Simple and perovskite oxides of transition-metals: Why some are metallic, while most are insulating

Some 76 simple and perovskite transition-metal oxides are classified as "metals," "insulators," and those exhibiting metal-insulator transitions. Using the framework of Zaanen, Sawatzky, and Allen and a simple ionic model to estimate the two relevant energies (Δ0 and U′0), we can find boundaries which separate the insulating oxides from two types of metals: low Δ0 metals and low U′0 metals. In addition, compounds with metal-insulator transitions are found to be on (or near) these boundaries. It is concluded that the large differences in conductivity behavior of oxides are largely due to differences in the ionization potentials of the transition metal cations. © 1991.