Superionicity in ammonium polyhydrides at extreme pressures
K de Villa1, X Wang2, E Zurek2
1Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA.
The Journal of Chemical Physics
|December 31, 2025
Summary
High-pressure ammonium polyhydrides exhibit hydrogen superionic diffusion when heated. At high proton fractions, these compounds are predicted to melt directly into liquids, rather than entering a superionic phase, especially under giant planet interior conditions.
Area of Science:
- Planetary Science
- Materials Science
- Computational Chemistry
Background:
- Novel polyhydride structures form at high pressures, relevant to giant planet interiors.
- Ammonium polyhydrides are metastable at 100-300 GPa, exhibiting complex phases.
Purpose of the Study:
- Investigate the behavior of ammonium polyhydrides under high pressure and temperature.
- Characterize solid, superionic, and liquid phases and transitions.
- Determine the influence of proton fraction on phase transitions.
Main Methods:
- Density functional molecular dynamics simulations.
- Crystal structure search methods.
- Analysis of internal energy, pressure, chemical species, and diffusion rates.
Main Results:
- High-pressure ammonium polyhydrides show hydrogen superionic diffusion upon heating.
- Solid-to-superionic and superionic-to-liquid transition temperatures decrease with increasing proton fraction.
- Above a proton fraction of ~0.97, direct melting to liquid is favored over a superionic phase.
Conclusions:
- Ammonium polyhydrides exhibit distinct phase transitions, including superionic behavior.
- Proton fraction significantly impacts the phase diagram of these compounds.
- Hydrogen-rich ammonium hydrides likely exist as liquids in ice giant interiors.
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