High-energy-density polymeric carbon oxide: Layered CxOy 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 ${C}_{x}{O}_{y}$, i.e., ${\mathrm{C}}_{5}{\mathrm{O}}_{2}$ $(P\text{\ensuremath{-}}4m2)$, ${\mathrm{C}}_{6}{\mathrm{O}}_{4}$ ($P\text{\ensuremath{-}}4m2$ and $I\text{\ensuremath{-}}4m2$) and ${\mathrm{C}}_{8}{\mathrm{O}}_{8}$ $(I\text{\ensuremath{-}}4m2)$, which are all layered semiconductors with high energy density, large bulk modulus, and high hardness. For the $\mathrm{C}:\mathrm{O}\phantom{\rule{0.28em}{0ex}}\mathrm{ratio}=1:1$, the ${\mathrm{C}}_{8}{\mathrm{O}}_{8}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ 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 ${\mathrm{CO}}_{2}$ release, ${C}_{x}{O}_{y}(x>y)$ crystals, namely ${\mathrm{C}}_{5}{\mathrm{O}}_{2}$ $(P\text{\ensuremath{-}}4m2)$ and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}$ ($P\text{\ensuremath{-}}4m2$ and I-4m2) are predicted above $40\ensuremath{\sim}50\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. 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 ${C}_{x}{O}_{y}$ structures by releasing ${\mathrm{CO}}_{2}$: ${\mathrm{C}}_{8}{\mathrm{O}}_{8}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2\ensuremath{\rightarrow}{\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2\ensuremath{\rightarrow}{\mathrm{C}}_{5}{\mathrm{O}}_{2}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2$, and ${\mathrm{C}}_{6}{\mathrm{O}}_{4}\text{\ensuremath{-}}I\text{\ensuremath{-}}4m2$ and ${\mathrm{C}}_{5}{\mathrm{O}}_{2}\text{\ensuremath{-}}P\text{\ensuremath{-}}4m2$ can remain stable at ambient conditions. Those structures may enrich the phase diagram of high-pressure ${C}_{x}{O}_{y}$, and provide clues for synthesis and exploration of new stable energetic materials besides polymeric nitrogen.