Raman studies of molecular-to-nonmolecular transitions in carbon dioxide at high pressures and temperatures

We have studied the molecular-to-nonmolecular phase transformations of carbon dioxide (CO₂) in laser-heated diamond-anvil cells by confocal micro-Raman spectroscopy. Our spatially resolved Raman data across the laser-heated area suggest that two distinct forms of extended phases, phase VIII and disordered phase, coexist with tetrahedrally coordinated phase V above 50 GPa but at somewhat lower temperatures. The existence of these phases are most evident from their characteristic Raman features, which include a strong mode at 1200 cm^-1 for phase VIII and a broad peak at 900 cm^-1 for disordered phase, both of which appear at substantially higher wave numbers than the νb(C-O-C) mode of phase V at ~800 cm^-1, around 50 GPa. Our Raman data further suggest that phase III is stable to at least 65 GPa at ambient temperatures and thus, emphasize that the stability field of a-carbonia, an amorphous extended phase, is only at elevated temperatures at pressures around 50 GPa or at room temperature above 65 GPa. This apparent extended stability of phase III may be due to strong kinetics associated with the molecular-to-nonmolecular transitions.