Reply to Datchi et al.: Recovered phase CO2-V at low temperature and a newly predicted 3D-extended CO2 phase.

In their letter, Datchi et al. (1) question (i) the recoverability of CO2-V at low pressure and (ii ) the identification of CO2-V as a Pna21 structure in our paper (2). We argue below that both of Datchi et al.’s (1) claims are incorrect. Regarding Datchi et al.’s (1) first claim, it is well known that polymeric phase V of CO2 (CO2-V) is metastable at low pressures below 10 GPa, where phase I is a thermodynamically stable phase. As such, in this pressure region CO2-V transforms back to phase I rather rapidly, especially below 3–5 GPa. This point is highlighted in figure 5A of our paper (2), showing a rapid conversion of CO2-V below 10 GPa, as indicated by the rapid reduction in Raman intensity of its signature vibron at ∼700 cm. This result clearly shows that nearly no phase V remain at 4 GPa at ambient temperature in a time-scale of experiments (seconds to minutes). In contrast, the CO2-V phase at 180 or 191 K at ambient pressure is quite different, showing “kinetically” controlled stability over hours to a day.Note that there is absolutely no change in the intensity of the 650-cm vibron of phase V, compared with that at 11 GPa. Thus, its rapid disassociation (or sublimation) occurs above ∼200 K; the melting point of dry ice phase-I at ambient pressure suggests that the apparent “low”melting temperature of CO2-V is likely of a kinetic origin, not the melting of extended CO2-V. The peak at low frequency (below 200 cm) may well be coming from solid phase I at 191 K at ambient pressure, although the observed peaks do represent some different intensities and peak positions. On the other hand, this is not at all unusual, because CO2-V is always surrounded by untransformed CO2-I. The important aspect of this finding is the fact that CO2-V do exist “happily” with CO2-I at ambient pressure and low temperature without CO2-V being transformed to CO2-I, again underscoring the kinetically controlled metastability. As to Datchi et al.’s (1) second claim, in our paper (2) we did not present the Pna21 as the conclusive structure of CO2-V, but suggested it as a potential new structure for extended CO2. We have also pointed out that there are some differences in the calculated X-ray pattern (3) from those of experimental CO2-V in our paper (2). Nevertheless, we have emphasized the presentation of a new potential Pna21 structure for CO2 in a 3D network with fourfold sp carbon atoms. Therefore, we just marked Pna21 as CO2-V’. Our predictions may provide insights into many potentially possible metastable phases in local energy minima, just like the CO2-VTR (P212121). Moreover, our computed Raman spectrum of Pna21 structure in Fig. 1 disagrees with the calculation reported from Datchi et al. (1). In addition, the citation of experimental Raman spectrum of phase V in figure 1 of Datchi et al. (1) is incorrect, because in Yoo et al. (3) there is no Raman spectrum of phase V.