The structural evolution and transformation pathways of carbon dioxide phase IV (${\mathrm{CO}}_2$-IV) are investigated over 13.0–76.7 GPa and 296–700 K using synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations. ${\mathrm{CO}}_2$-IV was obtained from ${\mathrm{CO}}_2$-I by isothermal compression to 13.0 GPa at 600 K. Subsequent single-crystal diffraction performed at 14.5 GPa and 296 K confirms that ${\mathrm{CO}}_2$-IV adopts a rhombohedral $\text{R}\overline{3}\text{c}$ structure, consistent with Datchi et al. [Phys. Rev. Lett. 103, 185701 (2009)], establishing its purely molecular character without evidence for an “intermediate bonding state”. At room temperature, deviatoric stress promotes a symmetry-lowering distortion of $R\overline{3}c$ ${\mathrm{CO}}_2$-IV at 14.6 GPa, yielding a distorted molecular configuration described by a triclinic $P1$ model, denoted here as an ${\mathrm{IV}}^{\prime }$-type distortion. From 296 to 600 K, the onset pressure of this distortion increases modestly, giving rise to a positively sloped kinetic onset line in the $P$–$T$ diagram. At 650 and 700 K, enhanced thermal relaxation substantially reduces the influence of deviatoric stress, and no symmetry breaking is detected before the molecular-to-nonmolecular transformation, stabilizing the $R\overline{3}c$ structure in the molecular regime. Upon further compression, the molecular solid transforms into metastable nonmolecular states containing mixed three- and fourfold-coordinated carbon atoms. The transformation pathway is temperature dependent: at 296–600 K, amorphization proceeds from the $P1$-distorted form of ${\mathrm{CO}}_2$-IV to a-${\mathrm{CO}}_2$, whereas at 650–700 K the transformation proceeds directly from $R\overline{3}c {\mathrm{CO}}_2$-IV to the more ordered metastable covalent phase VI. The onset pressure of the molecular-to-nonmolecular transformation decreases with increasing temperature. These results reveal that the high-pressure behavior of ${\mathrm{CO}}_2$ is governed by the interplay between deviatoric stress and thermal activation, which controls symmetry lowering within the molecular regime and the subsequent pathways toward amorphous and metastable covalent structures.

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• High-pressure studies
• Phase transitions
• First-principles calculations
• Raman spectroscopy
• X-ray diffraction