Recently, a research team led by Prof. WANG Xianlong from 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. The hydrogen diffusion inside materials are generally believed to decrease with increasing pressure. This material is therefore widely used for protecting diamond avail cell to achieve high-pressure 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.

Through investigations on the hydrogen diffusion behaviors in Al₂O₃ at high-pressure and high-temperature conditions, the researchers found in this research that hydrogen atom (H atom) tends to agglomerate into hydrogen molecule (H₂) in all the three investigated phases. 

Further study showed that 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. 

"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," said Prof. WANG.

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, according to WANG.

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.

Diffusion paths and energy barriers of the H atom in Al₂O₃. (Image by ZHU Yuanqin)