Auxetic behaviour in high pressure polymorphs of CO2 and H2O : a computational chemistry study

Abstract

In this work, a detailed study of the mechanical properties of the single crystal and polycrystalline aggregate of high-pressure polymorphs of H2O and CO2 will be carried out, paying particular attention to Poisson’s ratio of these systems. Using first principles density functional theory (DFT) calculations, this work will show for the first time that the high-pressure polymorphs ice VIII, ice X, CO2-V and CO2-II have the potential to exhibit a negative Poisson’s ratio with the auxetic behaviour of these systems increasing with increasing hydrostatic pressure. To adequately study these systems using DFT simulations, detailed convergence and benchmarking studies will be carried out. It will be shown that contrary to the single crystal, the Poisson’s ratio of the polycrystalline aggregate of these systems exhibits a positive Poisson’s ratio which increases with increasing hydrostatic pressure. The deformation mechanism for ice X, ice VIII and CO2-V will be studied through the application of stress, with the proposed mechanism being consolidated through the use of spectroscopy. In the case of ice X and ice VIII, the auxetic behaviour will be rationalised by studying the deformation of two orthogonally interpenetrating rhombi on application of a stress. In the case of CO2-V, the auxeticity will be explained from a 2D perspective by the relative rotation of semi-rigid projected squares which both rotate and deform on application of a stress. It will also be shown that these 2D squares are projections of 3D CO4 tetrahedra which rotate relative to each other whilst stretching and deforming. In the case of CO2-II, it will be shown that the application of a stress results in a non-continuous change in the structural parameters studied. Thus, a novel approach will be developed in this thesis, where the auxetic behaviour of the system will be rationalised by studying the variation of the Raman active and infrared active modes with varying hydrostatic pressure and comparing any shifts observed in these modes with shifts in the Poisson’s ratio.