Double superionicity in icy compounds at planetary interior conditions.

Area of Science:

• Planetary Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Ice giant planets (Uranus, Neptune) and sub-Neptune exoplanets are primarily composed of hydrogen, carbon, nitrogen, and oxygen.
• The behavior of water (H2O), ammonia (NH3), and methane (CH4) under high pressure and temperature is crucial for understanding planetary interiors but remains poorly understood.

Purpose of the Study:

• To investigate the high-pressure, high-temperature behavior of thirteen key H-C-N-O compounds.
• To elucidate the potential states and transitions these compounds undergo within ice giant planets.

Main Methods:

• Utilized ab initio computer simulations to model thirteen H-C-N-O compounds.
• Analyzed the structural and dynamic properties of these compounds at elevated temperatures and pressures.

Main Results:

• Demonstrated that thirteen H-C-N-O compounds adopt a superionic state at high temperatures, with mobile hydrogen ions within a stable heavy-ion sublattice.
• Observed a novel doubly superionic state in four compounds at even higher temperatures, where both hydrogen ions and some heavy nuclei become mobile.
• Identified first-order phase transitions for both superionic states and melting.

Conclusions:

• The superionic and doubly superionic states can significantly impact the internal structure of ice giants.
• These phase transitions may lead to the formation of additional mantle layers and influence convective patterns in planets like Uranus and Neptune.