The peculiar nature of molecular hydrogen vibration under high pressure

Jan 14, 2011
Traces of the positions of silane and hydrogen molecules over time at 32 GPa, obtained from molecular dynamics calculations. Hydrogen atoms at tetrahedral (white) and octahedral (red) sites are shown. Silicon atoms at so-called ‘face-centered cubic’ sites are shown by gray spheres. Credit: Reproduced, with permission, from Ref. 1 2010 The American Physical Society

Most of our Universe consists of hydrogen atoms, which are often found under extraordinarily high pressure as high as tens of millions of times the atmospheric pressure of Earth. Understanding the exotic physics of such a high-pressure regime will contribute to our understanding of planet formation, hydrogen storage, room temperature superconductivity and other fields, explains Toshiaki Iitaka from the RIKEN Advanced Science Institute in Wako, Japan.

Iitaka, along with colleagues from the Institute of in Singapore and the University of Saskatchewan in Canada, recently uncovered the physical basis underlying a newly discovered behavior of hydrogen molecules under high pressure.

This behavior was observed in a complex of hydrogen molecules, and hydrogen bound to silicon, which is called silane. Silane’s are under so-called 'chemical compression' by virtue of their being part of a chemical bond. In 2009, physicists found that the vibrational frequency of hydrogen molecules in silane–hydrogen complexes fell as the applied pressure rose. This anti-correlation was the opposite of previous observations of high-pressure hydrogen.

Iitaka and colleagues modeled the system using molecular dynamics simulations. They first optimized the relative arrangement of hydrogen and silane molecules inside a unit cell, finding that the hydrogen molecules tend to sit at octahedral and tetrahedral sites. They then computed the vibrational frequencies of the , and found two groups of vibrational modes, one at high energy and one at low energy.

The frequencies of the lower-energy group decreased monotonically as applied pressure increased. However, the frequencies of the higher-energy group increased with pressure until about 20.1 giga Pascals (GPa), after which they fell. This reproduced the experimentally observed anti-correlation between vibrational frequency and applied pressure, indicating that the simulation was accurate.

The simulations also revealed that this rise and fall in frequencies resulted from interactions between hydrogen and silane molecules. These interactions resulted from the overlap between the filled electron orbitals of one molecule and the empty orbitals of the other molecule. This overlap stabilizes the system, and its strength depends on the distance between the molecules. This distance, in turn, depends on the applied pressure.

The simulation results are another glimpse into the exotic physics that underpins the high-pressure regime, according to Iitaka. “We have shown that there is much more interesting new physics and chemistry to be explored in the world of high pressure.”

Explore further: Unexpected new mechanism reveals how molecules become trapped in ice

More information: Yim, W.-L., et al. Pressure-induced intermolecular interactions in crystalline silane-hydrogen. Physical Review Letters 105, 215501 (2010).
Read the full article here: prl.aps.org/abstract/PRL/v105/i21/e215501

add to favorites email to friend print save as pdf

Related Stories

Squashing Silane into Metal

Jan 09, 2009

(PhysOrg.com) -- Squeeze it hard enough and hydrogen, the most abundant and lightest element in our Universe, strangely takes on a metallic nature. During this state, as it loses hold of its electrons, hydrogen ...

On the path to metallic hydrogen

Aug 03, 2009

Hydrogen, the most common element in the universe, is normally an insulating gas, but at high pressures it may turn into a superconductor. Now, scientists at the Carnegie Institution in Washington D.C., US, ...

Heavy Pyridine Crystallizes Differently

Jan 07, 2009

(PhysOrg.com) -- The nuclei of ordinary hydrogen atoms contain only a single proton. If a neutron is added, the hydrogen becomes deuterium. In principle, molecules that contain deuterium in place of hydrogen ...

Recommended for you

Hide and seek: Sterile neutrinos remain elusive

19 hours ago

The Daya Bay Collaboration, an international group of scientists studying the subtle transformations of subatomic particles called neutrinos, is publishing its first results on the search for a so-called ...

Novel approach to magnetic measurements atom-by-atom

23 hours ago

Having the possibility to measure magnetic properties of materials at atomic precision is one of the important goals of today's experimental physics. Such measurement technique would give engineers and physicists an ultimate ...

Scientists demonstrate Stokes drift principle

Oct 01, 2014

In nature, waves – such as those in the ocean – begin as local oscillations in the water that spread out, ripple fashion, from their point of origin. But fans of Star Trek will recall a different sort of wave pattern: ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

lstone
not rated yet Jan 18, 2011
This is very interesting!!