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Lithium (Li) and beryllium (Be) form no compounds
under normal atmospheric pressure. But under high pressure at
least four ordered alloys of these elements are predicted. The
bottom left structure is the most unexpected predicted alloy and
may have potential for superconductivity.
Credit: Ji Feng, Richard G. Hennig, N.W. Ashcroft,
and Roald Hoffmann
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"We found that chemists working on inorganic
compounds and inorganic reactions under high pressure were interested
in the predictions and felt it would stimulate useful interactions
between theorists and experimentalists," said NSF Program Manager
Michael Clarke.
Of the four stable Li-Be alloys predicted by the scientists'
computational study, the alloy with the ratio of one Li atom to one Be
atom (LiBe) shows the greatest potential for superconducting
applications.
A most unexpected finding in the study is the predicted existence of
two-dimensional electron gas layers within a tightly compressed
three-dimensional LiBe compound.
"It's like taking a nice layer cake, squeezing the hell out of it, and
lo and behold, out of what would be expected to be a jumbled-up mess,
there emerges a neat hazelnut cream layer," said co-author Roald
Hoffmann, the 1981 chemistry Nobel laureate and Cornell's Frank H.T.
Rhodes Professor in Humane Letters Emeritus.
But it makes sense, according to co-author Neil Ashcroft, Cornell's
Horace White Professor of Physics Emeritus. When layers of Li and Be
are squeezed together at elevated pressures ranging from five to 10
times greater than the pressure at which diamond forms, outer
electrons from the Li layer get squeezed into the vicinity of the Be
layer, forming two-dimensional gas layers.
"It is extraordinary that such remarkably two-dimensional behavior
emerges from the conjunction of two such �simple' constituents. It is
actually a fine example of �emergent' phenomena," Ashcroft said. He
added that they do not yet know whether their theoretical Li-Be alloys
will become notable superconductors but creating and testing the
compounds would be relatively simple.
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At standard
atmospheric or ambient pressure, the lithium beryllium (LiBe)
alloy is unstable. However, at high density and at relatively high
pressure, the predicted alloy stabilizes. As the atoms are
squeezed in tightly, lithium's ionic cores (the larger of the two)
begin to overlap. This creates a sort of "wall" that forces the
outer (valence) electrons out of the lithium layer, and over to
the beryllium layer. It is there that the electrons form a curious
two-dimensional gas. In contrast, electrons in most metals bounce
about quite freely in a three-dimensional fashion.
Credit: Zina
Deretsky, National Science Foundation |
Ji Feng, now a postdoctoral researcher at Harvard, is lead author of
the Nature paper. Richard Hennig, a Cornell assistant professor in
materials science and engineering, is an additional co-author of the
paper.
The research was supported by NSF Division of Chemistry grant
#0613306; Division of Materials Research grant #0601461; and Division
of Earth Sciences grant #0703226.
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