Tokyo, Japan – At low temperatures, hydrogen atoms move less like particles and more like waves. This characteristic enables "quantum tunneling", the passage of an atom through a barrier with a higher potential energy than the energy of the atom. Understanding how hydrogen atoms move through potential barriers has important industrial applications. However, the small size of hydrogen atoms makes direct observation of their motion extremely challenging.
In a study to be published in Science Advances, researchers at the Institute of Industrial Science, The University of Tokyo report precise detection of quantum tunneling of hydrogen atoms in palladium metal.
Palladium is a metal that absorbs hydrogen. Palladium atoms are arranged in a repeating three-dimensional cubic pattern, otherwise known as a lattice. Hydrogen atoms can enter this lattice by occupying interstitial sites between the large palladium atoms. These sites are octahedral and tetrahedral in shape. Hydrogen sits stably in an octahedral site and can hop to another octahedral site via a tetrahedral site, which is metastable, i.e., less stable than an octahedral site.
Hopping between interstitial sites depends on the system temperature, which reflects the average kinetic energy of the atoms. Hopping between sites requires overcoming an energy barrier. At high temperatures, hydrogen atoms have sufficient kinetic energy to hop.
However, hydrogen hopping can also occur at low temperatures because of "quantum" effects. That is, hydrogen atoms act like waves, which can pass through the energy barrier via quantum tunneling. To do this, hydrogen atoms need help from phonons – vibrations of the lattice – or conduction electrons, the free-moving electrons in the host metal, palladium.
"To understand the quantum nature of hydrogen, we need to identify the hopping pathway," explains lead author, Takahiro Ozawa. "Typical probes, like X-rays and electron beams, can't be used to detect hydrogen because of its small cross-section. Thus, we employed channeling nuclear reaction analysis to locate hydrogen in the palladium lattice".
The team noted that hydrogen atoms injected into palladium first occupied the metastable tetrahedral sites and then moved to stable octahedral sites by tunneling. The tunneling rate was quantified by measuring the electrical conductivity, providing important clues about how tunneling occurs.
"Above 20 K, the tunneling rate slightly increased with the temperature, a signature of phonon effects," reports Katsuyuki Fukutani, senior author. "However, below 20 K, the tunneling rate slightly decreased with the temperature, signaling the involvement of conduction electrons that could not perfectly follow the motion of the hydrogen atoms."
The findings of the research team deepen our understanding of the quantum nature of hydrogen diffusion and pave the way for developing technologies to control atomic behavior based on quantum effects.
