Abstract
Experiments are described on the polymorphism of thallium halides formed as thin oriented layers by deposition in vacuo on to a variety of bases, in particular on to cleavage faces of crystals. The effect of varying the temperature of the base has been studied in some detail. Measurements have been made of the lattice constant of the abnormal rocksalt type form, using both the reflection and the transmission techniques. In the latter, which gives more accurate results, thallium halides were deposited on to a thin polycrystalline layer of alkali halide previously formed on an amorphous base.