Nonlinear carbon dioxide at high pressures and temperatures

Area of Science:

• Materials Science
• High-Pressure Physics
• Solid-State Chemistry

Background:

• Carbon dioxide (CO2) exhibits diverse solid phases under varying pressure and temperature conditions.
• Understanding CO2 phase transitions is crucial for geochemistry and planetary science.

Purpose of the Study:

• To investigate the high-pressure behavior of carbon dioxide beyond known phases.
• To characterize the structural and vibrational properties of novel CO2 phases.

Main Methods:

• Laser heating of CO2-III (Cmca) phase between 12 and 30 GPa.
• Quenching the sample to 300 K for analysis.
• Raman spectroscopy to analyze vibrational modes and structural changes.

Main Results:

• Discovery of a nonlinear molecular carbon dioxide phase IV.
• Observation of a triplet bending mode (ν2) near 650 cm⁻¹ in CO2-IV, indicating broken inversion symmetry.
• Softening of bending modes with increasing pressure, suggesting enhanced intermolecular interactions.
• Transition of CO2-IV into an extended amorphous solid at 80 GPa due to vibron-phonon collapse.

Conclusions:

• CO2-IV represents a novel high-pressure phase of carbon dioxide with broken inversion symmetry.
• Intermolecular interactions significantly influence the vibrational properties of CO2 at high pressures.
• CO2-IV transforms into an amorphous state at extreme pressures, impacting our understanding of dense CO2.