High-energy-density polymeric carbon oxide: Layered Cx Oy solids under pressure

Pressure-induced polymerization of carbon monoxide (CO) molecules may lead to next-generation high energy density materials. By combining structural search method, first-principles calculations and ab initio molecular dynamics (AIMD) simulations, we predict several polymeric carbon oxide CxOy, i.e., C5O2 (P-4m2), C6O4 (P-4m2 and I-4m2) and C8O8 (I-4m2), which are all layered semiconductors with high energy density, large bulk modulus, and high hardness. For the C:Oratio=1:1, the C8O8-I-4m2 phase is energetically more stable than the reported Cmca and Cmcm phases above 90 GPa, and its energy density is 4.51 kJ/g, which is higher than trinitrotoluene (TNT). Considering the mechanism of CO2 release, CxOy(x>y) crystals, namely C5O2 (P-4m2) and C6O4 (P-4m2 and I-4m2) are predicted above 40∼50GPa. At 100 GPa, phonon spectrum calculations and AIMD simulations indicate that they have good mechanical and dynamic stability. At 0 GPa, AIMD simulations also show the possible phase transition path of the four CxOy structures by releasing CO2: C8O8-I-4m2→C6O4-I-4m2 and C6O4-P-4m2→C5O2-P-4m2, and C6O4-I-4m2 and C5O2-P-4m2 can remain stable at ambient conditions. Those structures may enrich the phase diagram of high-pressure CxOy, and provide clues for synthesis and exploration of new stable energetic materials besides polymeric nitrogen.