Dislocations and grain boundaries in polycrystalline ice: a preliminary study by synchrotron X-ray topography

White-beam synchrotron X-ray topography has been used to image dislocations and grain boundaries in high-purity columnar-grained polycrystalline ice. It was found that screw, 30° and 60° basal dislocations with 〈 1 1 -2 0 〉 Burgers vectors far outnumber other dislocations: near free surfaces, the dislocations were bent because of image forces. Circular prismatic dislocation loops with [0 0 0 1] Burgers vectors and dipoles were also found. These probably formed due to thermal shock. In one sample, the dislocation structures of three grains were clearly observed simultaneously, although no dislocations were visible in the boundaries.