Stability and metallization of solid oxygen at high pressure.

Area of Science:

• Condensed matter physics
• Materials science
• Computational chemistry

Background:

• The phase diagram of solid oxygen under extreme conditions is not fully understood.
• Previous studies have not fully accounted for anharmonic effects and thermal disorder.

Purpose of the Study:

• To investigate the phase diagram of oxygen at high pressures (50-130 GPa) and temperatures (up to 1200 K).
• To identify stable phases and their properties using first-principles theory.
• To elucidate the role of anharmonic lattice dynamics and thermal disorder.

Main Methods:

• First-principles calculations employing hybrid exchange functionals.
• Inclusion of anharmonic lattice dynamics effects.
• Analysis of electronic, structural, and thermodynamic properties at 0 K and finite temperatures.

Main Results:

• A metallic molecular structure with P6₃/mmc symmetry (η' phase) is found to be thermodynamically stable above 50 GPa at elevated temperatures.
• The stability of the η' phase is dependent on incorporating anharmonic effects and thermal disorder.
• Calculations provide detailed electronic, structural, and thermodynamic properties for solid oxygen.

Conclusions:

• The η' phase represents a significant new phase in the solid oxygen phase diagram.
• Accurate theoretical predictions require accounting for complex physical phenomena like anharmonicity and thermal disorder.
• The study offers valuable insights for experimental verification and further theoretical investigations.