Assessing the stability fields of molecular and polymeric $\mathrm{C}{\mathrm{O}}_2$

We investigated the stability of polymeric $\mathrm{C}{\mathrm{O}}_2$ over a wide range of pressures, temperatures, and chemical environments. We find that the $I\overline{4}2d$ polymeric structure, consisting of a three-dimensional network of corner sharing $\mathrm{C}{\mathrm{O}}_4$ tetrahedra, forms at 40–140 GPa and from a $\text{CO-}{\mathrm{N}}_2$ mixture at 39 GPa. An exceptional stability field of 0–286 GPa and 100–2500 K is documented for this structure, making it a viable candidate for planetary interiors. The stability of the tetrahedral polymeric motif of $\mathrm{C}{\mathrm{O}}_2\text{−}\mathrm{V}$ is a consequence of the rigidity of $s{p}^3$ hybridized orbitals of carbon in a closed-packed oxygen sublattice.

Figure 1

X-ray diffraction patterns of ${\mathrm{CO}}_2$ after laser heating at 44 GPa. After heating, pressure dropped to 34 GPa. Initial ${\mathrm{CO}}_2\text{−}\mathrm{III}$ can be seen in unheated regions, while the patterns in the heated areas show the presence of pure ${\mathrm{CO}}_2\text{−}\mathrm{V}$ (red curve) and a mixture of ${\mathrm{CO}}_2\text{−}\mathrm{V}$ and $Pbcn\mathrm{Re}{\mathrm{O}}_2$ (green curve). Ticks correspond to the refined positions of the Bragg peak of the identified phases. The x-ray wavelength is 0.3344 $\acute{\AA }$.

Figure 2

X-ray diffraction patterns of $\mathrm{C}{\mathrm{O}}_2$ after pressure release to 0 GPa and warming up to 127(5) K. The patterns were obtained at different sample positions at nominally the same temperature just before the sublimation of $\mathrm{C}{\mathrm{O}}_2\text{-I}$ (top curve) and during the final XRD mapping (bottom curve). Vertical bars correspond to the refined positions and calculated intensities of the Bragg peaks of the identified phases. Asterisks indicate reflections from the Kapton windows of the cryostat. Red rectangles box single-crystal-like diffraction peaks of $\mathrm{C}{\mathrm{O}}_2\text{-V}$. The x-ray wavelength is $0.3445\acute{\AA }$.

Figure 3

X-ray diffraction patterns of $\mathrm{C}{\mathrm{O}}_2$ at 159 GPa (a) and 286 GPa (b),(c) after laser heating. Circles are the data, and the line is the le Bail fit. Ticks correspond to Bragg peaks of the fitted phases. The bottom panels in 3(a) and 3(b) are 2D images in rectangular coordinates. Red rectangles box single-crystal-like diffraction peaks of $\mathrm{C}{\mathrm{O}}_2\text{-V}$. Panel (c) is a mosaic where the patterns are taken at different DAC rotation angles around a vertical ω-axis, demonstrating a variety of (h k l) reflections. The single-crystal reflections are circled. The x-ray wavelength is $0.2952\acute{\AA }$ in (a) and $0.3344\acute{\AA }$ in (b) and (c).

Figure 4

Raman spectra at pressures between 159–286 GPa measured concomitantly with XRD patterns. The excitation laser wavelength is 532 nm for 159–237 GPa and 660 nm for 268 and 286 GPa.

Figure 5

Pressure dependence of the formula unit volume of $\mathrm{C}{\mathrm{O}}_2\text{-V}$ at room temperature. Filled symbols: this work (if not shown, the error bars are smaller than experimental uncertainties); open symbols: previous works [7, 18, 45]. Solid black curve representing the experimental data in the whole pressure range is a Birch-Murnaghan EOS with the following parameters: $V_0=23.8{\acute{\AA }}^3$, $B_0=130$ GPa, $B_0^{\prime }=4.8$.

Figure 6

Pressure dependence of the $A_1$ Raman mode frequency of $\mathrm{C}{\mathrm{O}}_2\text{-V}$ at room temperature. Filled symbols: this work (if not shown, the error bars are smaller than experimental uncertainties); open symbols: previous works. Solid black curve representing the experimental data in the whole pressure range is a fourth-order polynomial.

Figure 7

X-ray diffraction patterns of $\mathrm{C}{\mathrm{O}}_2$ synthesized at 38.6 GPa after laser heating of $\text{CO-}{\mathrm{N}}_2$ mixture at 39 GPa. Circles are the data, and a line is the le Bail fit. Ticks correspond to Bragg peaks of the fitted phases. The x-ray wavelength is $0.3445\acute{\AA }$.

Figure 8

Reconstructed reciprocal lattice plane (0 k l) of $\mathrm{C}{\mathrm{O}}_2\text{-V}$ at 38.6 GPa.

Figure 9

Phase diagram of $\mathrm{C}{\mathrm{O}}_2$ at low pressures. Symbols show the estimated P-T conditions of observations for $\mathrm{C}{\mathrm{O}}_2\text{-V}$ in this work and Ref. [13]. The arrows show the P-T path of this work.