WHY ARE SOME OXIDES METALLIC, WHILE MOST ARE INSULATING

A large variety of undoped binary and ternary transition metal oxides, including formally divalent, trivalent and tetravalent metal cations, have been examined. These 76 compounds are classified as either "metals", "insulators", or having a "metal-to-insulator" transition. In an attempt to understand these variations, the Zaanen-Sawatzky-Allen framework was used in which each compound can be characterized by three parameters: the Coulomb correlation or disproportionation energy (U'), the charge-transfer energy (DELTA) and the bandwidth (W). Assuming W is constant, we have calculated U' and DELTA using a simple ionic model, which includes only the gas phase ionization potentials and the bare electrostatic Coulomb interactions between the ions. With this model, the occurrence of metallic conductivity is remarkably well accounted for in these oxides.