Classical and quantum ordering of protons in cold solid hydrogen under megabar pressures.
Xin-Zheng Li1, Brent Walker, Matthew I J Probert
1London Centre for Nanotechnology and Department of Chemistry, University College London, London WC1E 6BT, UK.
Summary
Quantum effects significantly influence solid hydrogen phase transitions, particularly between phases I and II. Ab initio path integral molecular dynamics (PIMD) simulations reveal distinct quantum and classical behaviors in these transitions.
Area of Science:
- Condensed Matter Physics
- Quantum Mechanics
- Materials Science
Background:
- Solid hydrogen exhibits complex phases under high pressure.
- Understanding proton ordering and quantum effects is crucial for its phase diagram.
Purpose of the Study:
- To investigate classical and quantum ordering of protons in cold, high-pressure solid hydrogen.
- To explore the role of quantum nuclear motion and van der Waals forces in phases II and III.
Main Methods:
- Utilized state-of-the-art theoretical methods.
- Performed ab initio path integral molecular dynamics (PIMD) simulations.
- Employed the optB88-vdW density functional to include van der Waals forces.
Main Results:
- The phase I to II transition is strongly quantum, driven by competing anisotropic interactions and nuclear fluctuations.
- The phase II to III transition is primarily classical, with quantum motion playing a secondary role.
- A P2(1)/c structure is stable for phase II when including anharmonic quantum motion, accurately predicting vibron frequencies.
- A C2/c structure is supported as a candidate for phase III, remaining transparent up to 300 GPa with quantum effects.
- Including van der Waals forces improves agreement with experimental data compared to previous functionals.
Conclusions:
- Quantum nuclear motion significantly impacts solid hydrogen phase transitions.
- The inclusion of van der Waals forces enhances theoretical predictions.
- Specific structures are identified as stable candidates for phases II and III.
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