Crystalline polymeric carbon dioxide stable at megabar pressures

Abstract

Carbon dioxide is a widespread simple molecule in the Universe. In spite of its simplicity it has a very complex phase diagram, forming both amorphous and crystalline extended phases above 40 GPa. The stability range and nature of these phases are still debated, especially in view of their possible role within the deep carbon cycle. Here, we report static synchrotron X-ray diffraction and Raman high-pressure experiments in the megabar range providing evidence for the stability of the polymeric phase V at pressure-temperature conditions relevant to the Earth’s lowermost mantle. The equation of state has been extended to 120 GPa and, contrary to earlier experimental findings, neither dissociation into diamond and ε-oxygen nor ionization was observed. Severe deviatoric stress and lattice deformation along with preferred orientation are removed on progressive annealing, thus suggesting CO2-V as the stable structure also above one megabar.

Structural and Chemical Modifications of Carbon Dioxide on Transport to the Deep Earth

“…The new volume data and those obtained with CO 2 -V compressed in He (Datchi et al, 2012) could be fitted with an EOS that further lowers the bulk modulus to a value (114 GPa) very close to that reported in Santoro et al (2012). Interestingly, while the a lattice parameter nicely fits the evolution of the lower pressure data, the c parameter is considerably longer with the c/a ratio increasing from 1.655 at 43 GPa (Santoro et al, 2012) to 1.871 at 118 GPa (Dziubek et al, 2018), thus getting closer to the fully collapsed structure, which expects a c/a ratio equal to 2 (Figure 6.2).…”

“…Higher pressure experimental volume data up to 100 GPa (Seto et al, 2010) exceed this EOS by ≤5%. Very recently, the synthesis of CO 2 -V above one megabar allowed the EOS extension up to 120 GPa (Dziubek et al, 2018). The new volume data and those obtained with CO 2 -V compressed in He (Datchi et al, 2012) could be fitted with an EOS that further lowers the bulk modulus to a value (114 GPa) very close to that reported in Santoro et al (2012).…”

“…A triple point between the fluid, phase IV, and the extended phase V was identified at 33 GPa and 1720 K. Another triple point between phase V, fluid, and dissociated CO 2 is located around 35 GPa and 1900 K, thus making the stability region of extended CO 2 extremely limited at the P-T conditions typical of the mantle. As already stated, this claim was recently confuted by producing CO 2 -V at pressures and temperatures largely exceeding these conditions (Dziubek et al, 2018).…”

“…The Raman spectrum of this material is claimed to match that of orthorhombic (P2 1 2 1 2) carbonyl carbonate (COCO 3 ). These results have been recently challenged by the synthesis of CO 2 -V above 1 Mbar (Dziubek et al, 2018). In this work, phase V was synthesized by laser heating amorphized CO 2 at 85 GPa and 2700 K; the quenched phase V was laser heated again at the same temperature and a pressure just above 1 Mbar, the same conditions that should ensure the formation of i-CO 2 , but the only effect was the obtainment of a high-quality perfectly annealed phase V (Figure 6.2).…”

“…In all these cases, the sample was indirectly laser heated using Pt, B, or ruby as absorbers. Both dissociation and the formation of an ionic compound are challenged by the most recent studies, where CO 2 -V was synthesized with a remarkable quality by laser heating carbonia between 85 GPa, the same pressure as in Yoo et al (2011), and 115 GPa at temperatures in the order of 2500 K (Dziubek et al, 2018). Remarkably, repeated heating cycles have the only effect of producing a better crystallinity of the extended phase V, as expected in a normal thermal annealing process, and suggest that phase V is the thermodynamic stable phase in this portion of the phase diagram.…”