Comments about Cationic-Anionic Redox Competition in the Solid State. The Formation of Anion Associations in the Solid State

The formation of structural entities ranging from anions in simple contacts to true anionic polymers in the solid state is discussed in terms of a competition between d cationic levels and sp anionic ones. Transition metal dichalcogenides, which show two structural domains (layered on the left, tridimensional on the right of the periodic table), provide a wide series of phases to illustrate the process. Polymerization can occur in slabs or from slabs to slabs in the layered structures. Except a few particular cases, like MnTe2, most of the so-called pyrites are also anion-polymerized phases. The electronic transfer, which corresponds to a transfer to the cation of electrons taken from the top of the sp band, may result in the formation of metal-metal bonds. The holes created at the top of the sp band can remain isolated instead of condensing in anionic associations. CrTe3 shows a particular anion-anion exchange which causes the polymerization. Phases like P x VS2 or Si x CrSe2 are layered phases stabilized by an electronic transfer from the extra elements (Si or P) to the top of the valence band, impeding the formation of anionic associations.