Structure, Compressiblity and Thermal Behavior of CO2-IV to 24 GPa and 300-625 K

Sarah Palaich, University of California, Los Angeles

The study of the structure of solid CO2 at elevated pressures is important in many facets of science, including geology, condensed matter physics and material science. Like carbonate, high-pressure CO2 phases have been found to have a changing carbon coordination to tetrahedral at high pressures and temperature (P,T) (Oganov et al., 2008, EPSL, 12, 24). The phases, both metastable and stable, between CO2-I (dry ice, the CO2 phase stable at ambient P,T) and polyhedrally bonded CO2–V are poorly characterized. We used powder neutron diffraction in combination with diamond anvil cell (DAC) techniques to study CO2-IV to 24 GPa between 300 and 625 K.

We performed both isobaric and isothermal experiments on CO2-IV. The powdered sample was initially pressurized to 18.0(2) GPa at ambient temperature in a panoramic DAC. The resistive heating system was then initialized and heated to 625 K. Temperature was then adjusted to 525 K, 450 K, 375 K and 300 K to study the thermal expansion of CO2-IV. 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). Structure determination yielded aH = 8.532(6) Å, cH = 10.48(2) Å, V = 660.9(3) Å3, and ρ = 2.653 g cm-3 at 300 K and 18 GPa.

The second experiment compressed CO2-IV to 17 GPa, then heated to 625 K before cooling to 300 K in several steps. The sample was then compressed at ambient conditions to 24 GPa in 1 GPa increments. We find the compressibility of CO2-IV to fit well with a second-order Birch-Murnaghan equation of state yielding a bulk modulus of 27(2) GPa and V0 = 898(17) Angstroms with K’ set at 4. This is slightly less compressible than DFT models predicted in Datchi et al. (2009).

Abstract Author(s): Sarah Palaich, Adam Makhluf, Chris Tulk, Jamie Molaison, Malcolm Guthrie, Abby Kavner, Craig E. Manning