Splitting Water Creates Alloy

Zapping ice VII-a crystalline form of water that occurs at high pressure and ambient temperature-with X-rays cleaves the O-H bonds and creates a novel crystalline solid composed of molecular hydrogen and oxygen (Science 2006, 314, 636). The finding, reported by a multi-institutional team, opens up new avenues for high-pressure radiation chemistry.

"There are so many studies on water, you wonder, how can there be anything more to find?" says Wendy L. Mao, a postdoctoral fellow at New Mexico's Los Alamos National Laboratory who spearheaded the research. But the new material looks different from what has been seen before, she adds.

Mao's team—including her father, Ho-kwang Mao, and Russell J. Hemley of the Carnegie Institution of Washington, as well as Peter J. Eng of the University of Chicago—made the unexpected discovery while trying out a new high-pressure synchrotron X-ray technique that irradiates substances with moderately high-energy X-rays for long periods of time.

"At first, we saw these unexpected H₂ and O₂ peaks in our X-ray Raman spectra," Wendy Mao tells C&EN. "Then we released pressure from the diamond anvil cell, and we actually saw gas bubbling out."

Upon further investigation, they determined that at high pressure the X-rays dissociate H₂O and the resulting atoms recombine into a previously unknown solid of H₂ and O₂. This new "alloy" is spectroscopically distinct from a simple H₂-O₂ mix.

X-rays are known to create free radicals and instigate reactions at ambient pressure, but examples of similar transformations at high pressure are rare. "We managed to hit on just the right level of X-ray energy input," Hemley explains. "Any higher, and the radiation tends to pass right through the sample. Any lower, and the radiation is largely absorbed by the diamonds in our pressure apparatus."

The new alloy exhibits surprising stability, so long as it's kept at high pressure. Six months after the original experiments, the material hasn't reverted to ice VII, Mao says, even after heating it to 700 K and blitzing it with lasers and X-rays.