Spinel and Spinels

Abstract

This chapter discusses the crystal structure and mechanism for structural distortion of spinels. Spinels are materials that crystallize with a structure related to that of the mineral MgAl 2 O 4 , which is known as spinel. The gross features of the spinel structure may be most easily visualized as a nearly close-packed cubic array of oxygen, sulfur, or selenium anions within which the metal ions occupy certain interstitial positions. They all essentially involve thorough mixing of appropriate quantities of the pure component oxides and firing of the mixture at a high temperature, usually after hydraulic pressing into suitable compacts. The firing stages are particularly critical for obtaining the right proportions of metal ions of the correct valency in the final product, and careful control of the furnace atmosphere both at temperature and during cooling can be vital. Sulfide and selenide spinels require a different procedure because of the lower chemical reactivity of their constituents and lower decomposition temperatures. It is found that in contrast to the electrical insulating or hopping semiconductivity of the oxide spinels, sulfide and selenide compounds tend to possess a band structure conductivity ranging from semiconducting to metallic behavior according to composition.