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However, many questions still remained unanswered. Some reports suggest that in pressures up to 70 GPa, CO2 dissociates with a negative pressure-temperature slope of the reaction threshold. Still, no products of decomposition were identified in the samples studied at room temperature after the laser heating [1]. One of the supposed explanations for this observation was the possibility of a back transformation upon temperature quenching. Another assumption was that the dissociation is coupled with melting of the polymeric CO2-V phase, considering that a transition from a covalent network to liquid state can be associated with chemical bond cleavage. To date however, CO2 had not been characterised in situ at high temperature in the extreme pressure range.

In order to effectively explore this challenging pressure- temperature landscape, a compressed sample of CO2 was heated using a CO2 laser and a MgCO3 laser coupler, ensuring very efficient heating while eliminating the risk of sample contamination. Moreover, the gold lining of the rhenium gasket was used as a pressure gauge and to prevent a reaction between hot, dense CO2 and Re. In this environment, the sample compressed within the pressure range 37-106 GPa was brought to temperatures as high as 6200 K (Figure 4). Furthermore, to reduce to a minimum the likelihood of incomplete transformations, the isolation of kinetically trapped transient structures or unrelaxed stress, the duration of annealing was expanded

PRINCIPAL PUBLICATION AND AUTHORS Extending the Stability Field of Polymeric Carbon Dioxide Phase V beyond the Earth s Geotherm, D. Scelta (a,b), K.F. Dziubek (a), M. Ende (c), R. Miletich (c), M. Mezouar (d), G. Garbarino (d), R. Bini (a,b,e), Phys. Rev. Lett. 126, 065701 (2021); https:/doi.org/10.1103/PhysRevLett.126.065701 (a) LENS, Sesto Fiorentino (Italy) (b) CNR-ICCOM, Sesto Fiorentino (Italy) (c) Institute of Mineralogy and Crystallography, University of Vienna (Austria) (d) ESRF (e) Dipartimento di Chimica Ugo Schiff , University of Florence, Sesto Fiorentino (Italy)

REFERENCES [1] K.F. Dziubek et al., Nat. Commun. 9, 3148 (2018).

up to about 40 min per cycle. Detailed inspection of high- temperature diffraction patterns obtained at beamline ID27 revealed that all the Bragg diffraction peaks could be attributed either to the solid CO2-V phase or to MgCO3 and Au as other components in the sample chamber. It should be emphasised that no signs of dissociation were perceived in the explored pressure-temperature window (Figure 5).

On top of that, despite the noticeable variation in granularity of the Debye-Scherrer rings with time, in all the recorded diffraction patterns the signal of CO2-V was clearly visible. This observation casts a shadow of uncertainty over the calculated literature data on the CO2-V melting line. However, this result can be explained in various ways. If the melting conditions were not achieved in the experiment, the theoretically determined model of melting curves should be revisited. On the other hand, if only partial melting was realised, an explanation of discrepancy between the temperature recorded by pyrometry measurements and the predictions can be given by the substantial temperature gradient in the pressure chamber. Presuming the laser hot spot of ~25 μm, the X-ray beam focused down to ~3 μm (increased by the signal tails) and the initial thickness of the gasket of ~30 μm, the CO2 melt zone may form only in the fraction of the volume probed by the X-ray beam (Figure 5). While neither of these hypotheses can be excluded, these findings contribute to a better understanding of chemistry and physics in hot dense carbon dioxide.

Long-term storage of atmospheric CO2 in the deep Earth facilitated by poorly soluble carbonates

The atmospheric CO2 level controls our climate. While part of it can be transported via the subduction of carbonates into the deep Earth, its storage at depth has, so far, not been considered due to the high solubility of these phases. This work demonstrates that carbonate phases that are less abundant at the Earth's surface contradict this concept.

Water vapour and carbon dioxide are important greenhouse gases. The evolution of their concentration in the atmosphere determines the climate over geological time. These gases are strongly governed by the deep carbon and water cycles shown in Figure 6, starting with their outgassing from the deep Earth into the atmosphere, their sequestration on the surface due to the formation of biotic and abiotic carbonates and their transport back to depth via subduction. Subducting oceanic plates can indeed carry substantial amounts of water [1] and carbon [2,3] to depth in subduction zones (Figure 6).