Berkeley Researchers Eliminate One Theory in Mystery of Missing Xenon, but Find New Clues About Element's Behavior

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 core.

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.