Valency, valence degeneracy, ferroelectricity, and superconductivity

Applications of the oxidation state and valent state systems are discussed with particular emphasis on inorganic solids. Areas addressed include mixed valency, mixed electronic configurations, disproportionation and superconductivity, real charges, valence degeneracy, and stabilization of high oxidation states. The most significant advance to the oxidation state system since its inception is the bond valence approach where every bond between a cation and an anion is assigned a valence value. It is demonstrated that this bond valence approach can be used to understand ferroelectric transitions in oxides whether or not d orbitals are involved. Differences between the oxidation state system and the valent state system are delineated, and examples of common misuse of these systems are discussed.