Hydrogen diffusion in Al2O3 accelerates under high pressure, new research proves

Recently, a research team led by Prof. WANG Xianlong from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences systematically investigated the diffusion behavior of hydrogen in alumina (Al₂O₃) under high-pressure, providing a new perspective on the selection of hydrogen permeation barrier (HPB). 

The results were published in Physical Review B.

Al₂O₃ is an excellent HPB at ambient conditions due to its screw symmetric feature. Since the hydrogen diffusion inside of materials are generally believed to be decreased with pressure increasing, for protecting diamond avail cell to achieve high-pressure, Al₂O₃ is widely used as HPB in the hydrogen-related high-pressure experiments about metallic hydrogen, superhydride superconductors and so on. However, the effect of pressure-induced phase transition of Al₂O₃ on hydrogen diffusion behavior has not been well understood.

In this work, based on the first-principles methods, the diffusion behaviors in Al₂O₃ at high-pressure and high-temperature conditions are systematically investigated. The researchers find that in the investigated three phases, hydrogen atom (H atom) tends to agglomerate into hydrogen molecule (H₂). 

Unexpectedly, the hydrogen diffusion energy barriers in the high-pressure Al₂O₃ with Rh₂O₃(II) phase and CaIrO₃ phase are just about 10% and 50% of that in the atmospheric corundum phase. 

Researchers gave their explanation for this phenomenon: in the corundum phase with the screw symmetric feature hydrogen diffuses through a structure where the number of coordinated oxygen atoms changes as hydrogen diffuses (6-3-6 coordination), making it harder for hydrogen todiffuse. In contrast, the high-pressure phases have different coordination (4-4-4 and 5-4-5 coordination), which make it easier for hydrogen to diffuse.

In the CaIrO₃ phase, which has a layered structure, the diffusion energy barrier remains almost constant over a wide range of pressures (150 to 350 GPa), as hydrogen diffuse in the layers of the material. 

Their work provides insights for illustrating the diffusion behaviors of elements in the lower-mantle minerals, according to the team.