Structure of Polymeric Carbon Dioxide ${\mathrm{CO}}_2\mathrm{\text{−}}\mathbf{V}$

The structure of polymeric carbon dioxide (${\mathrm{CO}}_2\mathrm{\text{−}}\mathrm{V}$) has been solved using synchrotron x-ray powder diffraction, and its evolution followed from 8 to 65 GPa. We compare the experimental results obtained for a 100% ${\mathrm{CO}}_2$ sample and a 1 mol % ${\mathrm{CO}}_2/\mathrm{He}$ sample. The latter allows us to produce the polymer in a pure form and study its compressibility under hydrostatic conditions. The high quality of the x-ray data enables us to solve the structure directly from experiments. The latter is isomorphic to the $\beta$-cristobalite phase of ${\mathrm{SiO}}_2$ with the space group $I\overline{4}2d$. Carbon and oxygen atoms are arranged in ${\mathrm{CO}}_4$ tetrahedral units linked by oxygen atoms at the corners. The bulk modulus determined under hydrostatic conditions, $B_0=136(10)\mathrm{GPa}$, is much smaller than previously reported. The comparison of our experimental findings with theoretical calculations performed in the present and previous studies shows that density functional theory very well describes polymeric ${\mathrm{CO}}_2$.

Figure 1

XRD patterns collected at 41 GPa and 295 K after laser heating: (a) ${\mathrm{CO}}_2(1\%)/\mathrm{He}$ sample and (b) 100% ${\mathrm{CO}}_2$ sample. The x-ray wavelength was 0.3738 Å. Red circles, experimental data; black lines, full profile refinements; gray (blue) lines, fit residuals; green sticks, positions of diffraction lines. In (a), the background has been subtracted for easier visualization and diffraction lines from the He component and Re gasket are indicated. In (b), the grayed region has been excluded from refinement as it contains only reflections from untransformed molecular ${\mathrm{CO}}_2$. The insets show the respective Raman spectra collected from the same samples at 48 GPa (a) and 41 GPa (b). The * symbol indicates modes from unreacted molecular ${\mathrm{CO}}_2$. The weak modes of ${\mathrm{CO}}_2\mathrm{\text{−}}\mathrm{V}$ are more difficult to observe in (b) due to a larger luminescence background from the diamond anvils.

Figure 2

(a) Unit cell of polymeric carbon dioxide phase V: C atoms in white and O atoms in gray (red). The ${\mathrm{CO}}_4$ tetrahedral units are emphasized with gray polyhedra. (b)–(d) Evolution with pressure of the C-O bond distance, intratetrahedral, and intertetrahedral angles, respectively. Hollow and solid symbols represent experimental data from the 100% ${\mathrm{CO}}_2$ sample and the 1 mol % ${\mathrm{CO}}_2/\mathrm{He}$ sample, respectively; error bars come from statistical analysis of profile refinements [22], evaluated at the $3\sigma$ level. The larger uncertainties for the ${\mathrm{CO}}_2/\mathrm{He}$ sample are mostly due to the small content of ${\mathrm{CO}}_2$ which resulted in weaker signals [17]. Dashed lines and dash-dotted lines are from DFT calculations with LDA or GGA approximations, respectively.

Figure 3

Lattice parameters and volume of ${\mathrm{CO}}_2\mathrm{\text{−}}\mathrm{V}$ as a function of pressure. Hollow and solid circles are present experimental data from the 100% ${\mathrm{CO}}_2$ sample and the ${\mathrm{CO}}_2(1\%)/\mathrm{He}$ sample, respectively; the solid line is the Birch-Murnaghan fit to the ${\mathrm{CO}}_2/\mathrm{He}$ sample data. Dashed lines and dash-dotted lines represent DFT calculations using the LDA and GGA approximations, respectively. Up-pointing triangles and down-pointing triangles are measurements from Refs. 2, 7, respectively.