The disorder in silver bromide

In practice, the explicit calculation of the concentrations of all defects as functions of the independent thermodynamic variables (which is the basic problem in defect thermodynamics) in this general form is not an easy matter. However, it is also mostly unnecessary [4]. The reason for this is that by introducing disorder types (i.e. majority and minority defects) we can neglect the concentrations of the minority defects in the balance equations and in the conditions of electroneutrality. The various relationships then become immediately clear. 

In the following example concerning the equilibrium disorder in AgBr we shall illustrate the principles which we have been discussing, and vve shall illustrate the methods by which such problems may be treated. The methods will also apply for higher ionic crystals where only the number of components, and therefore the number of external equilibrium conditions, is increased. 

The disorder in AgBr will be discussed as a function of the component activities at given P and T. Because of the reaction equation Ag + 1/2 Br2 = AgBr, (d activities of 

Therefore, it makes no fundamental difference which of these two activities is chosen as the independent variable. In practice, is easily fixed. Therefore we shall choose this variable for the formulation of the reaction equations and the corresponding equilibria. This can be generalized as follows. In practice, the partial pressure of the electronegative component is frequently an easily controllable and determinable variable. Furthermore, silver bromide is always found in the standard state, since deviations from the stoichiometric composition AgBr are not chemically measurable for all values of the independent variables. Possible defects are Agj, Agi, V g, Br(, Br g, Var, and finally, the electronic defects e and h (i.e. excess electrons and electron holes which can be identified in the particle terminology with AgAg and Brir)- For the purpose of listing the possible disorder types (i. e. combinations of majority 

As external equilibrium condition (i.e. as the relationship between the chosen independent variable and the dependent defect concentrations at fixed P and T), the following reaction equation between components and structural elements may be written  

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