Constraints on the phase diagram of nonmolecular CO2 imposed by infrared spectroscopy

The investigation of the high-pressure phase diagram of CO2, specifically in the nonmolecular forms, is a challenging experimental issue, due to the large metastability of this material. Here we report on the determination of the pressure threshold for forming nonmolecular CO2 from the molecular crystal, in the P-T range of 10-53 GPa and 296-715 K, respectively. The study is based on a large set of IR spectroscopy measurements, which allows to detect, with a high sensitivity, the onset of the formation of nonmolecular, amorphous carbonia. This metastable form is first formed in the attempts to obtain nonmolecular, crystalline CO2, upon increasing pressure, for kinetic reasons. Therefore, it is identified an upper pressure bound for the real-phase boundary between the molecular solid and the nonmolecular, thermodynamically stable crystal, which results to be remarkably lower than those detected by other techniques. Also, it is shown how the hysteresis of the molecular-to-nonmolecular transformation is reduced upon increasing the temperature. The study adds constraints on the high-pressure, thermodynamic phase diagram of nonmolecular CO2.