Crystal symmetry controls hydrogen’s quantum tunnelling

Quantum tunnelling of hydrogen atoms is crucial in numerous processes, yet controlling this phenomenon remained a challenge. Researchers at the University of Tokyo have now discovered that the permeation of hydrogen through a material is influenced by the symmetry of the material's crystal structure. By manipulating this symmetry, scientists could potentially influence hydrogen's tunnelling, leading to more efficient hydrogen storage systems.

Katsuyuki Fukutani, the lead researcher, observed that hydrogen exhibits significant quantum tunnelling in highly symmetric crystal environments, while in less symmetric structures, tunnelling is significantly reduced. This discovery is significant as it identifies crystal symmetry as a fundamental factor in controlling hydrogen's quantum behavior in materials, opening new avenues for tailored hydrogen transport.

The researchers studied vanadium, a model hydrogen-storage material, at low temperatures. They employed two techniques: nuclear reaction analysis, which directly measures hydrogen's depth distribution with high resolution, and the measurement of electrical resistance, which monitors hydrogen's redistribution over time. Their findings indicated that hydrogen atoms start migrating through vanadium's crystal lattice at around 70 K, hopping between interstitial spaces.

At low hydrogen concentrations, this hopping occurs in a highly symmetric α-phase vanadium structure, allowing hydrogen atoms to tunnel between neighboring lattice sites easily. However, at higher hydrogen concentrations, vanadium's crystal lattice deforms into a β-phase, causing hydrogen atoms to need to overcome an energy barrier before they can tunnel to neighboring sites.

The researchers calculated the diffusion coefficient of hydrogen across a range of temperatures and used quantum-mechanical calculations to interpret these results. They found that crystal symmetry determines whether hydrogen moves through quantum tunnelling or classical thermal activation. In the α-phase, hydrogen's ground states are delocalized over tetrahedral sites thanks to tunnelling.

In the β-phase, however, the uniaxial strain produced by the lattice distortion leads to the localization of hydrogen's quantum states around certain sites in the material.

This finding is particularly exciting as it establishes crystal symmetry as a new design principle for tailoring hydrogen's quantum behavior in functional materials. The Tokyo researchers plan to extend their work to various hydrogen storage media, including metal alloys and oxide materials, in an effort to establish a universal framework describing how local atomic structure and crystal symmetry govern the quantum behavior of hydrogen.