Thermodynamics and Defect Chemistry of Some Oxide Solid Solutions Part I: Nearest‐Neighbour Interactions and the Effect of Substitutional Disorder

Abstract

AbstractThis paper is concerned with solid solutions of two isostructural ionic oxides. Relations are derived between activities and composition on the assumption that nearest‐neighbour interactions only are involved. For the solutions under discussion the consequences of this assumption are that the distribution of the cations present in any sublattice is random and that the activity of each kind of cations in a particular sublattice is equal to its concentration in that sublattice. The proof is given that such solutions will be ideal, if they show no substitutional disorder. A comparison is made with some experimental data given in the literature. Three types of disorder may in principle be present simultaneously in binary solutions of the spinels Mn3O4, Fe3O4 and Co3O4. (a) Exchange of electrons between cations of one element on different sites. (b) Exchange of electrons between cations of different elements. (c) Preference of the cations of one element for tetrahedral or octahedral sites. On the basis of these assumptions equations are derived controlling the equilibria which correspond to these types of disorder. A study of the literature shows that the system Mn3O4‐Co3O4 obeys these equations. This is not the case with the systems Fe3O4–Co3O4 and Fe3O4–Mn3O4. The possible reasons for this discrepancy between theory and experiment are discussed and lead to a more general theory, which will be presented in Part II.