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Hot dense carbon dioxide explored by in-situ high-temperature high- pressure studies

In-situ investigations in the pressure window 37-106 GPa, and at temperatures up to ~6200 K, indicate the thermodynamic stability of the extended carbon dioxide phase V and exclude its dissociation into chemical elements. This study sets the ground for further research on carbon dioxide at pressure- temperature extremes.

Carbon dioxide, CO2, is known in nature as the principal greenhouse gas, for its role in photosynthesis and as one of the most common volatile species in Earth s interior. At ambient pressure, it is considered a real thermodynamic sink for example dry ice, the solid form of CO2, is commonly used in fire extinguishers. This state is also conserved below ~40 GPa, where all known polymorphs of CO2 are built of linear molecules. However, above this pressure threshold, the molecules start to polymerise, forming an extended covalent network. While at temperatures below 700 K a conversion to an amorphous solid containing threefold and fourfold coordinated carbon atoms is observed, in the conditions of laser heating, a crystalline

Fig. 4: A schematic high-pressure high-temperature phase diagram of carbon dioxide. The striped area denotes the region between two computed melting curves of CO2-V phase. The shadowed rectangles indicate the temperature ranges achieved in the experiments.

Fig. 5: Top left: A representation of the laser-heated CO2 sample. Top right: 2D detector images of in-situ high-pressure high-temperature X-ray diffraction patterns. Bottom: Azimuthally integrated diffraction profiles at various pressure-temperature conditions with identification of Bragg peaks. Temperatures reported on the integrated patterns were measured on the hot spot.

covalent form, structurally related to the partially collapsed beta-cristobalite, is obtained. This modification, called CO2-V, was recently identified as the only stable crystalline structure of polymeric CO2 from ~40 GPa to 120 GPa [1].