New research delves into the chemistry of surprising hydrogen observations recently made, revealing remarkable parallel lines between graphene and hydrogen under high pressures
![This image is a comparison of the electron structure in graphene versus the structure in compressed hydrogen synthesized by Carnegie University researchers [courtesy of Ivan Naumov and Russell Hemley].](https://www.hayadan.org.il/images/content3/2014/12/PRHemleyHydrogenGrapheneFigureLarge.jpg)
[Translation by Dr. Nachmani Moshe]
A new Carnegie University study by researchers Ivan Naumov and Russell Hemley delves into the chemistry underlying surprising recent hydrogen observations, revealing remarkable parallels between graphene and hydrogen under high pressures. The findings were recently published in the scientific journal Accounts of Chemical Research.
Hydrogen is the most common element in the universe. As an element containing a single electron, it appears, deceptively, quite simple. As a result, hydrogen has been used as a basis for research in the fields of chemical bond theory since the birth of quantum mechanics about a hundred years ago. Understanding the chemical bond under extreme conditions is essential to expanding our understanding of the nature of matter across the wide range of conditions that exist in the universe.
Observing the behavior of hydrogen under very high pressures is a great challenge for researchers. However, recently research teams were able to reveal that at pressures 3.5-2 million times higher than atmospheric pressure, hydrogen atoms arrange themselves in an unexpected structure consisting of layered sheets, and not in a well-packed metal-like structure, as researchers predicted many years ago.
The hydrogen sheets are similar to the carbon compound graphene. The layers of graphene each consist of a honeycomb-like hexagonal structure that includes rings of six carbon atoms. This ordinary graphene, which was first synthesized about a decade ago, is very light-weight, and yet, it is extremely strong and conducts heat and electricity very efficiently. These features guarantee the development of revolutionary technologies, including electronic and optical components for screens, highly functional photovoltaic cells, and improved batteries as well as other energy storage devices.
The new research shows that the stability of the unusual hydrogen structure is due to the inherent stability of the hydrogen rings. These rings are formed due to the phenomenon known as 'aromaticity', which is properly structured for molecules containing carbon atoms, for example benzene and graphene. Aromatic structures have a ring-like shape that can be seen as carbon atoms linked together by alternating single and double bonds. However, what actually happens is that the electrons incorporated in these theoretical bonds become unlocalized and move within a common circle within the ring, increasing chemical stability.
The researchers showed that under such high pressures, the hydrogen initially turns into a black, metal-like substance with poor electrical conductivity, similar to graphite (which is also composed of carbon), and not into a normal shiny, good-conducting metal, as the researchers originally thought in light of theoretical calculations made back in the 30s of the last century with the help of early models of solids and based on quantum mechanics.
Although the discovery of this layered sheet of compressed hydrogen came as a surprise to many researchers, about thirty years ago certain chemists - even before the discovery of graphene - predicted this exact structure based on simple chemical considerations. The work of these chemists is being validated and expanded by the new findings.
"In general, our findings indicate that chemical binding occurs across a greater range of conditions than previously considered. At the same time, the structural consequences of this chemical link under these extreme conditions are very different from those observed under normal conditions and which are familiar to all of us," notes the lead researcher.
