experimentalists passed that threshold decades ago with no sign of a solid metal.
Silvera and Dias claim they've pushed their cell into an unexplored realm of low temperature and extreme
pressure, succeeding in part because they avoided continuous high-intensity laser monitoring that they say can
also cause an anvil's diamonds to fail. Eventually, as they neared 500 GPa, the black sample became shiny and
reddish. A low-intensity infrared laser—one that wouldn't risk stressing the diamonds—revealed a strong spike
in the sample's reﬂectance, as expected from a metal. Only then did the Harvard pair use a diﬀerent laser, in a
procedure called Raman spectroscopy, to verify the peak pressure in the diamond cell.
Silvera and Dias concede that their reddish silver speck could be a liquid rather than a solid, and they have not
dared to release it from their diamond-tipped vise. But they are conﬁdent it is a metal—a "very convincing" claim,
says Neil Ashcroft, a Cornell University physicist who predicted the superconductive state of hydrogen nearly 50
Eremets and others say they need more proof that the team has created a solid metal or even a metal at all.
"We see only one experiment. It should be reproduced," Eremets says. He also wonders whether the team
actually reached the claimed 495 GPa, because that is usually determined through continuous Raman laser
monitoring. Except for the ﬁnal 495-GPa Raman measurement, Silvera and Dias were forced to estimate
pressures from the number of turns of the screws on their anvils. Raymond Jeanloz, a high-pressure physicist at
the University of California, Berkeley, also wants to be sure the trapped speck is pure hydrogen, because the
gasket or the diamond coating could have broken down and reacted at high pressures. "It has fooled people in
the past," he says.
But Silvera remains steadfast. A comparison of reﬂectance measurements from the center of the hydrogen dot
and the surrounding gasket at 495 GPa suggests the hydrogen in the sample is pure, he says. As for the
pressure measurement, Silvera insists he and Dias have studied it closely and veriﬁed their calibration.
Silvera says they have just one experiment to report because they wanted to announce their result before
running further tests that could break their vise. Soon, he says, they plan to run additional Raman laser tests that
should reveal whether the sample has the regular atomic lattice expected of a solid metal. Eventually they will
unscrew the vice and see whether the metal is metastable.
Then, they will begin the experiment again. Claiming total victory in the "hydrogen wars," as Jeanloz calls them,
will require another round or two of evidence.