The crystal structure of ice Ih, showing the arrangement of molecules projected onto the plane. When a dislocation moves along a slip plane, material above the plane moves relative to the material below by an amount equal to the Burgers vector. This movement does not affect the arrangement of oxygen atoms as they remain in tetrahedral coordination with the surrounding oxygens. The disorder of protons is an infringement of the Bernal-Fowler rule . The propagation of a dislocation through the basal plane of an ice lattice.
Oxygen atoms remain in tetrahedral co-ordination after the dislocations have passed.
However, Bjerrum defects are produced by the passage of a dislocation leading to disordering in the co-ordination of the hydrogen atoms. Another possible re-arrangement of protons that avoids the creation of Bjerrum defects will occur if the hydrogen atom crosses the bond and joins the neighbouring oxygen before the dislocation disconnects the bond. This will result in the creation of a cation and a anion. The energy required to create a pair of ionisation defects is greater than that required for a Bjerrum defect so that ionisation defects are less likely to be created. The yield stress observed in shear experiments is more than three orders of magnitude less than the theoretical value.