Contact Eugene Gregoryanz at 202-478-8953, (email)

Or Olga Degtyareva, 202-478-8948, (email)%20">(email)

For image click here http://www.carnegieinstitution.org/sodiumunderpressure/

To access Physical Review Letters, visit: http://scitation.aip.org/dbt/dbt.jsp?KEY=PRLTAO&Volume=94&Issue=17

 

Washington, D.C. – When people pack an elevator, the passengers make more efficient use of the limited space. The same has been long observed in matter—when atoms and molecules are squeezed together by high pressures they form efficient and symmetrical arrangements. Sodium, one of the simplest metals and one of the two elements in everyday table salt, is defying this common wisdom and the long-held belief that when a material is compressed higher temperatures are needed to melt it. “We were very surprised by our results,” stated high-pressure researcher Eugene Gregoryanz of the Carnegie Institution’s Geophysical Laboratory and lead author of the study published on-line in Physical Review Letters for May 13. “Simple metal is forcing us to re-think some of the most basic assumptions in physics,” he mused.

This work and recent research on the sodium’s sibling elements, alkali metals, suggest that high pressures do not always make the molecular and atomic arrangements simpler and symmetrical. The sodium study, however, is the first to show that as pressures increase the temperature needed to melt a substance actually decreases over an unheard of range of pressures. Water acts similarly, however, only over a tiny pressure range. Melting causes ice to float because the water is denser and better packed than the solid. But sodium melts at decreasing temperatures over an enormous pressure range. “We don’t understand the melting process all that well,” commented Gregoryanz. “We do know that melting happens when the atoms or molecules are not kept together. Typically higher temperatures are needed to melt matter under high pressure because the atoms/molecules are more tightly connected and therefore need a bigger thermal push to move.” At ambient, “room,” pressures sodium melts at 208 degrees Fahrenheit (371 K). As the researchers increased the pressure to 300,000 times the atmospheric pressure at sea level (atmospheres), the melting temperature rose as expected. “After that strange things started to happen and the experiment became a roller coaster,” recounted Gregoryanz .

From 300,000 to almost 1.2-million atmospheres, the melting temperature actually decreased, eventually to 80 degrees Fahrenheit (300 K)—128 degrees Fahrenheit lower than that needed to melt sodium under normal conditions. “Perhaps the most interesting result was that under a pressure of 1-million atmospheres sodium melted at room temperature,” said Olga Degtyareva, crystallographer of the Geophysical Laboratory and co-author of the study. “Atoms in the frozen sodium at these pressures have a weird and complex arrangement.” The experiment took another turn from 1.2-million atmospheres to 1.3-million atmospheres. The melting temperatures began to rise again finally reaching 260 degrees Fahrenheit (400 K)—still an unusually low melting temperature.

The results of the research have spawned many new questions about how matter behaves. The scientists are particularly looking forward to examining the properties of the liquid made at room temperatures at 1 million atmospheres. They suspect they will find some very unusual properties—including the possibility that the liquid could be superconducting.



The research was conducted at the Carnegie-managed High Pressure Collaborative Access Team facility at the Advanced Photon Source, Argonne, Ill. Support for the work was provided by the U.S. Department of Energy, the National Science Foundation, DOD-TACOM, the W. M. Keck Foundation, and the Carnegie Institution. Authors of the paper are: Eugene Gregoryanz, Olga Degtyareva, Maddury Somayazulu, Russell Hemley, and Ho-kwang Mao.

The Carnegie Institution of Washington (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.