X-ray investigations on the growth of cadmium whiskers

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Whiskers of “pure” and “spectroscopically pure” cadmium grown in vacuum of the order of 10 -6 mm Hg and in an inert gas atmosphere were studied. Deliberate introduction of spec. pure tin and lead as impurities was tried in some cases. Tin did promote whisker growth but not through the VLS mechanism. Lead remained inactive. The direction of growth was mainly observed to be [ 1 ¯ 2 1 ¯ 0 ]. Three new directions of high index orientations were detected. The studies of imperfections were carried out with the Laue technique combined with a microfocus X-ray source. It was found that the imperfections vary much along the lengths and sides of the whiskers. Cases were detected where the structure of whiskers got drastically simplified towards the tip. Single twist around the growth axis of the whiskers was not found, but curvature of the planes inside the whiskers was commonly observed. They varied from several tenths to several tens of centimetres. They varied perfect whiskers showed some substructure (splitting of diffraction spots into doubletsin the nonequatorial reflections). It was found experimentally that the apparent simplicity of the equatorial layer spots is not a lattice property but is due to the geometry of the reflection itself. The structure of two kinked whiskers was also studied. In one case the kink occured as a continuation of the more perfect part of the crystal, in the other, a secondary crystal formation was the cause of the kinking.