Unusual mechanical properties of ice VIII: Auxetic potential in a high pressure polymorph of ice

Abstract

Water is known to have a complex phase diagram with numerous solid polymorphs, one of which is ice VIII, a proton ordered high pressure form of ice. In this work the mechanical properties of ice VIII were studied through the use of first principles density functional theory (DFT) simulations. In particular, it was shown that ice VIII has the potential of exhibiting a negative Poisson's ratio in the (110) plane on application of an on-axis stress. The auxetic behaviour of ice VIII was found to increase with an increase in hydrostatic pressure. This auxetic potential together with its pressure dependence was rationalised through the nanoscale deformation of this system when subjected to a series of stresses as well as through spectroscopic techniques. Our simulations suggest that the auxetic behaviour of ice VIII is due to the geometry of the oxygen sublattice in conjunction with an interplay between two deformation mechanisms namely stretching and hinging. As the hydrostatic pressure is increased the stretching mechanism becomes less significant resulting in the hinging mechanism becoming the predominate deformation mechanism.