Argonne scientists use unique diamond anvils to view oxide glass
structures under pressure
ARGONNE, Ill. (Nov. 9, 2007) – Researchers at the U.S. Department of Energy's
Argonne National Laboratory have used a uniquely constructed perforated diamond
cell to investigate oxide glass structures at high pressures in unprecedented
detail.
Argonne physicist Chris Benmore and postdoctoral appointee Qiang Mei, along
with colleagues at Arizona
State University,
used microscopic laser-perforated diamond anvil cells to generate pressures
of up to 32 gigapascals (GPa) – roughly
one-tenth the pressure at the center of the Earth. By "squashing" vitreous
(glassy) arsenic oxide samples between the anvils, the researchers were able
to determine the mechanism behind the structure's atypical behavior under high-pressure.
This research may have far-reaching affects in the geophysical sciences, Benmore
said, because oxide glasses and liquids represent a significant percentage
of the materials that make up the Earth. For example, knowing the atomic structure
of oxide materials at high pressures may give scientists a window on the behaviors
of magma during the formation of the early Earth and moon. "We now have
a technique where we can look a lot of different silicate glasses that are
relevant to the Earth's process and at the complex behaviors of the melts that
formed the Earth's mantle," he said.
During their investigation, Benmore and Mei noticed that if arsenic oxide
was subjected to high pressures the material underwent an unusual transformation
at about 20 GPa, as the color of the compound changed from transparent to red.
However, they did not know the atomic cause for this behavior.
By performing X-ray pair distribution function experiments at Argonne's Advanced
Photon Source (APS), however, Benmore and Mei were able to see the atomic reconfiguration
that produced the color change. Arsenic oxide, at normal pressures, typically
exists in isolated molecular "cages" in which four arsenic atoms
are surrounded by three oxygen atoms apiece – each of the six oxygen atoms
is bounded to two arsenic atoms. When the pressure rose above 20 GPa, however,
many of these molecular cages collapsed, creating new isomers in which each
arsenic atom was bonded to six oxygen atoms.
Regular diamond anvils could not be used because they caused a great deal
of background scattering that obscured the signal from the material. Previous
experiments on vitreous materials had used mechanically drilled diamond anvil
cells to create the high pressures, but these routinely failed at pressures
above 15 GPa. This experiment involved one of the first-ever uses of laser-perforated
diamond anvils combined with micro-focused high energy X-ray diffraction techniques,
which have the ability to generate high pressures without also producing background
noise.
Benmore hopes to extend his research to liquid oxides and silicates by heating
them pass their melting points. By doing so, he expects to gain a better understanding
of the structural transition, which is expected to occur more abruptly and
be reversible in the liquid phases of these materials.
Benmore and Mei's research was funded by the DOE Office of Basic
Energy Sciences.
Argonne National Laboratory brings
the world's brightest scientists and engineers together to find exciting and
creative new solutions to pressing national problems in science and technology.
The nation's first national laboratory, Argonne conducts leading-edge basic
and applied scientific research in virtually every scientific discipline. Argonne
researchers work closely with researchers from hundreds of companies, universities,
and federal, state and municipal agencies to help them solve their specific
problems, advance America 's scientific leadership and prepare the nation for
a better future. With employees from more than 60 nations, Argonne is managed
by UChicago
Argonne, LLC for
the U.S.
Department of Energy's Office
of Science.
For more information, please contact Angela Hardin (630/252-5501
or ahardin@anl.gov) at Argonne.
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