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

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Abstract
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.