Modelling of amorphous poly-CO structure with N and He

Abstract

Density functional theory (DFT) simulations of amorphous poly-CO were performed to understand the stability of the polymerized structure at low pressures (down to 100 bar) and to elucidate the weakest links of the structure. IR and Raman spectra of amorphous p-CO, calculated at 5.02 GPa from the dielectric tensor, are presented and show significant contributions of intact CO molecules, CO fragments decorating chains, and lactones of amorphous p-CO structures. DFT simulations of formation of amorphous polymeric structures were also done with the addition (as a result of replacement of CO molecules) of N or He atoms to the crystalline delta phase of CO. For the CO-N mixtures, the concentration of N was varied in the range from 6.25 % to 50% with different distribution patterns of N atoms in the unit cell. For all studied CO-N concentrations, isotropic compression led to CO polymerization beginning at a pressure of 11 GPa; the N was not incorporated in significant numbers (up to pressures of 20 GPa) in the random p-CO which starts to polymerize. This transition pressure is higher than that for pure p-CO to start to polymerize at 8 GPa. For the CO-He mixtures, the concentration of He atoms in the delta phase of CO was 12.5% of the number of atoms. Formation of random networks begins at 9 GPa and at 11 GPa all CO molecules have formed a combination of closed rings and chain type structures without any isolated CO molecules with a density of 2.40 g/cm3. He atoms appear to facilitate complete formation of the random structure at a lower pressure than that for pure poly-CO, which is almost completely polymerized at a pressure of 18 GPa. He atoms also help stabilize the structure while lowering the pressure down to 100 Bar with only few CO molecules detaching in the process. Without He atoms at the same pressure there are approximately ten times the number CO molecules occupying voids in the random network.