Diffusion-driven transient hydrogenation in metal superhydrides at extreme conditions

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Metal hydrides are key targets for high-pressure research due to their potential for near-ambient superconductivity.
• Superconducting properties of novel metal hydrides are extensively studied, but the atomic and electronic states of hydrogen remain underexplored.

Purpose of the Study:

• To investigate the atomic and electronic states of hydrogen in lanthanum superhydrides (LaH_x) under high pressure.
• To understand the long-term stability and decomposition mechanisms of synthesized metal superhydrides.

Main Methods:

• Synthesis of lanthanum superhydrides (LaH_x, x = 10.2–11.1) using laser heating at pressures >160 GPa.
• Combined proton ($\text{1H}$) and lanthanum ($\text{139La}$) Nuclear Magnetic Resonance (NMR) spectroscopy.
• Transport measurements to assess superconducting critical temperatures over time.

Main Results:

• Hydrogen in lanthanum superhydrides exists in a highly diffusive state at room temperature (diffusion coefficients ~10^-6 cm²s^-1).
• The superhydrides undergo dynamic de-hydrogenation over weeks, reverting to compositions similar to precursor materials.
• Superconducting critical temperatures decrease significantly over time, correlating with sample decomposition.

Conclusions:

• The high mobility of hydrogen atoms is a critical factor influencing the stability of metal superhydrides.
• Dynamic decomposition and de-hydrogenation challenge the long-term viability of these materials for practical applications.
• This study provides new insights into the stability limitations of metal superhydrides, addressing long-standing questions in the field.