Chapter 4 Oxide spinels

Abstract

This chapter discusses the crystal structure, chemistry, and magnetic properties of oxide spinels. The spinel structure is one of the most frequently encountered with the MM′2X4 compounds. X represents oxygen or some chalcogenic bivalent anion that may be partly substituted by proper monovalent anions; M, M′ are metallic ions whose valencies have to fulfil the electroneutrality requirements. Because of the large electronegativity of oxygen, the ionic type of bonds prevails in almost all oxide spinels. Therefore, the electrical resistivity is usually high and allows classifying the compounds as insulators even though sometimes the term low-mobility semiconductors would apply. The spinel structure is called after the mineral spinel MgAl204. It is formed by a nearly close-packed fcc array of anions with holes partly filled by the cations. There are two kinds of holes differing in coordination: tetrahedral and octahedral. From all these sites available in the elementary cube containing 32 anions, only eight of the tetrahedral and 16 of the octahedral are occupied by cations. The oxide spinels may usually be prepared at elevated temperatures by a direct solid-state reaction among the simple oxides. The relevant temperature range is about 800 to 1500 °C, depending on the type of cations.