ABSTRACT
Relations between the composition, structure and some physical properties of transition metal chalcogenides (sulphides, selenides and tellurides) are discussed in a qualitative way. A simple band model is given which explains the behaviour of lanthanide mono- and dichalcogenides. The model is also applied to transition-metal chalcogenides taking account of ligand-field effects. Due to the rather small electronegativity of the chalcogens, high oxidation states of transition metals are not stable in chalcogenides, but are reduced with creation of holes in the (chalcogen) valence band. These holes may (or may not) condense, for instance by the formation of chalcogenide pairs (X,)2-. d-Bands can be formed if the covalency is large or if metal-metal distances are short, for instance by the presence of metal chains or clusters. Chalcogen d orbitals participate in the bonding with transition metals which strengthens the metal-to-chalcogen bonds and favours trigonal-prismatic coordination of the chalcogen. The accessible d orbitals also cause the large polarizability of chalcogenide ions, which in turn gives rise to large defect concentrations and broad homogeneity ranges of transition-metal chalcogenides, stabilizes structures in which the chalcogen has a polar environment (for instance layer, fibre, channel and misfit structures) and allows distortions of the structures by formation of metal chains or clusters. Several examples are given.
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