Effects of pressure on hydrogen diffusion behaviors in MgO

Hydrogen, as the smallest atom and a key component of water, can penetrate into materials in various forms (e.g., atoms, molecules), which has significant effects on their properties; hence, the diffusion behavior of hydrogen has aroused widespread attention. One of the major compositions in the Earth's interior is MgO. Thus, the diffusion behavior of hydrogen in MgO under high pressure is vital for understanding the water cycle in the Earth's interior. However, the hydrogen diffusion behavior in MgO under high pressure is still poorly understood. Herein, the hydrogen diffusion behaviors in MgO with increasing pressure are systematically investigated in the framework of first-principles methods. Our results show that separated H atoms tend to converge to form H2 molecules, and H2 molecules tend to gather together. The energy barriers of both H and H2 diffusion in MgO increase with pressure. Notably, our results illustrate that hydrogen prefers to diffuse in solid MgO in its molecular state even under high pressure. Furthermore, the attempt frequency of hydrogen in MgO increases with temperature, while it decreases with pressure. This study will deepen our understanding of hydrogen diffusion behavior in MgO under high pressure and provide guidance for studies on particle diffusion in solid materials under extreme conditions.