Discontinuities at the melting point of bismuth

Abstract

During the last two or three years considerable attention has been given to the growth of single crystals of bismuth and to their physical properties. The result of this work has shown that the preparation of a good single crystal is exceedingly difficult, the process being easily disturbed in many ways, and the conclusion must be drawn that it has not yet been possible to obtain anything approaching a perfect crystal. In all cases where the grown crystal has been adequately examined, it has been necessary to admit that the supposed single crystal was full of discontinuities. This was shown by Kapitza* in his work on the growth and magneto­-resistance change of single crystals of bismuth. He found it very difficult to produce a crystal rod parallel to the hexagonal axis that was flexible. Almost all the crystals grown with this orientation were apparently brittle in that a very little strain would cause a sharp fracture parallel to the principal cleavage plane. Some crystals indeed broke up into small pieces 2 or 3 mm. long when dropped from a height of a few centimetres on a table. As the separate pieces proved to be fairly flexible this brittleness is not a real property of the crystal, but appears to be due to isolated cracks or discontinuities in the crystal (the word “crack” is used with the meaning defined by Kapitza in his paper, where further arguments for their existence from X-ray and resistance measurements are given). These cracks are not characteristic only of the brittle crystals, but appear to be present in many flexible crystals as well, their flexibility being due to a random distribution of a comparatively large number of cracks small in area compared to the dimensions of the rod.