Atomic changes occur in matter as it descends sufficiently under the
surface of the Earth or into the sun's core.
Metals have the potential to become nonconducting insulators due to the
increasing pressure within stars and planets. Squeezing sodium hard enough
has been proven to change it from a lustrous, gray metal into a transparent,
glass-like insulator.
Now, the chemical bonding behind this specific high-pressure phenomena has
been identified, thanks to research performed by the University at
Buffalo.
The electrons in sodium are thought to be basically squeezed out into the
gaps between atoms by high pressure, however quantum chemical calculations
performed by researchers reveal that these electrons are nonetheless
chemically connected to the surrounding atoms and remain a part of
them.
The co-author of the study, Eva Zurek, Ph.D., is a professor of chemistry
in the UB College of Arts and Sciences. "We're answering a very simple
question of why sodium becomes an insulator, but predicting how other
elements and chemical compounds behave at very high pressures will
potentially give insight into bigger-picture questions," says Zurek. How
does a star's inside look like? If there are magnetic fields on planets, how
are they produced? How do planets and stars evolve, too? This kind of study
brings us one step closer to providing answers to these queries."
The work is dedicated to the memory of the late, great physicist Neil
Ashcroft, and it both supports and expands upon his theoretical
predictions.
Before Ashcroft and Jeffrey Neaton's groundbreaking study two decades ago,
it was believed that materials always turned metallic at high pressure—like
the metallic hydrogen assumed to make up Jupiter's core. However, they
discovered that some materials, like sodium, can actually turn into
insulators or semiconductors when squeezed. They postulated that, at
tremendous pressure, the supposedly passive core electrons of sodium would
interact with one another and the surrounding valence electrons.
"Our work now connects it with chemical concepts of bonding, going beyond
the physics picture painted by Ashcroft and Neaton," says Stefano Racioppi,
Ph.D., the main author of the UB-led study and postdoctoral researcher in
the UB Department of Chemistry.
The researchers used supercomputers in UB's Center for Computational
Research to do calculations on how electrons behave in sodium atoms under
high pressure since it may be challenging to replicate the pressures seen
below Earth's crust in a lab.
An electride state is created when the electrons are trapped in the spaces
between the atoms. Because trapped electrons only let light to pass through,
free-flowing electrons absorb and retransmit light, causing sodium to
physically change from a shining metal to a transparent insulator.
Nevertheless, calculations made by the researchers revealed for the first
time that chemical bonding can account for the development of the electride
state.
Electrons in their respective atoms take up new orbitals due to the high
pressure. Thereafter, these orbitals cross across to create chemical bonds,
which result in localized charge concentrations in the interstitial
spaces.
The new calculations revealed that the electrons are still a part of the
surrounding atoms, contrary to the intuitive assumption put out by earlier
research that suggested high pressure forced electrons out of atoms.
It dawned on us that there were more than only solitary electrons that had
chosen to depart from the atoms. Rather, in a chemical link, the atoms share
the electrons," claims Racioppi. "They're quite special."
Malcolm McMahon and Christian Storm of the University of Edinburgh's School
of Physics and Astronomy and Center for Science in Extreme Conditions are
among the other contributors.
The institution for Matter under Atomic Pressure, a National Science
Foundation institution directed by the University of Rochester that
investigates how pressure within stars and planets may reorganize the atomic
structure of materials, provided support for this work.
"Obviously it is difficult to conduct experiments that replicate, say, the
conditions within the deep atmospheric layers of Jupiter," says Zurek, "but
we can use calculations, and in some cases, high-tech lasers, to simulate
these kinds of conditions."
Provided by
University at Buffalo
