Modeling the ‘free carbon’ phase in amorphous silicon oxycarbide

We model the ‘free carbon’ phase in silicon oxycarbide glasses using a low-density structure of a-SiCO, into which a part of the graphite structure is embedded. The complete structure comprises 158 atoms, Si48C16O64 + 30C, corresponding to the composition SiC0.33O1.33 + 0.62C, and is first fully optimized using density functional methods. Subsequently, the evolution of the composite model is investigated by ab initio (Car-Parrinello) molecular dynamic simulations at elevated temperatures. A treatment at 800 °C shows the reaction of the ‘free carbon’ phase with the surrounding a-SiCO host by multiple bond formation, resulting in a decrease of the total energy. Further annealing at 1600 °C decreases the energy further, but the more radical conditions also create a particular interface structure between segregation and host. The electronic structure of the final model is composed of the wide-band gap host material of SiCO and the semi-metallic graphitic segregation. We observe several gap states delocalized within the graphitic segregation as well as unpaired electrons, which are caused by strain at the interface between the graphitic segregation and the oxycarbide host. We also present vibrational spectra calculated for the oxycarbide phase with and without the embedded carbon structure. The composite not only exhibits a pronounced peak at 1400–1500 cm−1, resulting from the graphitic segregation, but also shows a depletion of density of vibrational states in the spectral range below 500 cm−1, indicating a stiffening of the glassy phase.