First direct band gap measurements of wide-gap hydrogen using inelastic X-ray scattering

January 26, 2021

Utilizing a newly developed state-of-the-art synchrotron technique, a group of scientists led by Dr. Ho-kwang Mao, Director of HPSTAR, conducted the first-ever high-pressure study of the electronic band and gap information of solid hydrogen up to 90 GPa. Their innovative high pressure inelastic X-ray scattering result serves as a test for direct measurement of the process of hydrogen metallization and opens a possibility to resolve the electronic dispersions of dense hydrogen. This work is published in the recent issue of Physical Review Letters.

The pressure-induced evolution of hydrogen's electronic band from a wide gap insulator to a closed gap metal, or metallic hydrogen, has been a longstanding problem in modern physics. However, hydrogen's remarkably high energy has prevented the electronic band gap from being directly observed under pressure before. Existing probes, such as electrical conductivity, optical absorption, or reflection spectroscopy measurements, are limited and provide little information on a wide-gap insulator. "All previous studies of the band gap in insulating hydrogen under compression were based on an indirect scheme using optical measurements," explains Dr. Mao.

The team used high-brilliance, high-energy synchrotron radiation to develop an inelastic x-ray (IXS) probe, yielding electronic band information of hydrogen in situ under high pressure in a diamond anvil cell (DAC). "The development of our DAC-IXS technique for this project took an international team of many experts in synchrotron inelastic X-ray spectroscopy, instrumentation, and ultra-high-pressure techniques over five years to complete," said Dr. Bing Li, the first author.

"Actually, the real beginning of this project can be traced back more than 20 years, and these results are the culmination of all that preparation and experimentation. A true testament to the enormous efforts and talents of the team involved," said Dr. Mao. The novel IXS probe technique enabled an inaccessible and extensive UV energy range of 45 eV to be measured, showing how dense hydrogen's electronic joint density of states and band gap evolve with pressure. The electronic band gap decreased linearly from 10.9 eV to 6.57 eV, with an 8.6 times densification from zero pressure up to 90 GPa.

These developments in state-of-the-art synchrotron capabilities with submicron to nanometer-scaled X-ray probes will only extend future experimental possibilities. Advances of IXS to higher pressure could place the semiconducting region of phases II-V within reach and enable the study of hydrogen metallization through direct and quantitative electronic band gap measurements.

This work overcomes formidable technical challenges, achieving direct experimental measurements of hydrogen's electronic band and its gap for the first time.
-end-
More information: Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90 GPa, Li et al., Phys. Rev. Lett. 126, 036402.

Center for High Pressure Science & Technology Advanced Research

Related Hydrogen Articles from Brightsurf:

Solar hydrogen: let's consider the stability of photoelectrodes
As part of an international collaboration, a team at the HZB has examined the corrosion processes of high-quality BiVO4 photoelectrodes using different state-of-the-art characterisation methods.

Hydrogen vehicles might soon become the global norm
Roughly one billion cars and trucks zoom about the world's roadways.

Hydrogen economy with mass production of high-purity hydrogen from ammonia
The Korea Institute of Science and Technology (KIST) has made an announcement about the technology to extract high-purity hydrogen from ammonia and generate electric power in conjunction with a fuel cell developed by a team led by Young Suk Jo and Chang Won Yoon from the Center for Hydrogen and Fuel Cell Research.

Superconductivity: It's hydrogen's fault
Last summer, it was discovered that there are promising superconductors in a special class of materials, the so-called nickelates.

Hydrogen energy at the root of life
A team of international researchers in Germany, France and Japan is making progress on answering the question of the origin of life.

Hydrogen alarm for remote hydrogen leak detection
Tomsk Polytechnic University jointly with the University of Chemistry and Technology of Prague proposed new sensors based on widely available optical fiber to ensure accurate detection of hydrogen molecules in the air.

Preparing for the hydrogen economy
In a world first, University of Sydney researchers have found evidence of how hydrogen causes embrittlement of steels.

Hydrogen boride nanosheets: A promising material for hydrogen carrier
Researchers at Tokyo Institute of Technology, University of Tsukuba, and colleagues in Japan report a promising hydrogen carrier in the form of hydrogen boride nanosheets.

World's fastest hydrogen sensor could pave the way for clean hydrogen energy
Hydrogen is a clean and renewable energy carrier that can power vehicles, with water as the only emission.

Chemical hydrogen storage system
Hydrogen is a highly attractive, but also highly explosive energy carrier, which requires safe, lightweight and cheap storage as well as transportation systems.

Read More: Hydrogen News and Hydrogen Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.