Scientists have a long and unsuccessful history of attempting to convert hydrogen
to a metal by squeezing it under incredibly high and steady pressures.
Metallic hydrogen is predicted to be a high-temperature superconductor. A superconductor
is a state of matter where electrons, and thus electricity, can flow indefinitely
and without resistance.
In a paper published this week in the online edition of the Proceedings of
the National Academy of Sciences, a team of scientists from Cornell
University and the State University of New York at Stony Brook announce a theoretical
study that predicts the metallization of hydrogen-rich mixtures at significantly
lower pressures.
By adding small amounts of lithium to hydrogen, the study calculates that the
resulting system may be metalized at around one-fourth the pressure required
to metalize pure hydrogen. Funding for the project was provided by the National
Science Foundation (NSF).
Hydrogen and lithium are the first and third lightest elements in the universe,
respectively. Under the temperature and pressures found on Earth, hydrogen is
a gas and lithium is a metal. In hydrogen gas, the atoms are robustly bonded
together in pairs and each hydrogen atom contributes one electron to the bonding.
In chemistry shorthand, hydrogen is called H2.
Hydrogen and lithium normally react with each other to form a stable compound.
This lithium-hydrogen compound, or LiH, is not metallic.
Metallic hydrogen is thought to be present in the interiors of planets like
Jupiter and Saturn because of the intense gravitational forces and pressures
that are found there.
On Earth, researchers have tried to pry loose hydrogen's electron by squeezing
it between the facets of a diamond anvil cell under pressures up to 3.4 million
atmospheres. The pressure at sea level is one atmosphere. The pressure at the
center of the Earth is around 3.5 million atmospheres. Scientists have not been
successful with this method of steady pressures. They have been, however, with
shock-wave methods.
To get around hydrogen's decidedly fixed stance of not becoming a metal under
currently accessible laboratory pressures, the research team used sophisticated
computer programs.
The programs theoretically calculate if hydrogen can be metalized by combining
a lithium atom with varying numbers of hydrogen atoms. The programs also compute
if metallic hydrogen can be made under pressures achievable in a laboratory.
The lithium and hydrogen combinations predicted by the study currently do not
exist on Earth.
One of the combinations predicted by the team contains one lithium atom for
every six hydrogen atoms or LiH6 (see top right image). The complex calculations
predict that in the hypothetical compound the Li atom is triggered to release
its lone outer electron, which is then distributed over the three H2 molecules.
Under pressure, the hypothetical reaction forms a stable and metallic hydrogen
compound.
The calculations also predict that LiH6 could be a metal at normal pressures.
However, under these conditions it is not stable and would decompose to form
LiH and H2.
"The stable and metallic LiH6 compound is predicted to form around 1 million
atmospheres, which is around 25 percent of the pressure required to metalize
hydrogen by itself," said Eva Zurek, lead author of the paper and an assistant
professor of chemistry at The State University of New York, Buffalo.
"Interestingly, between approximately 1 and 1.6 million atmospheres, all
the LiH combinations studied were stable or metastable and all were metallic,"
said Roald Hoffmann, co-author, recipient of the 1981 Nobel Prize in chemistry
and Cornell's Frank H.T. Rhodes Professor of Humane Letters, Emeritus.
Another one of the hypothetical compounds studied by the team was composed
of one lithium atom and two hydrogen atoms or LiH2 (see bottom right image).
"The theoretical study opens the exciting possibility that non-traditional
combinations of light elements under high pressure can produce metallic hydrogen
under experimentally accessible pressures and lead to the discovery of new materials
and new states of matter," said Daryl Hess, a program director in the NSF
Division of Materials Research.
"Once again, these researchers have taken chemistry to a new frontier,"
said Carol Bessel, a program director in the NSF Division of Chemistry. "They
have described, through their theories and calculations, molecules that test
our fundamental assumptions about atoms, molecules and structures. In doing
so, they challenge the experimentalists to make what they have imagined in their
minds a reality to be held in the hand."
The team members believe the information gleaned from the study suggests that
one may combine large amounts of hydrogen with other elements. The information
may also some day assist with the design of a metallic hydrogen-based superconductor.
"We have already been in touch with laboratory experimentalists about
how LiH6 might be fabricated, starting perhaps with very finely divided forms
of the common LiH compound along with extra hydrogen," said Neil W. Ashcroft,
co-author, and Cornell's Horace White Professor of Physics, Emeritus.
Additional authors include Artem R. Oganov, an associate professor, and Andriy
O. Lyakhov, a post doctoral research associate, of the State University of New
York at Stony Brook, Department of GeoSciences. Zurek was a postdoctoral associate
in Hoffmann's research group when the studies were completed.
Funding for the study was provided by the NSF Divisions of Chemistry and Materials
Research. The research was also supported in part by NSF through TeraGrid resources
provided by the National Center for Supercomputing Applications.
Posted October 5th, 2009
