X-ray topographical study of dislocations in pure and HF-doped ice

Abstract

Dislocations in ice have been studied by X-ray transmission topography, and dislocation densities in various types of ice have been determined. The Burgers vector, b, of the vast majority of basal (0001) dislocations is ⅓ ⟨1120⟩ confirming results of previous workers. Experimental image widths of the dislocations agree with theoretical calculations, and a bimodal intensity profile is observed in some reflections. In successive topographs taken at—10°C, dislocations are shown to have moved. Within approximately 100 μm of the surface, the dislocation density is much lower than in the bulk of the crystal and those dislocations that are present are usually perpendicular to the surface. At—18°C, when HF is diffused into a crystal, the total dislocation density is increased by a factor of about five, and many prismatic loops appear, grow, and then shrink. The rate of shrinkage is governed by the diffusion coefficient of HF in ice, rather than the vacancy diffusion coefficient. The increase in the number of b = ⅓ ⟨1120⟩ dislocations, due probably to a multiplication mechanism, can account for the mechanical softness of HF-doped ice.