Hydrogen confinement: Phase IV

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  • Published: Nov 1, 2014
  • Author: David Bradley
  • Channels: Raman
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Experiment IV

Combining hydrogen and its heavier sibling deuterium under pressure can create a novel, disordered,

Combining hydrogen and its heavier sibling deuterium under pressure can create a novel, disordered, "Phase IV" material as revealed by Raman spectroscopy according to US researchers.

Hydrogen is the most abundant element in the universe and a constant reminder of how even something apparently so simple can be so perplexing. For instance, hydrogen behaves very differently at extremes of temperature and pressure. Under ambient conditions, it forms diatomic gas molecules. But, scientists who have put it under pressure have so far identified four distinct solid phases for this element. Combine hydrogen with its heavier isotopic sibling, deuterium (with its neutron) and, according to Alexander Goncharov of Carnegie Mellon University, in Pittsburgh, Pennsylvania, USA, and colleagues and a novel, disordered, "Phase IV" material can be produced in which interactions between hydrogen molecules are like nothing seen before. The findings described in the journal Physical Review Letters could be invaluable in developing new superconducting and thermoelectric materials containing hydrogen.

Energy low

Phase IV is a dense solid of diatomic hydrogen and diatomic deuterium first identified by colleagues of Goncharov and others. Within this substance, the hydrogen molecules exhibit two very different behaviours. The first is a weak interaction with neighbouring molecules, the second a strong bonding with neighbours that leads to hexagonal atomic sheets not dissimilar in appearance to the all-carbon material graphene. Electronically, these layers have properties that lie between a semiconductor and a semimetal.

Now, Ross Howie of the University of Edinburgh and colleagues have combined experiment and theoretical calculations. They mixed the H2 and D2 in varying concentrations and subjected them to room temperature under different pressures, ranging from about 2000 times atmospheric pressure (0.2 gigapascals) to about 2.7 million atmospheres (270 GPa).

"Before conducting the experiments, we thought that the material could change under pressure by several different processes," explains Goncharov. "The mass differences of the molecules mean that they have very different low-energy states, which would affect the outcome. In one scenario, the physics could result in the ordered segregation of the dihydrogen and dideuterium molecules between strongly and weakly bounded layers."

Local

In a second theoretical scenario, however, the molecules might be distributed randomly, they might be disordered. In this case, one might imagine that the disordered state would inhibit the free propagation of atomic vibrations, or phonons, so-called Anderson localization. Phonon propagation through the disordered molecular maze would affect the electronic energy bands and, depending on the strength of the molecular bonds, the masses, or both, have an impact on just a few molecules at the local level.

By using Raman spectroscopy, the team was able to confirm this experimentally. Above 1.9 million atmospheres, the phonons do indeed show Anderson localization. The extent of this localization depends on the concentration of dihydrogen and dideuterium and whether these molecules belong to weakly or strongly bound layers. For instance in one layer, dihydrogen molecules vibrate in separate groups of two or three molecules at frequencies that depend only weakly on the local environment. As the dihydrogen concentration increases, however, clusters grow and begin to couple. This is, the team says, the first study to show that Anderson localization from vibrational energy occurs because of mass differences within a material.

"The Anderson localization of vibrational excitations in hydrogen mixtures provides a new mechanism for optimizing thermoelectric and electronic behaviour, for example in superconductivity," Goncharov adds.

Related Links

Phys Rev Lett, 2014, 113, 175501: "Phonon Localization by Mass Disorder in Dense Hydrogen-Deuterium Binary Alloy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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