Calcium Fluoride : Slip and Polygonization during Crystal Growth

Abstract

CALCIUM fluoride occurs in Nature as the crystal form known as fluorite. It can, however, be grown artificially as single crystals, and when doped with suitable impurities it is widely used as a laser material^1,2. For this purpose, calcium fluoride is usually doped with rare-earth impurities or uranium and grown by either the Czochralski³ or Stockbarger⁴ techniques. The Czochralski technique is inherently a high temperature-gradient method of growth, in which the crystal is pulled directly from the melt, whereas the Stockbarger process, in which the molten material is crystallized inside a crucible in a controlled manner, involves much lower temperature gradients. During etching investigations on crystals grown by these techniques, it was noticed that certain similarities and differences existed in the dislocation arrangements induced by the method of growth, and that some of these could be modified by subsequent annealing. These features are described and compared with the dislocation arrays observed in crystals of other materials grown using the Czochralski technique.