The octahedral cluster compounds of early transition metals: An original class of dielectric materials

Abstract

Octahedral clusters of early transition elements appear either in the face-capped M₆Lⁱ ₈L^a ₆ units for M = Mo, W, Re or in the edge-capped M₆Lⁱ ₂L^a ₆ ones for M = Nb, Ta (L = oxygen, halogen, chalcogen). These units constitute the building blocks of a large variety of crystal structures. In most of them, owing to the steric hindrance of the ligands, the M₆ clusters cannot interact and such materials are insulators. These units are usually centro-symmetric when all inner or apical positions are occupied by a same ligand. In contrast, in oxyhalides, chalcohalides and chlorofluorides, these positions are occupied by two different ligands either ordered or randomly distributed and thus, an acentric distribution of the anionic charges around the M₆ cluster can appear. In this case the units act as permanent dipoles and dielectric relaxations can be observed. In this paper we recall the crystal structures of M₆ cluster compounds built from acentric units and some relevant examples of unusual dielectric behaviour.

Key Words:

• Metallic clusters
• Early transition elements
• Crystal structures
• Halides
• chalco-halides
• oxyhalides
• Dielectrics