Squeezing H2 and O2 yields new compound.

Water, water, everywhere--and now a new property to think about.

Researchers studying mixtures of oxygen and hydrogen have found that the two gases--which normally combine explosively to form water (H2O)--appear to combust less readily under extreme pressure than they do under ordinary atmospheric pressure.

This unusual observation, reported in the Nov. 2 Nature by chemists Paul Loubeyre and Rene Le Toullec of the University of Paris, shows that even with such a highly combustible mixture as H2 and O2, "high pressures can lead to unexpected behavior."

Ordinarily, higher pressures enhance reactivity. Yet Loubeyre and Toullec found that oxygen and hydrogen, carefully pressurized in a diamond anvil container to 76,000 atmospheres at room temperature, resisted explosive condensation into droplets of water.

Instead, the gaseous molecules appeared to cluster quietly into a 14-atom compound containing three molecules of oxygen and four molecules of hydrogen. The researchers call this blend an "O2/H2 alloy."

"At this stage, we cannot tell whether this alloy is a disordered solid," they report, or some other type of mixture. Interestingly, they say, the spectroscopic measurements made of the pressurized oxygen-hydrogen mixture "are almost identical" to those of the separate elements in solid form--"which is quite surprising," they add.

Information from this experiment could prove useful in the development of novel energy-storage systems, including better rocket fuels, the two scientists speculate. It might also help explain unusual features of the interiors of the outer planets, such as Jupiter, whose cores contain much pressurized hydrogen, helium, oxygen, and ice.

That pressure caused the reaction rate of oxygen and hydrogen to slow down rather than speed up is "remarkable," writes Russell J. Hemley, a chemist at the Carnegie Institution of Washington, D.C., in Nature. "Perhaps the greatest implications are for fundamental chemistry." Although this reaction has been "exhaustively studied since the earliest days of chemistry," it continues to reveal new phenomena, he adds.

The atomic details of hydrogen-oxygen bonding, Hemley says, have remained "enigmatic." Yet a deeper understanding of the reaction's subtleties could affect our view of the solar system, given the abundance of these elements and their importance to planetary science.