Solid-state chemistry of early transition-metal oxides containing d(0) and d(1) cations

This paper presents a brief survey of the structures and properties of early transition-metal oxides containing d(0) and d(1)-d(0) electronic configurations. The metal-oxygen (MO6) octahedron, which is the essential structure building unit of these materials, exhibits a characteristic out-of-centre distortion for the d(0) configuration in many instances, the degree of distortion increasing with decreasing HOMO-LUMO gap. Several characteristic properties of d(0) oxides, which include low-dimensional structures (that give rise to intercalation, ion-exchange and acidity), ferroelectricity and non-linear optical response, arise from the out-of-centre distortion of d(0) MO6 octahedra. Oxides containing d(1)-d(0) electronic configurations exhibit an equally impressive array of electronic properties that owe their origin to the nature of d states near the Fermi level. While three-dimensional (3-D) oxides containing 5d and 4d electrons exhibit itinerant electron properties, 3d(1) oxides, especially with low-dimensional (low-D) structures, display localized electron magnetism and semiconduction. Low-dimensional oxides containing 4d electrons, typified by molybdenum bronzes and Mo4O11, exhibit charge-density-wave (CDW)-driven electronic instabilities arising from electron-photon interactions.