Ternary Transition Metal Chalcogenides AB2X4

Abstract

Ternary chalcogenides of the type AB2X4 (A, B = transition metals and X = S, Se, Te) generally adopt either the cubic spinel structure or defect structures related to NiAs. It will be shown that both the metal ions and the polarizability of the anion determine which of these structures is favored. In the cubic close-packed anion array of the thiospinel lattice (Fig. 11.1), the metal ions occupy both octahedral and tetrahedral interstices. However, in the nearly hexagonal close-packed array of the NiAs type structures, the metal ions occupy only octahedral interstices. Octahedral vs tetrahedral preference energies of the metal ions that occupy the interstices appear to influence the choice of structure. Spinel formation requires one kind of ion, e.g., Cu(II) 3d 9, which is stabilized on tetrahedral sites, and a second ion, e.g., Ti(III)(3d 1), which is stabilized on octahedral sites. Hence, an ion with a strong preference for tetrahedral sites, e.g., Cu(II), Zn(II), might result in formation of a spinel. However, when both metal ions are stabilized on octahedral sites, the NiAs-type structure can form. Furthermore, increasing the anion polarizability in the sequence S<Se<Te increases octahedral site stabilization, which in turn favors the NiAs-type structure over the spinel. In this presentation, the preparation and properties of the thiospinels will first be discussed, followed by a treatment of the ternary NiAs-type structures.