A manhole cover with a stylized hydrogen atom. The black plus in the center represents a proton and the yellow minus a negatively charged electron. (Credit: Flickr @ Joshua Kuhn http://www.flickr.com/photos/habitue01/)
A team of scientists from the Carnegie Institution for Science’s Geophysical Laboratory used intense infrared light to investigate the properties of the high-pressure hydrogen and has found out the details of a surprising new form of solid hydrogen. With hydrogen being the most abundant chemical substance, constituting roughly 75% of the Universe’s baryonic mass, the way it responds under extreme pressures and temperatures is crucial to our understanding of matter and the nature of hydrogen-rich planets.
At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2. Most of the hydrogen on Earth is in molecules such as water and organic compounds because hydrogen readily forms covalent compounds with most non-metallic elements.
However, the hydrogen molecules start to change as the pressure increases. States of matter, or phases, in physics are the distinct forms that matter takes on (like water, ice and steam). Hydrogen is known to have three solid phases. It has been theorized that at high pressures hydrogen even changes to a metal, which means it conducts electricity. It could even become a superconductor or a superfluid that never freezes—a completely new and exotic state of matter.
In a new paper published in Physical Review Letters [see footnote] , a team from Carnegie’s Geophysical Laboratory examined the structure, bonding and electronic properties of highly compressed hydrogen using intense infrared radiation.
Using a facility maintained by the Geophysical Laboratory at the National Synchrotron Light Source at Brookhaven National Laboratory, the team found the new form to be stable from about 2.2 million times normal atmospheric pressure and about 80 degrees Fahrenheit to at least 3.4 million times atmospheric pressure and about -100 degrees Fahrenheit.
Their experiments revealed that hydrogen takes a form under these conditions that differs remarkably from its other known structures. The new phase has two very different types of hydrogen molecules in its structure. One type of molecule interacts very weakly with its neighboring molecules—unusual for molecules under this type of very high compression. The other type of molecule bonds with its neighbors, forming surprising planar sheets.
The measurements also show that solid hydrogen under these conditions is on the borderline between a semiconductor, like silicon, and a semimetal, like graphite. The results disprove earlier claims that hydrogen forms a dense atomic metal at these pressures and temperatures.
“This simple element—with only one electron and one proton—continues to surprise us with its richness and complexity when it is subjected to high pressures,” Russell Hemley, Director of the Geophysical Laboratory, said. “The results provide an important testing ground for fundamental theory.”
Zha, C., Liu, Z., Ahart, M., Boehler, R., & Hemley, R. (2013). High-Pressure Measurements of Hydrogen Phase IV Using Synchrotron Infrared Spectroscopy Physical Review Letters, 110 (21) DOI: 10.1103/PhysRevLett.110.217402
