The Mechanical Properties of CO2‐II with Particular Emphasis on Its Auxetic Potential

Abstract

Carbon dioxide has a rich phase diagram boasting a number of different solid polymorphs. One such solid is CO2‐II, a high‐pressure polymorph of carbon dioxide. Herein, the structural, mechanical, and vibrational properties of CO2‐II are studied using first‐principles density functional theory simulations. In particular, it is shown that the CO2‐II single crystal has the potential to exhibit off‐axis auxetic behavior (negative Poisson's ratio) in the (010) and (001) plane. Moreover, the auxetic behavior is pressure dependent in the pressure range of 15–20 GPa, with an increase in pressure resulting in an increased auxetic behavior, for loading in certain directions, of this system. This anomalous mechanical property is also studied through the use of Raman and infrared spectroscopy. The results obtained suggest that the auxetic behavior of CO2‐II may be the result of the interplay between rotations and distortions of the system.