Physico-chemical properties of diatomic crystals in relation to the incorporation of foreign atoms with deviating valency

Abstract

If foreign ions of a valency deviating from that of ions of the base lattice are incorporated electro-neutrality is maintained. This is effected in a number of ways, some of which are already known. For example, electro-neutrality may be maintained by the formation of vacancies, or by the occupation of interstitial sites. Koch and Wagner gave as an example of this mechanism the case of AgCl containing some CdCl2. Another mechanism is that where the incorporation of the foreign ions is accompanied by a reduction or an oxidation of the base lattice, which prevents the formation of lattice imperfections. Here the compensation of charge is effected by the excess electrons or holes introduced in the lattice by the reduction or oxidation. (Verwey, Haaijman and Romeijn; Selwood; Wagner; Capps and Weijl; Hauffe et al.). It is shown that it is possible to consider the various ways of maintaining the electro neutrality from a general point of view. There are a number of factors which govern the way the electro-neutrality can be maintained. Among these are the concentration of the foreign ions, the tendency of the base lattice to form lattice imperfections, the position of the energy levels associated with the various lattice imperfections, and the width of the forbidden zone. Another factor of importance is the atmosphere in equilibrium with the compound. In such a way a third mechanism of maintaining the electro-neutrality is found. Sometimes, for the same compound two or three different mechanisms, depending on the composition of the surrounding atmosphere, may be found. It is explained how the manner of incorporation of foreign ions may differ for different base lattices. Application of these considerations to CdS as a base lattice gives a satisfactory agreement with experiment.