Isentropic magnetic compression and ab initio calculations on the compressibility and equation of state (EOS) for solid carbon dioxide were performed up to pressures of 540 GPa and 1.6 TPa, respectively. A method of isentropic compression by pressure of an ultrahigh magnetic field from an explosive magnetocumulative generator MC-1 was applied for the dynamic compression of solid ${\mathrm{CO}}_2$ samples to multimegabar pressures, with the simultaneous x-ray imaging of the compressed samples. An amplified magnetic field of the MC-1 generator plays the role of an intermediate medium converting the detonation waves into the smoothly increasing compression waves, thus achieving a high degree of isentropic conditions of the sample compression. This method is especially unique in achieving 500–700 GPa pressures in highly compressible substances, with sample heating only to $\sim 1000$ K, whereas known dynamic shock-wave methods do not allow for large compression in solids due to strong heating of the sample. As a result, eight points on the ${\mathrm{CO}}_2$ compressibility curve were obtained at pressures from 47 to 540 GPa. The maximum density of the ${\mathrm{CO}}_2$ sample achieved at this pressure was $6.87\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$. This corresponds to the highest compression ratio ever achieved for solid ${\mathrm{CO}}_2$ with $\rho /{\rho }_0=4.24$. The experimental results were compared with theoretical calculations of the EOSs of ${\mathrm{CO}}_2$ phases and showed their close agreement. This indicates that covalent polymeric phases of ${\mathrm{CO}}_2$ with crystal structures corresponding to theoretical calculations were obtained in the magnetic compression experiments. Accompanying first-principles theoretical calculations were carried out on solid ${\mathrm{CO}}_2$ phases at pressures up to 1.6 TPa and temperatures up to 5000 K in the quasiharmonic approximation. It was established that transition to phases with a sixfold coordination of carbon occurs at a pressure of about 1 TPa. No conductive solid phases were detected in the experiments or in calculations. The theoretical calculations also do not confirm dissociation of ${\mathrm{CO}}_2$ into carbon and oxygen. The obtained data clarify the compressibility and electrical conductivity of solid carbon dioxide at low temperatures and ultrahigh pressures and provide opportunities for constructing a wide-ranging EOS for this substance.

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• Dynamical phase transitions
• Electrical conductivity
• First-principles calculations
• Phase diagrams
• Pressure effects
• Thermoelasticity
• Crystal structures
• Density functional theory
• Hugoniot measurements
• X-ray imaging