Abstract
Cadmium single crystals (99.99% pure), prepared both by the Bridgman method and a solidification technique especially designed to obtain flat crystals, were studied at normal and elevated temperatures by means of X-ray diffraction topography. The former crystals contained a large number of subgrain boundaries, while the latter crystals generally tended to have their wide faces very close to the (0001) planes and contained a small number of subgrain boundaries as well as dislocations. A thermal-cyclic annealing, applied in a range of temperature between 493 and 553 K, and a subsequent isothermal annealing at 473 K prepared a large single crystal with a low dislocation density of less than 103 cm/cm3. When the cadmium crystals were grown and then cooled to room temperature, no dislocation loops were observed, but after 5 to 9 d of leaving in the ambient atmosphere several dislocation loops and dislocation dipoles appeared. The nucleation and growth of such dislocations were explained by considering that the cadmium crystals are oxidized on their surfaces and the formation of oxide layers supplies a lot of vacancies to the inner parts of the crystals. Dislocation loops having c and c+a types of Burgers vector increased their diameter during about 30 d (2.6 Ms) and then shrinked, and the dislocation loops with Burgers vector c extended 3 times faster than those with Burgers vector c+a. The behavior of such dislocation loops was discussed in terms of the vacancy saturation, the line tension of dislocation and the surface state of the oxide layer.
