Exploring the Phase Diagram SiO2-CO2 at High Pressures and Temperatures

Abstract:

CO₂ is an important volatile system relevant for planetary sciences and fundamental chemistry. Molecular CO₂ has doubly bonded O=C=O units but high pressure-high temperature (HP-HT) studies have recently shown its transformation into a three-dimensional network of corner-linked [CO₄] units analogous to the silica mineral polymorphs, through intermediate non-molecular phases. Here, we report P-V-T data on CO₂-IV ice from time-of-flight neutron diffraction experiments, which allow determining the compressibility and thermal expansivity of this intermediate molecular-to-non-molecular phase.¹ Aditionally, we have explored the SiO₂-CO₂ phase diagram and the potential formation of silicon carbonate compounds. New data obtained by laser-heating diamond-anvil experiments in CO₂-filled microporous silica polymorphs will be shown. In particular, these HP-HT experiments explore the existence of potential CO₂/SiO₂ compounds with tetrahedrally-coordinated C/Si atoms by oxygens, which are predicted to be stable (or metastable) by state-of-the-art ab initio simulations.^2,3 These theoretical predictions were supported by a recent study that reports the formation of a cristobalite-type Si_0.4C_0.6O₂ solid solution at high-pressures and temperatures, which can be retained as a metastable solid down to ambient conditions.⁴ Entirely new families of structures could exist based on [CO₄]^4- units in various degrees of polymerisation, giving rise to a range of chain, sheet and framework solids like those found in silicate chemistry.

References

[1] S. Palaich et al., Am. Mineral. Submitted (2015)

[2] A. Morales-Garcia et al., Theor. Chem. Acc. 132, 1308 (2013)

[3] R. Zhou et al., Phys. Rev. X, 4, 011030 (2014)

[4] M. Santoro et al. Nature Commun. 5, 3761 (2014)