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First X-ray pattern of hydrogen solid.

First X-ray pattern of hydrogen solid

Very high pressures can turn a free-wheeling gas into ahighly structured solid. Decades ago theorists began to predict the effects of pressure on solids made of condensed gases, but only recently have experimenters developed the tools to observe many of these effects firsthand.

At last week's meeting of the American Physical Society inNew York City, researchers from the Carnegie Institution of Washington (D.C.) reported the first X-ray diffraction patterns from single crystals of hydrogen under high pressure. Obtaining the patterns was somewhat of a technical feat because hydrogen atoms have only one electron with which to scatter the probing X-rays. "Many people thought this couldn't be done,' notes Carnegie's Russell Hemley. The group has also studied deuterium, xenon and neon.

Using a synchrotron to produce intense X-rays, Hemley andhis co-workers have shown that hydrogen retains a hexagonal, close-packed structure at up to 260 kilobars pressure. Until now, says Hemley, theorists didn't know whether hydrogen at these pressures would have a hexagonal or a cubic structure.

The researchers also found that the ratio of distancesbetween molecules lying along two different crystal axes decreased with increasing pressure. This compression effect, notes Carnegie's Robert Hazen, is unanticipated by normal theory and suggests that the hydrogen molecules, which rotate in random directions at low pressures, are beginning to align their rotation axes in one direction.

Hemley adds that at higher pressures, the hydrogen moleculesshould be pushed together so much that electrons are shared by surrounding molecules and the solid becomes metallic. He says that hydrogen studies of up to 1.5 megabars using other techniques have yet to reach this transition.

Knowing the structure of hydrogen at high pressures willprovide essential information for fine-tuning theoretical quantum calculations. And coupled with studies of helium and other noble gases, the hydrogen work may help physicists describe the evolution of planets by providing equations of state, which relate a gas's pressure, volume and temperature. "So far,' says Hemley, "planetary physicists have had to rely strictly on extrapolations from very low-pressure data on one end, and theoretical calculations of very high pressure on the other. Very little is known about the region, from several hundred kilobars to a couple of megabars, in between.'
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Publication:Science News
Date:Mar 28, 1987
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