Crystal Structure of the High-Pressure Phase of Solid CO ₂

Abstract

X-ray diffraction study of solid CO(2) at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in terms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4.657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/- 0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52 degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO(2) are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.

A solid–solid phase transition in carbon dioxide at high pressures and intermediate temperatures

“…The reported transition pressures at room temperature range widely from 10 GPa (ref. 20 17,25 , which is reproduced in Fig. 2 (the broken line separating phases II and III is a kinetic line).…”

“…3. In Table 1, the calculated lattice constants and tilt angle (j) at 11.8 GPa are compared with the experimental values 20 and with those obtained with the empirical force field by Kuchta and Etters 31 . Note that in addition to a, b, c, and j, the CO bond lengths in both CO 2 -I and III have also been optimized at each pressure in our calculations, which, therefore, constitute full optimization with only symmetry constraints imposed.…”

“…In fact, the high-pressure phase transition from I to III was computationally predicted by Kuchta and Etters [31][32][33] prior to the x-ray diffraction study 20 . Their calculated value of the CO 2 tilt angle j of 52°was in exact agreement with the observed.…”

“…More specifically, when Aoki et al 20 determined by x-ray diffraction the crystal structure of an orthorhombic (Cmca) phase, the dry ice or the cubic (Pa3) phase (I) had been the only established crystalline phase of CO 2 (Fig. 1).…”

Thermal Carbon Dioxide Splitting: A Summary of the Peer-Reviewed Scientific Literature

“…26,27 Both phases I and III are stabilized by quadrupolar interactions between the linear molecules. 28 Other polymorphs have been synthesized, including a bent molecular-phase IV, 29 a polymeric phase V, 30 , 31 and a dimeric paired phase II (Figure 1).…”