Calcium peroxide from ambient to high pressures.

Area of Science:

• Materials Science
• Geophysics
• Computational Chemistry

Background:

• Calcium peroxide (CaO2) is an important compound with potential applications and relevance in planetary science.
• Understanding its structural behavior under extreme conditions is crucial for various scientific disciplines.

Purpose of the Study:

• To investigate the structural phases of calcium peroxide (CaO2) across a wide pressure range (0-200 GPa).
• To determine the thermodynamic stability of CaO2 and its potential role in planetary geochemistry.

Main Methods:

• Utilized ab initio random structure searching (AIRSS) and density functional theory (DFT) calculations.
• Employed phonon calculations within the quasiharmonic approximation to assess stability at mantle conditions.

Main Results:

• Identified several stable CaO2 structures, with the ground state sensitive to the exchange-correlation functional at 0 GPa.
• Discovered new stable phases of CaO2 below 40 GPa that are thermodynamically stable against decomposition.
• Observed increasing stability with pressure, peaking at the CaO B1-B2 phase transition (65 GPa).
• Confirmed CaO2 stability at mantle temperatures and pressures, suggesting a role in planetary geochemistry.
• Mapped a phase diagram revealing at least five new stable phases within 0–60 GPa and 0–600 K.

Conclusions:

• Calcium peroxide exhibits diverse structural phases under high pressure, with significant stability.
• The findings support a potential role for CaO2 in planetary geochemistry, particularly within planetary interiors.
• The study provides a comprehensive phase diagram for CaO2, including new candidates for its zero-pressure ground state.