September 25, 1997
Scientists at Ernest Orlando Lawrence Berkeley National Laboratory
and the University of California, Berkeley, looking into the "mystery
of the missing xenon" have found strong evidence against one leading
theory and, along the way, discovered new information about the
behavior of the element.
The findings were published in the Aug. 15 issue of "Science" magazine.
A team of investigators headed by professors Steven Louie of UC
Berkeley and Berkeley Lab and Raymond Jeanloz of UC Berkeley used
both experimental and computational science to try to determine
if xenon, which makes up only 0.000009 percent of Earth's atmosphere,
could also be found elsewhere on Earth, such as inside the planet's
Two graduate students, Sander Caldwell of the Earth Sciences Department
at UC Berkeley and Bernd Pfrommer of UC Berkeley's Physics Department
who is also associated with the Materials Sciences Division and
the National Energy Research Scientific Computing Center (NERSC)
at Berkeley Lab, are key contributors to the project.
According to Caldwell, xenon is more abundant on the other rocky
planets (Mars, Venus and Mercury) and scientists have long thought
more of the noble gas should be present on Earth.
One theory is that xenon, usually found as a gas, could have bonded
with iron in the earth's core, and it was this theory that Caldwell
tested in his lab. Despite subjecting a sample of xenon and iron
to pressures up to 70 gigapascals (or 700,000 times atmospheric
pressure at sea level), the two elements did not form a compound.
Using the computational capabilities of a highly parallel CRAY
T3E supercomputer at NERSC and other parallel computer platforms,
Pfrommer performed quantum mechanical calculations and reached similar
conclusions. "With our calculations it is much easier to simulate
high pressures than in experiment," Pfrommer said. Even at pressures
as high as 500 gigapascals, the calculations showed no sign of a
chemical bond between xenon and iron.
Caldwell also used an industrial heating laser to heat his sample
of xenon and iron to try to cause the two elements to bond. While
this did not occur, comparisons of the samples at different pressures
and temperatures did clear up one mystery of the different phase
changes xenon goes through.
At low pressure, xenon's structure is face-centered cubic. At higher
pressures, above 75 gigapascals, the structure changes to a hexagonal
close-packed structure. In between, the thinking went, was a third
structural form that was not entirely understood.
However, by using calculations from NERSC and observing samples,
Caldwell and his colleagues determined that there is no third structural
form. Rather, at those pressures, xenon "can't decide which phase
it should be in." NERSC calculations showed that there was a very
small energy difference between the two phases. "In fact, we had
to keep crunching the numbers because the difference is so small,
it was hard to calculate," Caldwell said.
By heating the sample, Caldwell provided energy for the sample
xenon to change from one phase to the next, without going through
the predicted middle phase.
"We cleaned up the area in the middle," said Caldwell. "There
is no xenon II phase--it is actually part of the two known phases."
Ironically, it was the iron in the sample that absorbed enough energy
from the laser to tip the scales toward solving the missing phase
mystery. "Although the question of xenon's presence is still up
in the air, so to speak," Caldwell said, "we've probably ruled out
that it's sequestered in the core of the earth. Now we need to seek
another explanation." Also contributing to the article were Jeffrey
Nguynen and Francesco Mauri of UC Berkeley and Berkeley Lab.
NERSC (http://www.nersc.gov), established in 1974, provides high
performance computing services to DOE's Energy Research programs
at national laboratories, universities and industry. The facility
has been located at Berkeley Lab since May 1996. Berkeley Lab (http://www.lbl.gov)
is a U.S. Department of Energy national laboratory located in Berkeley,
CA. It conducts unclassified research and is managed by the University