October 12, 2009

Lithium Hydrides Could become Metallic Hydrogen at Four Times Less Pressure

Ball-and-stick image of hypothetical metallic crystal cells composed of one lithium, or Li, atom and six hydrogen, or H, atoms. The lithium-hydrogen compound is predicted to form under approximately 1 million atmospheres, which is one-fourth the amount of pressure required to metalize pure hydrogen. The pressure at sea level is one atmosphere and the pressure at the center of the Earth is around 3.5 million atmospheres. Li atoms are green and H atoms are white

From detailed assessments of electronic structure, researchers at the University at Buffalo, Cornell University, Stony Brook University and Moscow State University discovered that unexpected hydrides violating standard valence rules, such as LiH6 and LiH8, become stable metals at a pressure approximately one quarter of that required to metalize pure hydrogen.
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.

Metallic hydrogen is believed to be stable at high pressures and is theorized to be a superconductor at room temperature or higher.

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"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.

From detailed assessments of electronic structure, we find that a combination of significantly quantal elements, six of seven atoms being hydrogen, becomes a stable metal at a pressure approximately 1/4 of that required to metalize pure hydrogen itself. The system, LiH6 (and other LiHn), may well have extensions beyond the constituent lithium. These hypothetical materials demonstrate that nontraditional stoichiometries can considerably expand the view of chemical combination under moderate pressure.

11 pages of supplemental information.

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