Ultralow Melting Temperature of High-Pressure Face-Centered Cubic Superionic Ice
Caoping Niu1,2, Hanxing Zhang1,2, Jie Zhang1
1Key Laboratory of Materials Physics, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, Hefei 230031, China.
The Journal of Physical Chemistry Letters
|August 5, 2022
Summary
Superionic ice, crucial for understanding ice giant magnetic fields, may not exist in Uranus and Neptune. New simulations reveal a lower melting point, challenging previous theories and highlighting the need to account for superheating in phase diagram studies.
Area of Science:
- Planetary Science
- Materials Science
- Computational Physics
Background:
- Superionic ice with an oxygen face-centered cubic (fcc) sublattice is hypothesized as the source of magnetic fields in Uranus and Neptune.
- The presumed high melting temperature (Tm) of this phase, exceeding ice giant isentropes, supports this theory.
- Experimental measurement of the fcc-superionic phase is challenging, leading to reliance on computational models.
Purpose of the Study:
- To accurately determine the melting temperature (Tm) of fcc-superionic ice.
- To investigate the potential existence of fcc-superionic ice within the interiors of Uranus and Neptune.
- To address the issue of superheating in computational simulations of superionic phases.
Main Methods:
- Ab initio molecular dynamics simulations incorporating a model with H2O vacancies.
- Computational methods designed to avoid superheating effects.
- Two-phase simulations to validate findings on the phase diagram.
Main Results:
- A significantly lower melting temperature (Tm) for fcc-superionic ice was obtained compared to previous reports.
- The calculated Tm suggests that fcc-superionic ice cannot exist under the conditions found in the interiors of Uranus and Neptune.
- The study demonstrates that superheating can lead to an overestimation of Tm in computational models.
Conclusions:
- Fcc-superionic ice is unlikely to be the origin of magnetic fields in Uranus and Neptune.
- Superheating effects must be carefully considered in simulations of hydrogen-related superionic states.
- Accurate phase diagrams are crucial for understanding the properties of ice giants, Earth, and Venus.
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