DEFECT STRUCTURE OF CUBIC SOLID-SOLUTIONS OF ALKALINE-EARTH AND RARE-EARTH FLUORIDES

In this paper we will consider the disorder in some cubic solid soluti ons consisting of one of the alkaline earth fluorides and one of the r are earth fluorides. This is an attractive group of model materials, b ecause these materials have a rather simple overall cubic structure. W e will discuss the most important features of the structure of these s olid solutions, which have been obtained using various spectroscopic m ethods. From these experiments we will conclude that, depending on the radii of the ions in the host alkaline earth fluoride crystal and the trivalent impurities the defect structure varies significantly. In so me solid solutions evidence for appreciable preferential clustering ha s been found. On the other hand there is substantial evidence that for some other materials there is little or no preference towards cluster ing for impurity concentrations upto even a few mol%; here, the defect structure is determined more or less by statistics.