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) electroni c configurations. The metal-oxygen (MO6) octahedron, which is the esse ntial structure building unit of these materials, exhibits a character istic out-of-centre distortion for the d(0) configuration in many inst ances, the degree of distortion increasing with decreasing HOMO-LUMO g ap. Several characteristic properties of d(0) oxides, which include lo w-dimensional structures (that give rise to intercalation, ion-exchang e and acidity), ferroelectricity and non-linear optical response, aris e from the out-of-centre distortion of d(0) MO6 octahedra. Oxides cont aining d(1)-d(0) electronic configurations exhibit an equally impressi ve 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, dis play localized electron magnetism and semiconduction. Low-dimensional oxides containing 4d electrons, typified by molybdenum bronzes and Mo4 O11, exhibit charge-density-wave (CDW)-driven electronic instabilities arising from electron-photon interactions.