Under pressure: Viewing how hydrogen transforms
Researchers have determined how hydrogen molecules are packed at extremely high pressures. Hydrogen is the most abundant element in the Universe and the research solves the long-standing mystery of the structure of its dense form, known as phase IV.
An international team, including Wendy Mao of Stanford University’s School of Earth, Energy & Environmental Sciences (Stanford Earth), succeeded in determining how hydrogen molecules are packed at extremely high pressures. Their work solves the long-standing mystery of the structure of the dense form of hydrogen, called phase IV. The research was published in Nature Sept. 25.
“It is remarkable that something as basic as the structure of hydrogen in this high-pressure phase was not determined until now,” said Mao, a professor of geological sciences. “This just demonstrates how much there is still left to discover about element one in the periodic table.”
Hydrogen is the most abundant element in the Universe. Under compression, the tenuous hydrogen gas solidifies and then transforms to a number of dense, solid forms. It has been predicted that hydrogen will eventually turn into a ‘wonder’ material that contains the highest energy density, is a room temperature superconductor that conducts electricity without any resistance, flows uphill as a superfluid and may represent a novel state of matter governed by unknown new physics.
Relentless pursuit of this novel form of hydrogen over the past century has brought us close to the pressure range where this ‘wonder’ material was expected, and several new forms of hydrogen have been discovered along the way. However, the structure of hydrogen under extreme conditions is very difficult to identify, and these newly discovered hydrogen phases remained undefined.
‘‘We must understand phase IV, which plays a pivotal role linking ordinary hydrogen to the exotic metallic form,” said lead author Cheng Ji, a researcher with the Center for High Pressure Science and Technology Advanced Research in Beijing, China (HPSTAR). ‘‘This study was very challenging. It took me five years to resolve the many technical problems, so it is very exciting that we can finally solve the structure unequivocally.’’
Using the intense X-rays from large synchrotron facilities including the Advanced Photon Source in the U.S. and the Shanghai Synchrotron Radiation Facility in China, Mao’s team overcame the previous challenge that hydrogen is relatively invisible using X-rays, and solved the crystal structure of one of the new forms of hydrogen, called phase IV. Surprisingly, the hydrogen molecules are packed with a hexagonal symmetry, like a snow flake. The hexagonal crystals are flattened under compression, leading to the electronic transformation that forms phase IV.
‘‘Hydrogen under extreme condition has been one of the main focuses and grand challenges of the physics community for many decades,’’ added Ho-Kwang Mao. ‘‘At HPSTAR, we gathered experts on hydrogen, with the aim to not only discover new forms of hydrogen, but also provide reliable characterization in order to understand the novel physics of these high pressure phases. The current work is one such example which paves the road for understanding the metallization process of hydrogen.’’
News and Views
Learn more about how the researchers' experiments revealed key details about the arrangement of molecules in several of hydrogen's high-pressure phases.
Wendy Mao is also a professor of photon science and principal investigator of the Stanford Institute for Materials and Energy Sciences. Other co-authors on the study include Jinfu Shu, Junyue Wang, and Wenge Yang of HPSTAR; Wenjun Liu, Jesse S. Smith, Guoyin Shen, Yue Meng, Ruqing Xu, and Xianrong Huang of Advanced Photon Source, Argonne National Laboratory, Vitali B. Prakapenka and Eran Greenberg of University of Chicago, Stanislav Sinogeikin of DACTOOL. Inc, and Arnab Majumdar, Wei Luo, and Rajeev Ahuja of Uppsala University.
This research was supported by the Department of Energy (DOE), the National Science Foundation (NSF), the National Natural Science Foundation of China (NSFC) and the Swedish Research Council.
This story was adapted from a press release by the Center for High Pressure Science and Technology Advanced Research in Beijing, China.
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