Crystal Structures of the Compounds RX

Abstract

In generating Laue patterns for RX compounds we had to limit ourselves to some specific types, because several hundreds of such compounds exist and many of which have imperfectly known structures. We selected the four types: NaCl, CsCl, cubic ZnS and hexagonal ZnS — which have a large number of representatives. Most of the data for these structure types have been drawn from Wyckoff [1] and Landolt-Börnstein [2]. On account of the different scattering factors f of specific compounds the Laue patterns reproduced in this book have mainly an illustrative purpose in contrast from those appropriate to the elements. Even if we assume identical crystal structures for a compound RX and an element A, their respective Laue patterns might be quite different. This is caused by the fact that the atomic scattering factor f of an R-atom may be different from that of an X-atom. This means there might be a net intensity in those directions where the Laue pattern of a crystal of element A shows complete extinction. Thus the coordinates of the Laue spots of a compound RX are equal to those of an element with the same crystal structure. The corresponding intensities, however, are usually different. In calculating the structure factor for a given structure type we assumed a fixed ratio of the scattering factors of an R-atom and of an X-atom. This factor, given in Chapter 9, is different for different compounds. Therefore, the plots given in Chapter 9 represent in most cases only a rough approximation with respect to the actual intensity distribution from specific compounds.