How
hydrogen—the most abundant element in the cosmos—responds to extremes
of pressure and temperature is one of the major challenges in modern
physical science. Moreover, knowledge gleaned from experiments using
hydrogen as a testing ground on the nature of chemical bonding can
fundamentally expand our understanding of matter. New work from Carnegie
scientists has enabled researchers to examine hydrogen under pressures
never before possible. Their work is published online in Physical Review Letters.
To
explore hydrogen in this new domain, the scientists developed new
techniques to contain hydrogen at pressures of nearly 3 million times
normal atmospheric pressure (300 GPa) and to probe its bonding and
electronic properties with infrared radiation. They used a facility that
Carnegie manages and operates at the National Synchrotron Light Source
(NSLS) at Brookhaven National Laboratory in partnership with NSLS.
Observing
hydrogen’s behavior under very high pressures has been a great
challenge for researchers, because it is in a gas state under normal
conditions. It is known that it has three solid molecular phases. But
the structures and properties of highest-pressure phases are unknown.
For
example, a transition to a phase that occurs at about 1.5 million times
atmospheric pressure (150 GPa) and at low temperatures has been of
particular interest. But there have been technological hurdles in
examining hydrogen at much higher pressures using static compression
techniques.
It
has been speculated that under at high pressures, hydrogen transforms
to a metal, which means it conducts electricity. It could even become a
superconductor or a superfluid that never freezes—a completely new and
exotic state of matter.
In
this new work, the research team, which included Carnegie’s Chang-sheng
Zha, Zhenxian Liu, and Russell Hemley, developed new techniques to
measure hydrogen samples at pressures above 3 million times normal
atmospheric pressure (above 300 GPa) and at temperatures ranging from
-438 F (12 K) to close to room temperature..
“These
new static compression techniques have opened a window on the behavior
of hydrogen at never-before-reached static pressures and temperatures,”
said Hemley, director of the Geophysical Laboratory.
The
team found that the molecular state was stable to remarkably high
pressures, confirming extraordinary stability of the chemical bond
between the atoms. Their work disproves the interpretations of
experiments by other researchers reported last year indicating a
metallic state under these conditions. Evidence for semimetallic
behavior in the dense molecular phase was found in the new study, but
the material must have electrical conductivity well below that of a full
metal.
Meanwhile, in another paper also published in Physical Review Letters,
a team from the University of Edinburgh and including Carnegie’s
Alexander Goncharov report evidence for another phase of molecular
hydrogen. They found it at the relatively high temperature of 80 F (300
K) and under pressures above 220 GPa. They suggest that the structure of
hydrogen in this new phase is a honeycomb made of six-atom rings,
similar to the carbon structure of graphene.
Source: Carnegie Institution
