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

Novel extended tetrahedral forms of CO2 have been synthesized recently under high-pressure conditions. We perform ab initio density functional theory calculations to investigate whether doping with Si can extend the stability range of such tetrahedral forms of CO2 to ambient pressure. Calculations are performed with a simple cubic cell containing eight formula units in a ββ-cristobalite-like structure. Though we find that all the SixC1−xO2 structures considered by us are thermodynamically unstable with respect to decomposition into the end members at ambient pressures, the energy differences are small, suggesting that it might be possible for such phases to exist in metastable forms. At higher pressures, the heat of formation is found to be negative. The bonding between C and O atoms is more covalent than that between Si and O atoms. We also find indications that some C atoms may prefer three-fold coordination at low pressure.