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

Octahedral clusters of early transition elements appear either in the face-capped M6L81L6a units for M = Mo, W, Re or in the edge-capped M6L121L6a 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.