Physical Properties and Cation Arrangement of Oxides with Spinel Structures I. Cation Arrangement in Spinels

Abstract

By means of a detailed x‐ray study of a large number of oxides having a spinel structure, it has been established whether they crystallize in the ``normal'' spinel‐type structure, XY<sub>2</sub>O<sub>4</sub>, or in the ``inversed'' type of structure, Y(XY)O₄, discovered by Barth and Posnjak. Certain rules can be derived which are also applicable to more complicated spinels or to solid solutions of different spinel oxides. Madelung constants have been calculated for different spinel types in order to investigate their stability on the basis of an ionic concept of the chemical bond (Born's lattice theory). The experimental results are only partially explainable on this basis. All exceptions to the electrostatic principles can be understood by assuming that Fe³⁺, Ga³⁺, In³⁺, Zn²⁺, and Cd²⁺ favor a configuration having the coordination number 4, because of a considerable covalent contribution to the chemical bond for that configuration.