Growth and perfection of ice crystals

Abstract

Attempts were made to grow perfect, or nearly perfect, ice single crystals for solid state studies. Dislocation densities as low as 10 2 cm -2 were found in crystals grown by the modified Bridgman method whereas using the Czochralski method the lowest densities found were of the order of 10 4 cm -2. The configurations and structures of dislocations in the crystals grown by the different techniques and under various conditions were examined by X-ray diffraction topographs. Characteristic features of dislocation images in topographs are interpreted as showing that the Peierls trough is shallow for dislocations having a 1 3 〈11 20〉 Burgers vector on the basal plane. Thus, dislocations in the basal plane extend perpendicular to the growth interface due to the preferential line tension when the crystal was grown in the direction perpendicular to the c-axis. The generation of dislocation densities of the order of 10 4 cm -2 in Czochralski grown crystals is mainly attributed to those inherited from the large area of seed crystals and in addition to the thermal stresses caused by steep temperature gradients in the crystal. Reduction of dislocation densities to the order of 10 2 cm -2 in Bridgman grown crystals is achieved by limiting the inheritance of dislocations or by the selection of a single mosaic from the seed crystal through a neck of the growth cell. Dislocation loops of [0001] Burgers vector and stacking faults found both in NH 3-doped ice are described.