Under conditions of high pressure and temperature (about 35 GPa at 1500 K), carbon dioxide (CO2) transforms into polymeric extended solid, opposed to the more familiar molecular form. The stability of this polymeric phase (termed CO2-V) extends to the top of Earth's lower mantle and could play a significant mineralogical role. Until now, the structure of CO2-V has remained controversial, but affiliates of the Deep Carbon Observatory have published the first definitive structural solutions to this long-standing problem.
Using high-pressure diamond anvil cells combined with high-temperature laser heating, the authors of both studies formed CO2-V and probed the crystal structure using synchrotron x-ray diffraction measurements. Through Rietveld analysis methods, both groups determined that CO2-V is structurally related to the b-cristobalite SiO2 polymorph, comprised of CO4 tetrahedral units. Interestingly, the CO4 units possess O-C-O bond angles that average 109.5°, in stark contrast to molecular carbon dioxide. Structural determination as a function of pressure revealed the bulk modulus of CO2-V to be about 136 GPa, significantly less than the 365 GPa reported previously.
The structural information obtained for CO2-V may now be used to examine interactions with other mineral phases and provide insights concerning segregation of carbon and oxygen in Earth's mantle. Deep Carbon Observatory investigator Alexander Goncharov stated "it is truly a great achievement that the structure of this phase has finally been solved."
Figure Caption: The structure of CO2-V at 43 GPa showing partially collapsed arrangement of CO4 tetrahedra. From Santoro and others (2012). Reproduced with permission.