Upping The Pressure

May 23, 1997

New Data Shows How Diamonds Bend At High Pressure

Russell Hemley, Dave Mao, and Guoyin Shen at Carnegie's Geophysical Laboratory, collaborating with scientists in France,* have developed techniques to image directly the deformation of such strong materials as diamond under ultrahigh pressures (200-300 Gigapascals**). The measurements, reported in this week's Science magazine, show that diamond, the strongest material known, can be bent (and not break) at high pressures without failing. The results suggest ways of improving high-pressure techniques to attain still higher pressures in the laboratory.

The ability to confine and study materials at ultrahigh pressures is the result of continued refinement in the diamond-anvil cell. In this apparatus, a sample under study is contained between two facing diamonds mechanically forced together. The transparency of the diamonds allows x-ray, infrared, and other radiation to penetrate inside. Many of the advancements in the diamond-anvil cell have occurred over the past 25 years at the Geophysical Laboratory, a leading high-pressure facility and home to the NSF Center for High Pressure Research. Despite these advances, scientists haven't been able to determine the three-dimensional distribution of stresses and strains in diamond and other materials at ultrahigh pressures. Consequently, they have had to rely on indirect measurements and extrapolations from lower pressures, which are typically unreliable.

The behavior of materials at high pressure, where they are stressed and strained in unique ways, differ considerably from their behavior at lower pressures. For example, diamond-anvil cell studies a number of years ago at the Geophysical Laboratory showed that diamond can flow-- deform plastically--under pressures approaching 200 GPa.

The new work documents a much more extensive elastic deformation under pressure. The Carnegie/France group were able to image and measure (using intense x-ray beams from a synchrotron source) the stress-strain distribution in the load-bearing components of the diamond- anvil cell, including the diamond anvils and the gaskets that hold the samples being measured. Because most gaskets absorb x-rays, the group developed a special kind of gasket made out of beryllium, allowing the x-ray beams to pass through from the side, as well as through the diamond. This allowed a "three-dimensional" view of the strain of the sample.

By directly imaging the topography of the diamond-anvil surface in situ to pressures of about 300 GPa, the group found that diamond can accommodate remarkably large strains localized over small areas in the anvil tip. Although the diamonds bent as much as 16 degrees over a distance of 300 micrometers, they still did not fail. The group also performed complementary experiments using the three-dimensional diffraction geometries, and found similar results. It is thus possible to identify the maximum amount of strain accommodated by the diamond anvil tips before they fail.

Another important question--from the standpoint of both fundamental knowledge and the development of stronger high-pressure apparatuses--is the ultimate strength of materials under pressure. Using the beryllium gasket, the group measured stresses and strains in samples of iron and tungsten subjected to pressures in the 200-300 GPa range. They found that the strength of these materials is enhanced at ultrahigh pressures by up to two orders of magnitude. In other words, applied pressure makes these already strong materials much, much stronger.

Rus Hemley, the lead author on the Science paper, says that such measurements in effect allow researchers to perform "tomography" of the stresses and strains in materials at ultrahigh pressures. "Detailed modeling should further permit us to optimize the design of the apparatus. In a broader sense, the study shows us that the elasticity and strength of materials at high pressure can differ in extraordinary ways in comparison to ordinary conditions."

This work was carried out with the support of the National Science Foundation.

The Geophysical Laboratory, led by Charles Prewitt, is one of five research departments of the Carnegie Institution of Washington, a nonprofit research and educational organization founded in 1902 by Andrew Carnegie. Scientists at the Laboratory perform physicochemical studies of geological problems, with particular emphasis on the processes involved in the formation and evolution of the Earth's crust, mantle, and core. In 1991, NSF established The Center for High Pressure Research at the Laboratory, a cooperative venture between Carnegie, SUNY at Stony Brook, and Princeton.

*Russell J. Hemley, Ho-Kwang (Dave) Mao, Guoyin Shen of the Geophysical Laboratory, James Badro and Philippe Gillet of the Laboratoire de la Terre, Ecole Normale Superieure du Lyon, and Michael Hanfland and Daniel Hausermann of the European Synchrotron Radiation Facility in Grenoble.

**One GPa equals about 10,000 times atmospheric pressure at sea level; 364 GPa is the pressure at the Earth's core.

Carnegie Institution for Science

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