Hydrogen is
deceptively simple. It has only a single electron per atom, but it
powers the sun and forms the majority of the observed universe. As such,
it is naturally exposed to the entire range of pressures and
temperatures available in the whole cosmos. But researchers are still
struggling to understand even basic aspects of its various forms under
high-pressure conditions.
Experimental difficulties contribute to the lack of knowledge about
hydrogen's forms. The containment of hydrogen at high pressures and the
competition between its many similar structures both play a part in the
relative lack of knowledge.
At high pressures, hydrogen is predicted to transform to a metal, which means it conducts electricity. One of the prime goals of high pressure research, going back to the 1930s, has been to achieve a metallic state in hydrogen. There have been recent claims of hydrogen becoming metallic at room temperature, but they are controversial.
New work from a team at Carnegie's Geophysical Laboratory makes significant additions to our understanding of this vital element's high-pressure behavior. Their work is published in two papers by Proceedings of the National Academy of Sciences and Physical Review B.
New theoretical calculations from Carnegie's Ronald Cohen, Ivan Naumov and Russell Hemley indicate that under high pressure, hydrogen takes on a series of structures of layered honeycomb-like lattices, similar to graphite. According to their predictions the layers, which are like the carbon sheets that form graphene, make a very poor, transparent metal. As a result, its signature is difficult to detect.
"The difficulty of detection means that the line between metal and non-metal in hydrogen is probably blurrier than we'd previously supposed," Cohen said "Our results will help experimental scientists test for metallic hydrogen using advanced techniques involving the reflectivity of light."
At high pressures, hydrogen is predicted to transform to a metal, which means it conducts electricity. One of the prime goals of high pressure research, going back to the 1930s, has been to achieve a metallic state in hydrogen. There have been recent claims of hydrogen becoming metallic at room temperature, but they are controversial.
New work from a team at Carnegie's Geophysical Laboratory makes significant additions to our understanding of this vital element's high-pressure behavior. Their work is published in two papers by Proceedings of the National Academy of Sciences and Physical Review B.
New theoretical calculations from Carnegie's Ronald Cohen, Ivan Naumov and Russell Hemley indicate that under high pressure, hydrogen takes on a series of structures of layered honeycomb-like lattices, similar to graphite. According to their predictions the layers, which are like the carbon sheets that form graphene, make a very poor, transparent metal. As a result, its signature is difficult to detect.
"The difficulty of detection means that the line between metal and non-metal in hydrogen is probably blurrier than we'd previously supposed," Cohen said "Our results will help experimental scientists test for metallic hydrogen using advanced techniques involving the reflectivity of light."
This image
shows the predicted optical absorption of a 1 ¼m of hydrogen in a high
pressure diamond anvil cell for different crystal structures at a
pressure of 300 GPa (3 million times normal atmosphere—similar to the
pressure in the center of the Earth). At these pressures hydrogen no
longer forms molecules, but instead forms in sheets, as shown in the
figure. Scientists use optical absorption to look for metallization in
hydrogen, based on the assumption that metallic hydrogen would be opaque
as most metals are. But the team's analysis shows that it may very well
actually be transparent. Absorption units on the graph (AU) are in
factors of 10, meaning 2 AU lets just 1% of the incident light pass
through the structure (quite dark!). The graphite structure is an ideal
structure that is not expected to be observed in reality. The proposed
high-pressure forms, phase 3 (at low temperatures) and phase 4 (at room
temperature), are both predicted to be transparent in the near infrared
and optical frequencies of light, although phase 4 is poor metal. The
Cmca structure is a similar structure, but is predicted to be a better
metal and opaque, and to form at higher pressures. Graph is courtesy of
Ronald Cohen. (Credit: Courtesy of Ronald Cohen, Carnegie Institution
for Science)
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