Carbon dioxide is one of the most stable molecules at ambient conditions, but under extreme conditions of pressure and temperature it collapses into extended covalently-bonded "polymeric" polymorphs. The stable phase of CO2 above 40 GPa is a partially collapsed cristobalite-like structure with tetrahedral carbon coordination similar to silicon in SiO2. However several other non-molecular structures have been reported experimentally above 30 GPa and the determination of the phase diagram is hindered by slow kinetics and extensive metastability. At higher pressures, seeming similarities with compressed silica are disproved by ab-initio simulations, which yield for the crystalline phases evidence for the stability of tetrahedral CO4 units over a much wider pressure range than in silica.

Polymeric carbon dioxide at high pressures

253rd National Meeting (2017)

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