Perspective on Engineering Transition-Metal Oxides

Engineering transition-metal oxides depends on understanding a few general concepts. Three of these are discussed: (1) orbital mixing and the roles of cation-d/O-2p covalent bonding as distinct from on-site cation orbital hybridization; (2) cooperativity in ordering (a) localized orbitals to remove an orbital degeneracy, (b) ferroic atomic displacements, and (c) bond lengths in a charge-density wave; and (3) cation-site expansion at the crossover from itinerant to localized electron behavior. The latter can stabilize a first-order transition to a ferromagnetic metallic phase on the approach to crossover from the itinerant-electron side or, in a single-valent compound, an intermediate charge-density-wave phase on the approach to crossover from either the localized- or itinerant-electron side. In a mixed-valent compound, a two-phase segregation at a first-order crossover may be static or mobile, and a mobile second phase may become ordered at low temperature to stabilize high-temperature superconductivity.