Structure and Thermal Behavior of CO2-IV at 18 GPa from 300-625 K

Abstract:

The study of the structure of solid CO₂ at elevated pressures is important in condensed matter physics and material science, as well as geophysics—especially in understanding the structure of the interiors of planetary bodies. The high-pressure phases, including CO₂-IV, and their stability between CO₂-I (dry ice, the CO₂ phase stable at ambient P,T) and polyhedrally bonded CO₂–V are poorly characterized. Yoo et al. (2001, PRL, 86, 3) and Park et al. (2003, Phys Rev B, 68,1) concluded that the CO₂ molecule is bent in phase IV, with C=O bond lengths of up to 1.3-1.5 Å, longer than the 1.168 Å in CO₂-I. In contrast, Gorelli et al. (2004, PRL, 93, 20) and Datchi et al. (2009, PRL, 103, 18) inferred that the CO₂ molecule in phase IV is linear, and the double-bond length is nearly identical to that of CO₂-I. The studies also disagree on the structure of CO₂-IV: tetragonal P4₁2₁2, orthorhombic Pbcn and rhombohedral R-3c have all been reported.

We performed neutron diffraction experiments at SNAP, Spallation Neutron Source, ORNL. The powder sample was pressurized to 18.0(2) GPa at ambient temperature in a panoramic DAC. The resistive heating system was then heated to 625 K. Temperature was adjusted to 525 K, 450 K, 375 K and 300 K to study the thermal expansion of CO₂-IV and provide data for a P,T equation of state. At 18 GPa and 625 K the structure of CO₂-IV is well indexed by the R-3c structure found by Datchi et al. (2009, PRL, 103, 18). The rhombohedral symmetry was retained as temperature was lowered through the proposed stability fields of both CO₂-II and CO₂-III to 300 K. Structure determination yielded a_H = 8.532(6) Å, c_H = 10.48(2) Å, V = 660.9(3) ų, and ρ = 2.653 g cm^-3 at 300 K and 18 GPa. Preliminary linear thermal expansion parameters using the Holland-Powell (1998, J Metamorph Geol, 16, 3) model range from 5.7(6) x 10^-6 K^-1 to 1.39(1) x 10^-5 K^-1 for the lattice planes indexed. A fit to the volume yields a thermal expansion of 3.530(2) x 10^-5 K^-1. These results indicate that CO₂-IV is comprised of linear molecules, supporting the structure determined by Datchi et al. (2009). Furthermore, phases II and III were not observed, suggesting that these phases are metastable at these conditions. These insights help us to more fully understand the behavior of CO₂ at high P,T in Earth and planetary bodies.