Within and out
Pressures in the cosmos
span more than 60 orders
of magnitude, from the
near-vacuum of intergalactic space to the crushing
force at the center of a
neutron star.
Hydrogen gas in
intergalactic space
Interplanetary space
Internal pressure at the
Large Hadron Collider
100 kilometers
elevation
Sea level
10–32
Pressure (atmospheres)
10–24
10–16
10–8
1
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
TIMELINE, FROM LEFT: GERARD LODRIGUSS/PHOTO RESEARCHERS, INC.; MARK GARLICK/PHOTO RESEARCHERS, INC.; CERN/PHOTO RESEARCHERS, INC.;
SCIENCE SOURCE; SURAWACH5/SHUTTERSTOCK; DORLING KINDERSLEY/GETTY IMAGES; NASA; CHRIS BUTLER/PHOTO RESEARCHERS, INC.
