Solid-state chemistry of early transition-metal oxides containing d0 and d1 cations

This paper presents a brief survey of the structures and properties of early transition-metal oxides containing d0 and d1–d0 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 d0 configuration in many instances, the degree of distortion increasing with decreasing HOMO–LUMO gap. Several characteristic properties of d0 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 d0 MO6 octahedra. Oxides containing d1–d0 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, 3d1 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–phonon interactions.