In the search for superconductors, finding ways to compress
hydrogen into a metal has been a point of focus ever since scientists predicted
many years ago that electricity would flow, uninhibited, through such a
material.
Liquid metallic hydrogen is thought to exist in the high-gravity
interiors of Jupiter and Saturn. But so far, on Earth, researchers have been
unable to use static compression techniques to squeeze hydrogen under high
enough pressures to convert it into a metal. Shock-wave methods have been
successful, but as experiments with diamond anvil cells have shown, hydrogen
remains an insulator even under pressures equivalent to those found in the
Earth’s core.
To circumvent the problem, a pair of Univ. at Buffalo chemists has proposed an alternative
solution for metalizing hydrogen: Add sodium to hydrogen, they say, and it just
might be possible to convert the compound into a superconducting metal under
significantly lower pressures.
The research, published in Physical
Review Letters, details the findings of UB Assistant Professor Eva Zurek
and UB postdoctoral associate Pio Baettig.
Using an open-source computer program that UB PhD student David
Lonie designed, Zurek and Baettig looked for sodium polyhydrides that, under
pressure, would be viable superconductor candidates. The program, XtalOpt,
is an evolutionary algorithm that incorporates quantum mechanical calculations
to determine the most stable geometries or crystal structures of solids.
In analyzing the results, Baettig and Zurek found that NaH9,
which contains one sodium atom for every nine hydrogen atoms, is predicted to
become metallic at an experimentally achievable pressure of about 250 gigapascals—about
2.5 million times the Earth’s standard atmospheric pressure, but less than the
pressure at the Earth’s core.
“It is very basic research,” says Zurek, a theoretical
chemist. “But if one could potentially metalize hydrogen using the
addition of sodium, it could ultimately help us better understand
superconductors and lead to new approaches to designing a room-temperature
superconductor.”
By permitting electricity to travel freely, without resistance,
such a superconductor could dramatically improve the efficiency of power
transmission technologies.
Zurek, who joined UB in 2009, conducted research at Cornell Univ. as a postdoctoral associate under
Roald Hoffmann, a Nobel Prize-winning theoretical chemist whose research
interests include the behavior of matter under high pressure.
In October 2009, Zurek co-authored a paper with Hoffman and
other colleagues in the Proceedings of
the National Academy of Sciences predicting that LiH6—a compound containing
one lithium atom for every six hydrogen atoms—could form as a stable metal at a
pressure of around 1 million atmospheres.
Neither LiH6 and NaH9 exists naturally as stable compounds on
Earth, but under high pressures, their structure is predicted to be stable.
“One of the things that I always like to emphasize is that
chemistry is very different under high pressures,” Zurek says. “Our
chemical intuition is based upon our experience at one atmosphere. Under
pressure, elements that do not usually combine on the Earth’s surface may mix,
or mix in different proportions. The insulator iodine becomes a metal, and
sodium becomes insulating. Our aim is to use the results of computational
experiments in order to help develop a chemical intuition under pressure, and
to predict new materials with unusual properties.”
