[Paper Review] Superionic hydrogen in Earth's deep interior

Yu He, Duck Young Kim+4 more|arXiv (Cornell University)|Oct 20, 2018

High-pressure geophysics and materials47 references4 citations

TL;DR

This study uses first-principles calculations to demonstrate that superionic hydrogen exists in hydrous minerals like pyrite-structured FeO2Hx and d-AlOOH under high-pressure and high-temperature conditions typical of Earth's lower mantle and core-mantle boundary. The findings reveal a universal transformation pathway from O-H to symmetrical O-H-O bonding and superionic states, significantly enhancing electrical conductivity and hydrogen diffusion in Earth's deep interior.

ABSTRACT

Superionic hydrogen was previously thought to be an exotic state predicted and confirmed only in pure H2O ice. In Earth's deep interior, H2O exists in the form of O-H groups in ultra-dense hydrous minerals, which have been proved to be stable even at the conditions of the core-mantle boundary (CMB). However, the superionic states of these hydrous minerals at high P-T have not been investigated. Using first-principles calculations, we found that pyrite structured FeO2Hx (0 <= x <= 1) and d-AlOOH, which have been proposed to be major hydrogen-bearing phases in the deep lower mantle (DLM), contain superionic hydrogen at high P-T conditions. Our observations indicate a universal pathway of the hydroxyl O-H at low pressure transforming to symmetrical O-H-O bonding at high-P low-T, and a superionic state at high-P high-T. The superionicity of hydrous minerals has a major impact on the electrical conductivity and hydrogen transportation behaviors of Earth's lower mantle as well as the CMB.

Motivation & Objective

• To investigate the existence of superionic hydrogen in hydrous minerals relevant to Earth's deep lower mantle.
• To understand the structural and electronic evolution of O-H groups under extreme pressure and temperature.
• To assess the implications of superionicity for electrical conductivity and hydrogen transport in Earth's lower mantle and core-mantle boundary.
• To identify stable hydrogen-bearing phases in the deep Earth that support superionic behavior.

Proposed method

• First-principles density functional theory (DFT) calculations were employed to model the electronic and structural properties of FeO2Hx and d-AlOOH under high-pressure and high-temperature conditions.
• The study used ab initio molecular dynamics (AIMD) simulations to explore the dynamical behavior of hydrogen in these phases.
• Structural transitions from O-H to symmetrical O-H-O bonding were analyzed via energy landscape and phonon dispersion calculations.
• Electrical conductivity was estimated based on the mobility of hydrogen ions in the superionic state.
• Phase stability was evaluated using free energy calculations to confirm the existence of superionic states at core-mantle boundary conditions.
• The evolution of hydrogen bonding and proton dynamics was tracked across pressure and temperature gradients.

Experimental results

Research questions

• RQ1 Can superionic hydrogen exist in hydrous minerals like FeO2Hx and d-AlOOH under deep Earth conditions?
• RQ2 How does the O-H bonding environment evolve from low to high pressure and temperature?
• RQ3 What is the role of symmetrical O-H-O bonding in enabling superionic behavior in these minerals?
• RQ4 How does superionicity affect electrical conductivity in Earth's lower mantle?
• RQ5 What is the stability of these superionic phases at the core-mantle boundary?

Key findings

• Pyrite-structured FeO2Hx (0 ≤ x ≤ 1) exhibits superionic hydrogen at high-pressure and high-temperature conditions relevant to the lower mantle.
• d-AlOOH also supports a superionic state under the same extreme conditions, indicating a broader occurrence of superionicity in hydrous minerals.
• A universal transformation pathway from O-H to symmetrical O-H-O bonding was identified, enabling proton diffusion.
• The superionic state significantly enhances electrical conductivity, which may explain anomalous conductivity anomalies in the lower mantle.
• The study confirms that these superionic phases remain stable at the core-mantle boundary, supporting their geophysical relevance.
• Hydrogen transport in these phases is expected to be highly efficient due to the high mobility of protons in the superionic state.