Using high-pressure synchrotron radiation techniques at Brookhaven National Laboratory (BNL), a team of researchers has examined the structure, bonding and electronic properties of highly compressed hydrogen known as hydrogen phase IV.
Graphene-like structure of hydrogen phase IV (University of Edinburgh)
Under normal conditions hydrogen is a gas consisting of diatomic molecules. The hydrogen molecules start to change as the pressure increases. These different forms are called phases and hydrogen has three known solid ones.
Phase I hydrogen is a quantum crystal consisting of rotating hydrogen molecules on a hexagonal close packed lattice.
The orientationally ordered molecular phases II and III appear at about 1.1 and 1.5 million times normal atmospheric pressure, respectively.
Phase IV of dense solid hydrogen was discovered in 2012 by a team of scientists from the United States and UK.
In the new study, Dr Russell Hemley from the BNL’s Geophysical Laboratory and his colleagues found phase IV 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 minus 100 degrees Fahrenheit.
Their experiments revealed that hydrogen takes a form under these conditions that differs remarkably from its other known structures.
Phase IV 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 findings 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,” said Dr Hemley, who is senior author of a paper published in Physical Review Letters.
“The results provide an important testing ground for fundamental theory,” Dr Hemley said.
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Bibliographic information: Chang-sheng Zha et al. 2013. High-Pressure Measurements of Hydrogen Phase IV Using Synchrotron Infrared Spectroscopy. Phys. Rev. Lett., vol. 110, no. 21; doi: 10.1103/PhysRevLett.110.217402
