The crystal structures of the recently discovered nonmolecular phases of ${\text{CO}}_{2}$ are still the subject of intense debate. Based on density-functional theory calculations, we show that a layered structure (space group $P{4}_{2}/nmc$) with carbon in tetrahedral coordination is thermodynamically stable between 200 and 900 GPa. The Raman spectrum for this phase agrees with that measured for ${\text{CO}}_{2}\text{-VI}$. Contrary to ${\text{SiO}}_{2}$, where octahedral coordination for silicon starts with stishovite at about 10 GPa, we find that structures with carbon in octahedral coordination are unlikely to be thermodynamically stable in ${\text{CO}}_{2}$ at pressures currently reachable in the laboratory. We attribute the exceptional stability of tetrahedral structures in ${\text{CO}}_{2}$ to the small atomic size of the carbon atom, which allows it to occupy the tetrahedral interstitial sites of the close-packed oxygen sublattices.