Crossover from melting to dissociation of CO2 under pressure: Implications for the lower mantle

The discovery of nonmolecular carbon dioxide under high-pressure conditions shows that there are remarkable analogies between this important substance and other group IV oxides. A natural and long-standing question is whether compounds between CO2 and SiO2 are possible. Under ambient conditions, CO2 and SiO2 are thermodynamically stable and do not react with each other. We show that reactions occur at high pressures indicating that silica can behave in a manner similar to ionic metal oxides that form carbonates at room pressure. A silicon carbonate phase was synthesized by reacting silicalite, a microporous SiO2 zeolite, and molecular CO2 that fills the pores, in diamond anvil cells at 18–26 GPa and 600–980 K; the compound was then temperature quenched. The material was characterized by Raman and IR spectroscopy, and synchrotron X-ray diffraction. The experiments reveal unique oxide chemistry at high pressures and the potential for synthesis of a class of previously uncharacterized materials. There are also potential implications for CO2 segregation in planetary interiors and for CO2 storage.

https://www.pnas.org/content/pnas/108/19/7689.full.pdf

Silicon carbonate phase formed from carbon dioxide and silica under pressure

Extreme conditions permit unique materials to be synthesized and can significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct with respect to its heavier homologues in forming oxides. Here we report the synthesis of a crystalline CO2-SiO2 solid solution by reacting carbon dioxide and silica in a laser-heated diamond anvil cell (P = 16-22 GPa, T>4,000 K), showing that carbon enters silica. Remarkably, this material is recovered to ambient conditions. X-ray diffraction shows that the crystal adopts a densely packed α-cristobalite structure (P4(1)2(1)2) with carbon and silicon in fourfold coordination to oxygen at pressures where silica normally adopts a sixfold coordinated rutile-type stishovite structure. An average formula of C0.6(1)Si0.4(1)O2 is consistent with X-ray diffraction and Raman spectroscopy results. These findings may modify our view on oxide chemistry, which is of great interest for materials science, as well as Earth and planetary sciences.

Carbon enters silica forming a cristobalite-type CO2–SiO2 solid solution

The decomposition of silicon–carbon–oxygen (SiCO)-based materials into their binary oxides is studied at different pressure conditions by first-principles total energy calculations. We evaluate how the influence of pressure modifies the enthalpy of this reactive process as the carbon concentration increases for a number of compounds with variable stoichiometries within the general chemical formula SixCyO24 (x = 4–11, y = 1–8, x + y = 12). At low carbon content, pressure destabilizes all SiCO structures examined, whereas at high carbon content, pressure tends to reduce the exothermic character of the decomposition reaction. After the evaluation of the equation of state parameters for these crystals, we found a similar trend for the bulk modulus with the highest values found for the richest carbon compounds. According to our results, we propose SiC2O6 as the most plausible stoichiometry.

First-principles study of structure and stability in Si–C–O-based materials

Oxides containing both carbon and silicon had been elusive. One such oxide was synthesized under high pressure in 2011, but its structure was not known. An extensive computational search enabled by an evolutionary algorithm finds SiC${}_{2}$O${}_{6}$ to have a crystalline structure that is stable under pressure as high as two hundred times the ambient pressure.

/pdf/unraveling-crystalline-structure-of-high-pressure-phase-of-1mw2q9fuo5.pdf

Suman S. Dhayal

Papers

SixC1−xO2 alloys: A possible route to stabilize carbon-based silica-like solids?