Basal dislocations in proton-ordered hexagonal ice

Abstract

We use molecular dynamics modeling to investigate the core structures and critical resolved shear stresses for glide of basal dislocations in proton-ordered hexagonal ice. Straight and kinked dislocations of 60° and screw character residing on shuffle and glide set planes are investigated. Except for the 60° shuffle, all these dislocations are sessile: they do not move for modulus-normalized resolved shear stresses up to 0.11 or higher. Straight 60° shuffle dislocations move at different critical resolved shear stresses, depending on the orientation of the core relative to molecules in the crystal as well as on the sense of loading. The lowest modulus-normalized stress to move a straight 60° shuffle dislocation is 0.044: comparable to the Peierls barrier of diamond. Depending on their structure, kinks can either pin the dislocation or lower its critical resolved shear stress to a modulus-normalized level of 0.037. Our work shows that proton ordering does not ease dislocation glide in hexagonal ice. We discuss the consequences of these findings for theories of plastic flow in ice Ih and ice XI.